Patent Publication Number: US-8124359-B2

Title: Use of additives for the reduction of non-specific binding in assays

Description:
FIELD OF THE INVENTION 
     The present invention relates to methods for reducing non-specific binding in assays for the detection and/or measurement of an analyte in a sample. In particular, the invention relates to the use of additives to reduce non-specific binding in assays. 
     BACKGROUND OF THE INVENTION 
     Fluorescence polarization and fluorescence intensity measurements provide a powerful means by which macromolecular association reactions can be studied. These fluorescent techniques have been applied to study antigen-antibody, hapten-antihapten, protein-ligand, and protein-DNA interactions. 
     The inherent sensitivity of fluorescence measurements can be used in monitoring the extent of reaction as a fluorescent reactant, F, combines with its macromolecular partner, R: 
                         
where k 1  is the forward reaction and k −1  is the back reaction such that (k 1 )/(k −1 )=K (eq) .
 
     The investigator can choose to follow changes in the fluorescence polarization (FP) and/or the fluorescence intensity (FI). If the reactants do not have natural fluorescence, as in the case of many antigen-antibody systems, one of the reactants can be covalently labeled with a fluorescent tag. An increase in the fluorescence polarization of F usually occurs during combination with R, even if there are no concomitant changes in the fluorescence intensity. This is because the polarization increase reflects a slowing down of the rotary motion of the smaller ligand, F, when it becomes attached to the larger species, R. R is in many instances an antibody or a fragment of an antibody, such as an F ab  or F ab2  (dimer). Equilibrium fluorescence polarization and intensity measurements can be determined in a direct readout polarometer capable of measuring both the degree of fluorescence polarization and the fluorescence intensity of a solution. 
     Immunoassays have been used in an effort to improve upon the success in detecting substances at very low levels. For example, the use of such techniques has been prompted by the extraordinary successes that have been achieved in the measurement of biological substances by specific immunological reagents and techniques. Available evidence indicates that specific antibodies can be obtained against even low molecular weight organic compounds, such as pesticides or other haptens. 
     Any means of applying an immunochemical reaction to a detection problem ultimately relies upon a reaction occurring between a substance (antigen or hapten) and its specific antibody. One means by which this interaction can be employed in measurement and detection has come to be known as “competitive binding assay”. In principle, this method requires two reagents. These are a labeled form of the substance to be detected or measured, and an antibody or receptor specifically directed against the substance. The principle of the assay involves a preliminary measurement of the binding of the labeled hapten or antigen (substance being detected) with its antibody and then, a determination of the extent of the inhibition of this binding by known quantities of the unlabeled hapten or antigen, which corresponds to the unknown. From these data, a standard curve can be constructed which shows the degree of binding by the labeled hapten or antigen under certain specified conditions as a function of concentration of the unlabeled hapten or antigen or unknown added. 
     One way of implementing an immunoassay is to employ a fluorescent label. Usually, fluorescent labeling of one of the reagents e.g. the hapten is important in carrying out of the immunoassay by means of fluorescence polarization and/or fluorescence intensity measurements. Unlike other immunoassays, such as ELISA, no physical separation of bound from free forms of the labeled hapten is necessary. Therefore a simple rapid optical measurement yields the essential information without physical separation of bound and free labeled materials. 
     One problem associated with immunoassays, as well as other assays, has been non-specific binding. Ideally, in an immunoassay, the investigator wants to follow a simple biomolecular reaction occurring between a labeled substance (antigen or hapten) and its specific antibody. However, the investigator must often contend with non-specific binding, such as antibody-antibody interactions, antigen-antigen interaction, hapten-hapten interaction, or interactions between the antibody or antigen (or hapten) and interfering substances in the assay. Such non-specific binding can make it extremely difficult, if not impossible to measure specific binding reactions, especially when the equilibrium and rate constants for non-specific binding reactions are a significant fraction of those of the specific binding reactions. 
     Therefore, there is a need to provide improved and more sensitive assays for detecting the presence and/or amount of an analyte in a sample. In particular, it would be advantageous to provide competitive and non-competitive assays in which non-specific binding in the assays is substantially reduced. This would allow, for example, the investigator to successfully follow the specific binding of a labeled antigen or hapten (substance being detected) with its antibody, an unlabeled antibody with a tracer, an unlabeled analyte with a labeled antibody, and many other analytes or labeled tracers with an appropriate “binding partner”, such as a fragment of an antibody, a receptor, or other proteins. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to methods for reducing non-specific binding in assays which rely upon a reaction occurring between a substance and its specific binding partner. In some embodiments, the method involves providing a reaction mixture, which includes or is suspected to include a first component and a second component capable of binding to each other in a specific binding reaction. The method further includes adding non-physiological amounts of at least one additive to the reaction mixture before, during or after binding in a sufficient amount to reduce non-specific binding in the reaction mixture. Moreover, the method includes monitoring or measuring the presence and/or concentration of at least one of the first and second components in the presence of the at least one additive. If the first and second components do not have natural fluorescence, one of the first and second components can be covalently labeled with a fluorescent tag. 
     In some embodiments, the present invention provides competitive binding fluorescence assays. In particular, the invention provides a method for reducing non-specific binding in a competitive-type fluorescence assay for the detection and/or measurement of an analyte in a sample. This method employs various reagents. For example, the method includes providing a fluorescent conjugate of an analyte of interest; and providing a component that specifically binds to the analyte and its fluorescent conjugate. The method further includes combining the fluorescent conjugate and the specific binding component with a sample which includes or is suspected to include the analyte under conditions suitable for the specific binding component to specifically bind to the analyte and its fluorescent conjugate. Importantly, the method also includes adding non-physiological amounts of at least one additive to the sample before, during or after the combining step to minimize non-specific binding in the assay. Significantly, the method includes monitoring for the inhibition of the binding of the fluorescent conjugate to the specific binding component by the sample. 
     In the presence of the unlabeled analyte, a smaller percentage of the labeled analyte is bound to the specific binding component. A standard curve can be constructed from this type of data, which would show fluorescence measurements for certain standard chosen experimental conditions plotted as a function of the amount of unlabeled analyte. An unknown amount of analyte in the sample can then be determined from this standard curve. 
     The present invention further provides a kit for detecting and/or measuring an analyte of interest in a sample. The kit includes a labeled conjugate of the analyte; and an antibody or receptor that specifically binds to the analyte. The kit also includes at least one additive. The kit may optionally also include an unlabeled form of the analyte, which is useful for constructing a standard curve. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a graph of the results of competitive inhibition of the binding of F-thyroxine to its specific antibody by unlabeled thyroxine (T4). 
         FIG. 2  is a graph of the effects of an additive on specific and non-specific binding. 
         FIG. 3  is a graph of a kinetic curve measurable in the presence of an additive using a rhodamine-labeled thyroxine/anti-thyroxine system. 
         FIG. 4  is a graph and table of the influence of sodium benzoate combined with a solvent on the reaction rate of a labeled thyroxine with anti-thyroxine and on the non-specific binding. 
         FIG. 5  is a graph of the effects on a fluoresecein-T4/anti-T4 reaction performed in a PBS buffer containing an 5% 2-propanol and differing amounts of sodium benzoate in the absence of non-specific binding substances. 
         FIG. 6  is a graph of the effects on a fluorescein-T4/anti-T4 system using differing concentrations of sodium benzoate combined with 5% 2-propanol in the presence of the non-specific binding substance BSA. 
         FIG. 7  is a graph of the effects on a fluorescein-T4/anti-T4 system using differing concentrations of guanidine hydrochloride combined with 5% 2-propanol. 
         FIG. 8  is a graph of the effects on a fluorescein-T4/anti-T4 system using differing concentrations of sodium dichloroacetate combined with 5% 2-propanol. 
         FIG. 9  is a graph of the effects on a fluorescein-T4/anti-T4 system using differing concentrations of potassium benzoate combined with 4.75% 2-propanol. 
         FIG. 10  is a graph of the effects on a fluorescein-T4/anti-T4 system using differing concentrations of sodium chloroacetate combined with 4.75% 2-propanol. 
         FIG. 11  is a graph of the effects on a fluorescein-T4/anti-T4 system using differing concentrations of sodium dichloroacetate combined with 4.75% 2-propanol. 
         FIG. 12  is a graph of the effects on a fluorescein-T4/anti-T4 system using differing concentrations of sodium salicylate combined with 4.75% 2-propanol. 
         FIG. 13  is a graph of the effects on a fluorescein-T4/anti-T4 system using differing concentrations of N-hydroxsuccinimide combined with 4.75% 2-propanol. 
         FIG. 14  is a graph of the effects on a fluorescein-T4/anti-T4 system using differing concentrations of sodium chloride combined with 4.75% 2-propanol. 
         FIG. 15  is a graph of the effects on a rhodamine-T4/anti-T4 system using differing concentrations of sodium benzoate combined with 10% 2-propanol. 
         FIG. 16  is a graph of a Standard Calibration Curve of a rhodamine-T4/anti-T4 in the presence of sodium benzoate combined with 10% 2-propanol. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides an improved assay by which macromolecular association reactions are studied. The improvement to previous technology is that non-physiological amounts of a one or more additives selected from salts, alcohols, solvents or combinations thereof are added to a reaction mixture in a sufficient amount to minimize or reduce the non-specific binding, as defined herein, in the reaction mixture such that the assay is easier to perform, easier to interpret, easier to carry out successfully and improves results giving increased accuracy. The term “additive” as used herein is intended to include, but is not limited to, chaotropes, cosmotropes, salting-in and salting-out agents as outlined by the Hofmeister series 1 , organic salts, inorganic salts, non-ionic organic compounds, organic solvents and/or combinations thereof.  1 Hofmeister F:  Zur Lehre von der Wirkung der Salze . Arch Exp Pathol Pharmakol 1888, 24:247-260. 
     The reaction mixture includes a first component and a second component capable of binding to each other in a specific binding reaction. The use of at least one additive in non-physiological amounts reduces non-specific binding, i.e. undesired binding or cross-reactivity, thereby allowing the investigator to follow a simple biomolecular reaction occurring between the first and second components. If the reactants (i.e., first and second components) do not have natural fluorescence, as in the case of many antigen-antibody or hapten-antibody systems, one of the reactants can be labeled with a fluorescent-tag. 
     The term “non-specific binding” as defined herein is the binding and or cross-reactivity of the first and/or second components in the reaction mixture to anything other than each other. In some embodiments, the at least one additive reduces non-specific binding between molecules of the first component. In other embodiments, the at least one additive reduces non-specific binding between molecules of the second component. In still other embodiments, the at least one additive reduces or minimizes non-specific binding and or cross reactivity occurring between the first or second component and an interfering substance in the reaction mixture. Examples of interfering or cross-reacting substances include a variety of binders, such as bovine serum albumin (BSA) and immunoglobulins, which may or may not have a definable relationship with the analyte; or the walls of a container in which the analyte is measured; or other random biomolecules that have no definable relationship with the analyte of interest. 
     It is understood that the additive or additives added to the reaction may reduce all binding to a certain extent. However, it has been observed that the additive or combinations of additives are capable of minimizing non-specific binding relative to specific binding. Seeing that the intended specific binding may be slightly affected by the additive, this side effect is minimal in comparison to the reduction of the non-specific binding caused by the additive, which permits more accurate and more precise measurements of the intended specific binding. In other words, this minimization of non-specific reactions or binding can occur at an extent equal to or much greater than the minimization of the specific binding. This leads to the ability to measure specific binding free of interfering non-specific interactions. 
     In one embodiment, the assay of the present invention takes advantage of the well-known highly specific binding of antibodies to their corresponding haptens or antigens. As mentioned above, non-specific binding is a problem often associated with immunoassays. For example, antigen-antigen (hapten-hapten) and antibody-antibody interactions can occur. Moreover, the antibody can form immune complexes with substances other than its specific antigen or hapten, which may be present in the assay. In particular, an antibody only recognizes a small part of the antigen molecule, the so-called epitope. Any molecule containing such an epitope accessible for the antibody, will bind as if it were the analyte of interest. The impact is largely dependent on the relative affinity to the antibody and the relative concentration of the antibody molecule in comparison with the affinity and concentration of the analyte (i.e., the antigen or hapten). 
     Before the present invention, the cross-reactivity in antigen-antibody or hapten-antibody binding of structural analogues could not always be controlled in a manner that only a single analyte would react with the antibody. The improvement in the assay of the present invention includes conducting an assay, such as an immunoassay, in the presence of non-physiological amounts of at least one additive. This reduces non-specific binding without disturbing the specific immunochemical reaction, relative to specific binding. 
     In one embodiment, the at least one additive is present in the assay in an amount of about 0.2 M to about 2.5 M or higher. In another embodiment, the at least one additive is present in an amount of about 5% to about 20% (weight/volume) of the reaction mixture. 
     Suitable additives include salts of an anion selected from the following: chloride, bromide, iodide, salicylate, trichloroacetate, thiocyanate, perchlorate and benzoate. In some embodiments, the additive is selected from the following agents: 8-anilino-1-napthalene-sulfonic acid, 2-Guanidinobenzimidazole, 2,3,5-triacetylguanosine, Benzimidazolylurea, acetamide, 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid), sodium trichloroacetate, sodium deoxycholate, creatine benzimidazole, sodium p-toluene-sulfonate, sodium dichloro acetate, sodium iodide, sodium fluoride, sodium chloroacetate, 5-benzimidazolecarboxylic acid, Salicylamide, guanidine hydrochloride, sodium chloride, 2-benzimidazole-proprionic acid, 2-benzimidazolemethanol, Sodium chlorodifluoroacetate, 4-guanidinobenzoic acid, 3-chlorobenzoic acid, N-hydroxy succinimide, guanidine and Potassium benzoate, organic solvents and combinations thereof. The present inventors have found that, when present in non-physiological amounts, these additives disrupt non-specific binding, but essentially do not disturb the specific binding reaction. Non-physiological amounts of the salt agents include ranges of 0.2 M to 2.5 M, desirably 0.3 to 2.0 M, more desirably 0.4 to 2.5 M and even more desirably 0.5 to 1.5 M. However, the upper limit salt concentration may go beyond these ranges, provided that it does not have deleterious effects on the assay and is largely dictated by practical considerations, such as unwanted precipitation and/or denaturation of reaction components, excessive precipitation of salts, general interference with the results, among other practical considerations. 
     In one embodiment, the additive affects the order of reaction with respect to the first component and the second component in the reaction mixture. In one preferred embodiment, the additive makes the binding reaction first order with respect to the concentration of each of the first and second components. This allows the investigator to follow a simple second order reaction between the first and second components. This is described in further detail below. 
     In some embodiments, at least one of the first or second specific binding components is fluorescently-labeled. Use of a fluorescent label allows the methods of the present invention to be carried out either by fluorescence polarization measurements or, in some cases, by fluorescence intensity measurements. In some embodiments, a method of the present invention is selected from the following: a fluorescence polarization assay, a fluorescence-based enzyme-linked immunosorbent assay (ELISA) and a Polarized Fluorescence Intensity Difference (PFID) assay. In one preferred embodiment, a method of the present invention is a fluorescence polarization assay. 
     As described above, an increase in fluorescence polarization or a change in fluorescence intensity via enhancement or quenching of a fluorescent reactant usually occurs during the combination with its macromolecular partner. An increase in polarization reflects a slowing down of the rotary motion of the fluorescent reactant when it becomes attached to its macromolecular partner. This increase in fluorescence polarization occurs even if there are no concomitant changes in the fluorescent intensity. 
     In the methods of the present invention, equilibrium fluorescence polarization and intensity measurements can be made in a direct readout “polarometer”. Moreover, kinetic measurements of slow kinetic processes (10 seconds or greater) can also be made in a direct readout polarometer. 
     Polarometer denotes an instrument for measuring the degree of polarization as contrasted to optical rotation. The solution to be measured is first excited in a standard cell by linearly polarized light of appropriate wavelength. The emission fluorescent beam (with appropriate filters) then passes through a rapidly rotating polarizer and onto a photomultiplier tube whose output is fed into a computer which calculates the fluorescence polarization, p=(V−H)/(V+H), Polarized Fluorescence Intensity Difference or PFID (V−H), and the total fluorescence intensity, V+H. V and H denote intensities of vertically polarized and horizontally polarized components in fluorescent light. Alternatively, a “T-format” polarometer using two photomultiplier tubes set at right angles to the excitation source and each having polarizing filters place in a mutually orthogonal position. Provision is made for automatic deduction of the blank. Temperature control of the cell compartment is maintained with an appropriate thermostat. 
     Direct readout polarometers are available commercially. For example, such instruments are available from the following vendors: BMG Labtech GmbH, Offenburg, Germany; JASCO Corporation, Tokyo, Japan; Tecan Schweiz AG, Hännedorf, Switzerland; Bioscan, Inc, Washington, D.C.; Molecular Devices Corporation, Sunnyvale, Calif.; Perkin Elmer Life and Analytical Sciences, Inc., Wellesley, Mass.; Photon Technology International, Inc., Birmingham, N.J.; Abbott GmbH &amp; Co. KG, Wiesbaden, Germany; Diachemix Corp. (USA), Whitefishbay, Wis.; and Invitrogen Corp., Carlsbad, Calif. 
     As described above, the inherent sensitivity of fluorescence measurements can be used in monitoring the extent of reaction as a fluorescent reactant, F, combines with its macromolecular partner, R: 
                         
where k 1  is the forward reaction and k −1  is the back reaction such that (k 1 )(k −1 )=K (eq) .
 
     The ratio of bound to free fluorescent material in Eq. (1) above can be directly related to fluorescence polarization and intensity parameters, as shown in the equations below (Dandliker, et al, (1969) Immunochemistry, 6, 125): 
     
       
         
           
             
               
                 
                   
                     
                       F 
                       b 
                     
                     
                       F 
                       f 
                     
                   
                   = 
                   
                     
                       
                         Q 
                         f 
                       
                       
                         Q 
                         b 
                       
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           P 
                           - 
                           
                             P 
                             f 
                           
                         
                         
                           
                             P 
                             b 
                           
                           - 
                           P 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     2 
                     ) 
                   
                 
               
             
             
               
                 
                   
                     
                       F 
                       b 
                     
                     
                       F 
                       f 
                     
                   
                   = 
                   
                     
                       
                         Q 
                         f 
                       
                       - 
                       Q 
                     
                     
                       Q 
                       - 
                       
                         Q 
                         b 
                       
                     
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     3 
                     ) 
                   
                 
               
             
           
         
       
     
     In equations (2) and (3) above, the symbols or subscripts are as follows: f and b, denote free and bound forms, respectively; p denotes the polarization of fluorescence; F denotes fluorescent-labeled material; and Q denotes the ratio of fluorescence intensity to molar concentration of fluorescent-labeled material. 
     If the binding sites on the fluorescent reactant&#39;s macromolecular partner are uniform, the results can be treated by the Scatchard form of the mass law, 
                       F   b       F   f       =     K   ⁡     (       F     b   ,   max       -     F   b       )               Eq   .           ⁢     (   4   )                 
where F b,max  is the maximum value of F b  and is taken to be equal to the initial binding site concentration of R. R in equation 4 is equal to (F b,max −F b ). K is the association constant for the reaction and the sum, F b +F f =M, is the known concentration of fluorescent reactant.
 
     The interpretation of kinetics in terms of fluorescence polarization was carried out by initial rate equations: 
                       (       ⅆ     F   b         ⅆ   t       )     0     =       -       (       ⅆ     F   f         ⅆ   t       )     0       =         Q   f       Q   b       ⁢     (       F     f   ,   0           P   b     -     P   f         )     ⁢       (       ⅆ   P       ⅆ   t       )     0                 Eq   .           ⁢     (   5   )                     (       ⅆ   P       ⅆ   t       )     0     =         Q   b       Q   f       ⁢     (       P   b     -     P   f       )     ⁢     k   ⁡     (     F     b   ,           ⁢   max       )       ⁢     (     N   1     )     ⁢     (     F     f   ,   0       )     ⁢     (       N   2     -   1     )               Eq   .           ⁢     (   6   )                 
where k is the usual second order rate constant. For constant F b,max  but varying (F f ) o 
 
                       log   ⁡     (       ⅆ   P       ⅆ   t       )       0     =         (       N   2     -   1     )     ⁢     log   ⁡     (     F     f   ,   0       )         +   constant             Eq   .           ⁢     (   7   )                 
Alternatively, for constant (F f,o ), but varying (F b,max ):
 
     
       
         
           
             
               
                 
                   
                     
                       log 
                       ⁡ 
                       
                         ( 
                         
                           
                             ⅆ 
                             P 
                           
                           
                             ⅆ 
                             t 
                           
                         
                         ) 
                       
                     
                     0 
                   
                   = 
                   
                     
                       
                         ( 
                         
                           N 
                           1 
                         
                         ) 
                       
                       ⁢ 
                       
                         log 
                         ⁡ 
                         
                           ( 
                           
                             F 
                             
                               b 
                               , 
                               max 
                             
                           
                           ) 
                         
                       
                     
                     + 
                     constant 
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     8 
                     ) 
                   
                 
               
             
           
         
       
     
     In the kinetic equations, P is the value of the polarization, N 2  is the order of the reaction with respect to the fluorescent reactant concentration, and N 1  that with respect to the concentration of binding sites on its macromolecular partner. The subscript “0” refers to zero time. Equations (7) and (8) are especially useful in determining the order of a reaction, which is an important characteristic to establish when investigating kinetic relationships. If the order with respect to the concentration of each reactant proves to be constant over a wide concentration range, then it is likely that the path of the reaction is also remaining the same which gives some assurance that derived kinetic constants have some simple physical meaning. Also, equations (7) and (8) are in a form easy to use, since it is not necessary to know the absolute, but only the relative concentrations of each reactant. Dealing with initial rates while focusing only upon the initial stages of reaction accomplishes some simplification by avoiding the back reaction and the inhibition by product. 
     In some embodiments of the present invention, a non-physiological amount of a suitable additive affects the order of the reaction with respect to a fluorescently-labeled reactant (e.g., fluorescently-labeled analyte) and its macromolecular partner (e.g., antibody). In one example, the additive makes the binding reaction first order with respect to the concentration of each of the fluorescent reactant and its macromolecular partner (i.e., N 1  and N 2  will each be 1). 
     As described above, there are situations where fluorescence polarization and intensity measurements can not be easily made. For example, p, V−H and V+H measurements are difficult if not impossible in situations where there is non-specific binding occurring in the reaction mixture. This is described further in the examples below. The present invention is directed to overcoming the problem of non-specific binding by employing non-physiological amounts of at least one additive in the reaction mixture. Preferably, the additive is present in an amount of about 0.5 M to about 1.5 M or higher. 
     In one embodiment, the method of the present invention is a competitive inhibition-type assay. The method is useful for detecting and/or measuring unknowns. For example, the assay is useful for the detection of a hapten or antigen in a sample. In one embodiment, the substance to be detected is a low molecular weight organic contaminant of environment concern. Such contaminants may be in a food or soil sample, for example. In another embodiment, the substance to be detected is a biological substance in a patient sample, e.g., blood or serum. 
     In one embodiment, the method of the present invention is a non-competitive type assay where the presence or absence of a protein moiety (e.g., enzyme, antibody or receptor), an oligonucleotide, or other macromolecule is detected. The method is useful for detecting and/or measuring the concentration of the macromolecule. Furthermore, other parameters, such as the equilibrium and rate binding constants of the macromolecule with a partner of interest, can be determined. 
     In some other embodiments, a competitive inhibition type assay of the present invention employs a fluorescent conjugate of the substance (analyte) of interest, together with an antibody to the analyte. In some embodiments, the antibody is first exposed to the fluorescent-labeled form of the analyte. Then, a study of the inhibition of the binding of this fluorescent conjugate by a sample thought to contain the analyte of interest is performed. The assay is done in the presence of non-physiological amounts of at least one additive. 
     In particular, the invention provides a general method for reducing non-specific binding in a fluorescence assay for the detection and/or measurement of an analyte in a sample. This method includes providing a fluorescent conjugate of the analyte; and providing a component that specifically binds to the analyte and its fluorescent conjugate. The method further includes combining the fluorescent conjugate and the specific binding component with a sample which includes or is suspected to include the analyte under conditions suitable for the specific binding component to specifically bind to the analyte and its fluorescent conjugate. Moreover, the method includes adding non-physiological amounts of at least one additive before, during or after the combining step to reduce non-specific binding in the assay; and monitoring for the inhibition of the binding of the fluorescent conjugate to the specific binding component by the sample. 
     The monitoring step can include measuring the binding of the fluorescent conjugate to its specific binding component and determining the extent of inhibition of this binding by different known quantities of unlabeled analyte. The monitoring step can also include constructing a standard curve which shows the degree of binding by the fluorescent conjugate as a function of the quantity of the unlabeled analyte. The amount of analyte in the sample can then be determined by measuring the binding of the fluorescent conjugate to its specific binding component in the presence of the sample and determining the amount of analyte in the sample from the standard curve. 
     One competitive-type assay of the present invention is preferably a fluorescence polarization assay. Other assays include radioimmune assays, fluorescence intensity assays, enzyme immunoassays (e.g., ELISA), gold-colloidal assays, latex-bead assays, or any other homogeneous or heterogeneous type assays consisting of the binding between a macromolecule and a binding partner. In one desired embodiment, the monitoring step involves monitoring for a change in the initial rate of polarized fluorescence intensity difference (PFID), defined as the absolute difference in the polarized fluorescence in the vertical and horizontal directions, i.e. (V−H), or d(V−H)/dT as a function of an amount of analyte in the sample. For example, V−H (PFID) versus time is measured before and then after the sample is added, and the total difference between these values over a set time period is calculated. The total change in PFID is then compared to a standard curve to determine the amount of analyte. Such a standard curve is constructed by monitoring for a change in the initial rate of polarization as a function of different known quantities of unlabeled analyte. The amount of analyte in the sample can be determined from the standard curve. 
     Suitable additives to employ in a competitive-type assay of the present invention are the same as those described above. Preferably, the additive is present in these assays in an amount of about 0.5 M to about 1.5 M or higher, or from about 5% to about 20% w/v (of the assay mixture). 
     In one embodiment of the competitive assay, the additive affects the order of reaction with respect to the analyte and the component that specifically binds to the analyte. For example, in one embodiment, the additive makes N 1 =1 in eq. 8 and N 2 =1 in Eq. 7. That is, in one embodiment, the additive is present in a sufficient amount to make the binding reaction first order with respect to the concentration of each of the analyte and its specific binding partner. 
     In one embodiment of the competitive assay, the additive reduces non-specific binding between molecules of the analyte. In another embodiment, the additive reduces non-specific binding between molecules of the analyte&#39;s specific binding partner (i.e., its specific binding component). In yet another embodiment, the additive reduces non-specific binding between an interfering substance in the assay and at least one of the analyte and its specific binding component. 
     The particular fluorescent moiety chosen to form the conjugate with the analyte is selected from any number of fluorescent moieties. The choice of fluorescent moiety is to a large extent a matter of convenience once a coupling chemistry has been selected. Virtually any fluorophore having a fluorescence lifetime of between 0.1 and 50 nanoseconds and having an excitation wavelength of 350 to 800 nanometers is suitable for purposes of the present invention. For a detailed listing of fluorophores, which are commercially available, see Handbook of Fluorescent Probes and Research Chemicals, ed. Karen Larison, by Richard P. Haugland, Ph.D., 5 th  ed., 1992, published by Molecular Probes, Inc. Some examples of suitable fluorescent moieties include the following: 7-AAD, Acridine Orange, Alexa 488, Alexa 532, Alexa 546, Alexa 568, Alexa 594, Aminonapthalene, Benzoxadiazole, BODIPY 493/504, BODIPY 505/515, BODIPY 576/589, BODIPY FL, BODIPY TMR, BODIPY TR, Carboxytetramethylrhodamine, Cascade Blue, Coumarin, CY2, CY3, CY5, CY9, Dansyl Chloride, DAPI, Eosin, Erythrosin, Ethidium Homodimer II, Ethidium Bromide, Fluorescamine, Fluorescein, FTC, GFP (e.g. yellow shifted mutants T203Y, T203F, S65G/S72A), Hoechst 33242, Hoechst 33258, IAEDANS, Indopyras Dye, Lanthanide Chelate, Lanthanide Cryptate, Lissamine Rhodamie, Lucifer Yellow, MANT, MQAE, NBD, Oregon Green 488, Oregon Green 514, Oregon Green 500, Phycoerythrin, Porphyrin, Propidium Iodide, Pyrene, Pyrene Butyrate, Pyrene Maleimide, Pyridyloxazole, Rhodamine 123, Rhodamine 6G, Rhodamine Green, SPQ, Texas Red, TMRM, TOTO-1, TRITC, YOYO-1, Vitamin B12, flavin-adenine dinucleotide, 6-carboxy-X-rhodamine, nicotinamide-adenine, and dinucleotide. Preferably, the fluorescent conjugate would have a fluorescent wavelength different from competing fluorescent substances which may occur in host samples of interest, e.g., blood serum, urine, tissue and extracts thereof. 
     In one embodiment, the analyte of interest is an organic contaminant. The organic contaminant can be one of environmental concern. For example, in one embodiment, the organic contaminant is a fungal or microbial toxin. Other analytes that can be detected with a method of the present invention include, but are not limited to, drugs, steroids, hormones, proteins, peptides, lipids, sugars, receptors, nuceic acids, vitamins, etc. For example, in one embodiment, the analyte of interest is thyroxine, which is a major hormone secreted by the follicular cells of the thyroid gland. 
     In some embodiments, the component that specifically binds to the analyte of interest is an antibody. It is well known that a substance, when injected into an animal, stimulates the animal to produce antibody. The antibody is capable of reacting with the injected substance in a highly specific manner. These antibodies belong to a group of serum proteins known as immunoglobulins. The production of these antibodies as a result of the injection of the antigen takes place over a period of many weeks, and depends upon the immunization schedule. In general, “good” antigens are usually of large molecular size (greater than 20,000 MW), partially digestible by enzymes and are recognized as being foreign by the antibody-producing animal. 
     Many compounds of environmental concern do not have a large molecular weight, and would, therefore, appear to be incapable of stimulating antibody formation. However, this is not the case. So-called partial antigens or haptens can be produced and are capable of reacting with specific antibody. Haptens or partial antigens are defined as antigens which alone cannot induce antibody formation, but in conjugation with a suitable carrier can produce antibody against themselves, as well as against the carrier-hapten complex. Examples of carriers include ovalbumin, bovine serum albumin, fibrinogen, and many others. Conjugation may be carried out by methods known in the art (Coligan, J. E. et al. (Eds.) Current Protocols in Immunology, Chapter 9, Wiley Intersciences, New York, 1999). 
     The hapten, once conjugated with a suitable carrier, can stimulate antibody production. Some antibody will be produced which is highly specific in its reaction with the hapten alone. Therefore, by employing hapten-specific antibodies, the methods of the present invention can be used in the detection and quantitation of even low molecular weight organic compounds, such as pesticides. 
     Antibodies suitable for use in the methods of the present invention include polyclonal and monoclonal antibodies. Polyclonal antibodies can be prepared in accordance with known methods (Coligan, J. E, et al. (Eds.), Current Protocols in Immunology, Wiley Intersciences, New York, 1999). 
     Monoclonal antibodies may be produced by methods known in the art. These methods include the immunological method described by Kohler and Milstein in Nature 256:495-497 (1975) and by Campbell in “Monoclonal Antibody Technology, The Production and Characterization of Rodent and Human Hybridomas” in Burdon et al. (Eds.), Laboratory Techniques in Biochemistry and Molecular Biology, Volume 13, Elsevier Science Publishers, Amsterdam (1985); and Coligan, J. E, et al. (Eds.), Current Protocols in Immunology, Wiley Intersciences, New York, (1999); as well as the recombinant DNA method described by Huse et al., Science 246:1275-1281 (1989). 
     Antibodies against markers in normal human tissue and neoplasms are commercially available, for example, from Invitrogen Corporation (Carlsbad, Calif.), Advanced Immunochemical Inc. (Long Beach, Calif.) and RDI Division of Fitzgerald Industries, Intl. (formerly Research Diagnostics, Inc., Concord, Mass.). These include, but are not limited to, monoclonal and polyclonal antibodies against the following classes of analytes: proteins (e.g., enzymes, growth factors, cytokines), peptides, receptors (e.g., CD markers), toxins, infectious agents (e.g., viruses), steroids, hormones, lipids and lipoproteins). For example, antibodies against angiogenesis markers are commercially available, and such antibodies include those against the following receptors: CD31, CD34, Vascular Endothelial Growth Factor (VEGF), Vascular Endothelial Growth Factor C (VEGF-C) and Vascular Endothelial Growth Factor Receptor 3 (VEGFR-3). Also, antibodies against cancer markers are commercially available, such as those against the following: ALK, ALK 400, c-kit (CD117), COX-1, COX-2, EZH2, Ezrin, MAGE-A, Mesothelin, MTA1, NY-ESO-1, PDEF, PRAC, PSMA, RCAS1, thymidylate synthase and tyrosinase. Furthermore, antibodies against markers for specific types of cancer are commercially available. For example, commercially available antibodies against breast cancer markers include those against the following: BCA-225, Bcl-2, c-Met, Cathepsin D, CD63, cytokeratins, E-cadherin, EGFr, estrogen receptor, progesterone receptor, HER2, HER4, p53 and phospho-MAP kinase. Moreover, markers for colon cancer include, but are not limited to, CA 19-9, CEA, COX-1, Ezrin, MLH1, MSH2, MSH6, Platelet Derived Endothelial Cell Growth Factor (PD-ECGF), PRLr and Thymidylate Synthase (TS). Antibodies against these colon cancer markers, as well as markers for other specific cancers, diseases or disorders are also commercially available. 
     Antibodies against apoptosis markers are also commercially available, and these markers include, for example, Bax, Bcl-2, Bcl-XL and PARP. Moreover, antibodies against cell cycle and cell proliferation markers are commercially available. Such cell cycle and cell proliferation markers include, but are not limited to, BrdU, Cyclin D1, Cyclin E, p21, Proliferating Cell Nuclear Antigen (PCNA) and S-Phase Kinase-Associated Protein 2 (SKP2). Antibodies against cellular proteins, such as, but not limited to, calcitonin, HLA DR, MGMT, nitrotyrosine and nNOS are also commercially available. Moreover, antibodies against cytoskeletal proteins, such as α-Tubulin, β-Tubulin, Actin, Desmin, GFAP, Myosin, Ubiquitin and Vimentin, are commercially available. Furthermore, antibodies against markers for lipoprotein metabolism are commercially available; Such antibodies include, for example, those against HDL, LDL, APOE, ApoE2, LDL receptors, etc. The abbreviations used herein for the markers are well known in the art. 
     Furthermore, antibodies against pesticides and herbicides are commercially available from such companies as Guildhay, Ltd. (Guildford, Surrey, England). For example, antibodies against 2,4-D, aldrin, atrazine, chlortoluron, diuron, isoproturon, MCPA, mecoprop, paraquat, simazine, solanine, etc. are commercially available. 
     The present invention is not, in any way, limited to the specific examples provided herein of analytes and antibodies that specifically bind to the analytes. 
     In other embodiments of the present invention, the component that specifically binds to the analyte is a receptor. For example, if a hormone carrying a fluorescent label attaches to its receptor, the hormone is thereby largely immobilized and this immobilization is registered by an increase in the polarization of the emission from the attached fluorescent label. 
     In addition to antigen-antibody, hapten-antibody and hormone-receptor interactions, the methods of the present invention can also be applied to enzyme-substrate, protein-DNA, peptide-antibody and ligand-receptor interactions. 
     Table A below provides some further examples of analytes, which may be detected and/or measured using the methods of the present invention. In some embodiments, a fluorescent conjugate of the analyte can be employed in methods of the present invention. The analysis may be performed, for example, in water, serum, blood, urine or other bodily fluids. Moreover, the analysis may be performed in milk, wine, juices and food extracts. Table A is for illustrative purposes only and is not intended to limit the scope of the present invention. 
     
       
         
           
               
               
             
               
                 TABLE A 
               
               
                   
               
               
                 CLASS 
                 EXAMPLES 
               
               
                   
               
             
            
               
                 Pesticides 
                 Acetochlor and Other Acetanilides, atrazin, simazine, 
               
               
                   
                 triazine, 2,4-D, 2,4,5-T, dichlorprop, MCPA, MCPB, 
               
               
                   
                 Triclopyr, Pentachlorophenol, DDT, Isoproturon, 
               
               
                   
                 Methabenzthiazurone, Metasulfuron-methyl, Chlorsulfuron, 
               
               
                   
                 Acetochlor, Propanil, Paraquat, Parathion-methyl, BTEX, 
               
               
                   
                 Nonylphenol, LAS 
               
               
                 Lipids 
                 DHET 
               
               
                 Sugars 
                 Bacterial sugars, polysaccharides 
               
               
                 Peptides, Proteins, Receptors 
                 IGG, Albumin, Receptors, KLH, LPH Myoglobin, Feto 
               
               
                   
                 proteins, AT1 
               
               
                 Oligonucleotides, DNA, 
                 Bacterial, animals, plants, specific sequences 
               
               
                 RNA 
               
               
                 Toxins 
                 Zearalenone, Deoxynivalenol, T-2, Aflatoxins, Ochratoxin, 
               
               
                   
                 Fumonisins, Patulins, Trichothecene, citrinin, Cyclopiazonic 
               
               
                   
                 acid, monoliformin, sterigmatocystin, alternaria-mycotoxins, 
               
               
                   
                 ergot alkaloids 
               
               
                 Vitamins 
                 B 12, Vitamin C, Folic acid 
               
               
                 Small Molecules 
                 Chlorinated compounds, metals 
               
               
                 Therapeutic drugs, e.g., 
                 Theophylline, doxorubicin, methotrexate, disopyramide, 
               
               
                 antiasmatics, antineoplastics, 
                 lidocaine, procainamide, propranolol, quinidine, N-acetyl- 
               
               
                 antiarythmics, 
                 procainamide, Phenobarbital, phenotoin, pridon, valproic 
               
               
                 anticonvulsants, antibiotics, 
                 acid carbamazepine, ethosuximide, cephalosporins, 
               
               
                 antiarthritics, antidepressants, 
                 erythromycin, tetracyclin, vancomycin, gentamicin, 
               
               
                 etc. 
                 amikacin, chloramphenicol, streptomycin, tobramycin, 
               
               
                   
                 penicillin, salicylate, nortriptyline, amitriptoline, imipramine, 
               
               
                   
                 desipramine, liodcaine, histamine, thyroxin, triiodothyronine, 
               
               
                   
                 serotonin, etc. 
               
               
                 Drugs of abuse 
                 Morphine, heroin, hydromorphone, dihydrocodeine, 
               
               
                   
                 pholcodine, Amphetamine, etc. 
               
               
                 Steroids and hormones 
                 Esterone, estradiiol, cortisol, testosterone, progesterone, 
               
               
                   
                 chenodeoxycholic acid, digoxin, cholic acid, digitoxin, 
               
               
                   
                 deoxycholic acid, lithocholic acid, etc. 
               
               
                 Prostaglandins 
                 PGE, PGF, PGA others 
               
               
                 Components of binding and 
                 Antibodies, receptors, enzymes (i.e., proteases, nucleases, 
               
               
                 reaction studies 
                 phosphatases) 
               
               
                   
               
            
           
         
       
     
     In one embodiment of the present invention, the component that specifically binds to the analyte and its fluorescent reactant is on a solid substrate. For example, a specific binding component, such as an antibody, can be deposited on a glass, plastic or paper substrate. Substrates can include various microporous filters, such as PVDF filters, nitrocellulose filters, cellulosic filters and the like. In one example, the antibody can first be bound to a substrate, such as PVDF. Second, the antibody on the substrate can be exposed to a fluorescent-labeled form of an analyte of interest. Then, a study of the inhibition of the binding of this fluorescent conjugate by a sample thought to contain the analyte of interest is performed. In particular, one can observe a change in p, V−H and/or V+H which occurs in the presence of the sample. 
     The competitive-inhibition assay of the present invention can be applied to the simultaneous analysis of multiple analytes in a sample using analytes labeled with different fluorescent wavelength conjugates. This would reduce the time and effort involved in multi-analyte, multi-sample analyses. 
     As described above, the present invention also provides a kit for detecting and/or measuring an analyte of interest in a sample. The kit includes the following components: a fluorescent conjugate of the analyte; an antibody or receptor that specifically binds to the analyte; and at least one additive. The additive in the kit may be a salt of an anion selected from the following: chloride, salicylate, trichloroacetate, thiocyanate, perchlorate and benzoate. In one embodiment, the kit further includes an unlabeled form of the analyte. 
     The following examples are for illustrative purposes only, and are not to be construed as limiting the present invention. 
     EXAMPLES 
     Example 1 
     Materials 
     Preparation of Buffers 
     The influence of at least one additive on a reaction occurring between specific binding molecules was investigated in pH 7.5 buffer solutions. These buffer solutions were denoted “Buffer 1” and “Buffer 2” and each contained a different amount of sodium benzoate. 
     Buffer 1: 
     0.5 M sodium benzoate in Phosphate Buffered Saline (PBS), adjusted to pH 7.5. The final concentration of sodium benzoate in buffer 1 was about 7.2% w/v. 
     Buffer 2: 
     1.0 M sodium benzoate in PBS, adjusted to pH 7.5. The final concentration of sodium benzoate in buffer 2 was about 14.4% w/v. 
     Preparation of Anti-Sera Stock Solution 
     An initial stock solution of anti-thyroxine sera was prepared. In particular, 1 ml of polyclonal anti-thyroxine sera containing 10 mg/ml IgG (Sigma) was diluted 1:5 in PBS buffer, pH 7.5. The final concentration of IgG in the stock solution was about 2 mg/ml. 
     Preparation of Thyroxine Stock Solution 
     An initial stock solution of an analyte (thyroxine) was prepared. Unlabeled thyroxine at 1 mg/ml in 0.5 M NaOH was diluted 100-fold in PBS. The final concentration of thyroxine in the stock solution was about 10 μg/ml. 
     Preparation of Fluorescein-Labeled Thyroxine Stock Solution 
     An initial stock solution of fluorescein-labeled analyte (F-thyroxine) was prepared. Fluorescein-labeled thyroxine at 1248 g/mol in a lyophilized form was obtained from emp Biotech GmbH. First, 5 mg (4×10 −6  moles) of F-thyroxine was diluted to 10 ml in PBS to get a 4.1×10 −4  M solution of F-thyroxine. This solution was then further diluted 100-fold in PBS. The final concentration of F-thyroxine in the stock solution was about 4.1×10 −6  M. 
     Example 2 
     Kinetic Curve Measurement Methodology 
     The following is a protocol useful for performing a fluorescein polarization assay of the present invention. All steps are performed at ambient temperature. The total final reaction volume after all additions is 3030 μl.
         1. Add 20 μl or 0 μl (control) anti-sera stock solution to about 2980 μl of Buffer 1.   2. Pipette the diluted anti-sera solution prepared in step 1 up and down from 1 to 10 seconds to thoroughly mix.   3. Add between 0 and 20 μl thyroxine stock solution (e.g., 0, 0.5, 2.0, 5.0 and 20 μl) to the mixed anti-sera from step 2.   4. Pipette the mixture formed in step 3 up and down for 1 to 10 seconds and then incubate for 30 to 300 seconds.   5. Add 10 μl of the Fluorescein-labeled thyroxine stock solution to the incubated mixture from step 4.   6. Pipette the mixture formed in step 5 up and down for 1 to 10 seconds and begin measuring polarization (P), Polarized Fluorescence Intensity Difference (V−H) or quenching (V+H total intensity) versus time for between 10 and 7200 seconds after end of mixing in a direct readout polarometer or between 0.1 and 7200 seconds after end of mixing with a stopped-flow polarometer.   7. Determine the rate of reaction (initial rates) and plot versus amount of thyroxine stock solution added in step 3 to produce a standard curve.   8. Against a standard curve, an unknown amount of thyroxine can be measured by comparison of initial rate measurement (speed of reaction) of unknown with the standard curve.       

     Example 3 
     Kinetic Curve Measurable in the Presence of an Additive Using a Fluorescein-Labeled Thyroxine/Anti-Thyroxine System 
     This example demonstrates that a reaction between F-thyroxine and its specific antibody was only capable of being followed when the reaction was performed in the presence of an additive. The additive in the present example was sodium benzoate The thyroxine/anti-thyroxine system is well known to be a very difficult system to measure due to thyroxine&#39;s capacity to bind to many things non-specifically. It was for this reason that the thyroxine-anti-thyroxine system was chosen as a model system to demonstrate the present invention. 
     The materials described in Example 1 and the protocol described in Example 2 were used for each of Experiments 1 to 5 shown in Table 1 below, except that for Experiments 1 to 3, the reaction was performed in PBS instead of Buffer 1. Experiment No. 1 is a control to establish baseline measurements for polarization (P), V−H, and V+H. 
                                     TABLE 1               Experiment   Antiserum   Thyroxine   F-thyroxine           No.   (μl)   (μl)   (μl)   Additive                                                    1   0   0   10   −       2   20   0   10   −       3   20   20   10   −       4   20   0   10   +       5   20   20   10   +                    
Results of Experiment 1
 
     Polarization, V−H and V+H were measured versus time for between 10 and 600 seconds. During this time period, fluorescence polarization baseline measurements were constant. In particular, V−H values were between 0.198 to 0.193 V, V+H values were between 2.135 to 2.130, and V and P values were between 91 to 90 mP. 
     Results of Experiment 2 
     Attempts were made to measure P, V−H and V+H versus time for between 30 seconds and 600 seconds. However, the reaction was immediate and the values obtained for P, V−H and V+H did not change over this time period. In particular, V−H was at 1.51 V, V+H was at 8.118, and V and P values were at 184 mP throughout the measurement period. 
     The results of Experiment 2 indicate that it was not possible to measure a kinetic curve in the absence of an additive. In particular, it was not possible to obtain meaningful information during the measurement of the reaction between the fluorescently-labeled thyroxine and its specific antibody in the absence of benzoate. 
     Results of Experiment 3 
     Attempts were made to measure P, V−H and V+H versus time for between 30 seconds and 600 seconds. However, the reaction was immediate and the values for P, V−H and V+H were constant over the measurement period. In particular, V−H was at 0.1580 V, V+H was at 5.243, and V and P values were at 30 mP throughout the measurement period. 
     The results of Experiment 3 indicate that it was not possible to obtain meaningful information during the measurement of a kinetic curve in absence of an additive due to non-specific binding. In particular, it was not possible to follow the binding between F-thyroxine and its specific antibody, nor the inhibition of this binding by the unlabeled thyroxine in the absence of benzoate. 
     Results of Experiment 4 
     Polarization, V−H and V+H values were measured for between 10 and 600 seconds. During this time course, a kinetic curve was obtained using an anti-thyroxine/F-thyroxine system, in which no unlabeled thyroxine was added. In particular, the polarization, V−H and V+H values increased during the 600 second time period. 
     The increase in polarization was due to the binding occurring between the F-thyroxine and its specific antibody. The results of Experiment 4, which are provided in  FIG. 1 , indicate that it was possible to obtain meaningful information during the measurement of a kinetic curve in the presence of an additive. 
     Results of Experiment 5 
     A study of the competitive inhibition of the binding of the F-thyroxine to its specific antibody by unlabeled thyroxine in the presence of sodium benzoate was performed. 
     The results of this competitive inhibition type assay of the present invention indicated that inhibition of the binding of F-thyroxine to its specific antibody by unlabeled thyroxine occurred. These results are shown in  FIG. 1 . In particular, in Experiment 5, a standard inhibition curve was obtained using various amounts of unlabeled thyroxine from 0 μl to 20 μl using the protocol in Example 2. This standard curve is used to measure unknown quantities of analyte. This also demonstrates it is possible to perform an immunoassay in the presence of an additive. 
     Example 4 
     Effects of an Additive on Specific and Non-Specific Binding in a F-Thyroxine/Anti-Thyroxine System 
     With reference to  FIG. 2 , experiments were performed to determine the effects of an additive on specific and non-specific binding. The additive used in the present example was sodium benzoate. The materials described in Example 1 and the measurement methodology described in Example 2 were used. In particular, 50 μl of human sera was added to 3000 μl of Buffer 2 or PBS at pH 7.5 (no additive control) and thoroughly mixed by pipetting for 1 to 10 seconds. To this was added 0 μl of unlabeled thyroxine and 0 μl, 20 μl or 50 μl of anti-sera stock solution. The mixture was thoroughly mixed by pipetting for 1 to 10 seconds. Then, 5 μl of the fluorescein-labeled thyroxine stock solution was added. This was followed by mixing for 1 to 10 seconds by pipetting. Then, d(V−H)/dt measurements were made versus time for between 10 and 600 seconds. This protocol was followed for Experiments 6-9 in Table 2. This table shows the experimental set-ups, and the corresponding symbols used in  FIG. 2  for each experiment. 
                                     TABLE 2               Experiment   Antiserum   Human Sera   F-thyroxine           No.   (μl)   (μl)   (μl)   Additive                                                        6   (♦)   0   50   5   +       7   (▪)   20   50   5   +       8   (▴)   50   50   5   +       9   (-X-)   20   50   5   −                    
Results
 
     As shown in  FIG. 2 , it was only possible to measure an increase in PFID (V−H) values over the 600 second time course in the presence of an additive (▴ and ▪). An increase in P and V+H values was also observed over this time course in the presence of the additive. V−H values did not change in the absence of the additive (—X—; data not shown). 
     The data further show that the reaction only proceeds in the presence of anti-sera (see constant V−H values for ♦, where no anti-sera was present). The overall results indicated that the additive inhibits non-specific binding, but does not inhibit specific binding. 
     Example 5 
     Kinetic Curve Measurable in the Presence of an Additive Using a Rhodamine-Labeled Thyroxine/Anti-Thyroxine System 
     This example demonstrates the successful use of a non-fluorescein “red”-labeled thyroxine/anti-thyroxine system for fluorescence polarization and polarized fluorescence intensity difference PFID (V−H) measurements as a substitute system for a fluorescein-labeled thyroxine/anti-thyroxine system. The protocol used is described below. The additive was sodium benzoate combined with an organic solvent (5% isopropanol).
         1. Use clean glassware, washed with isopropanol.   2. Make a 0.5 M solution of sodium benzoate.   3. Make a solution of 5% isopropanol in 0.5 M sodium benzoate (i.e., 2.5 ml 2-propanol in 47.5 mL 0.5 M aqueous sodium benzoate). This will be named BUFFER.   4. 800 μl of BUFFER added to approximately 200 μg 5-carboxy-X-rhodamine-thyroxine (ROX-T4). This will be named STOCK ROX-T4 solution.   5. 600 μl Buffer added to 1 mL curvette. The absorbance spectrum is measured using a UV-VIS spectrophotometer, 400 nm to 800 nm. This spectrum is used to define the absorbance of the background as zero and to control that it is actually zero.   6. Add approximately 20 μl of STOCK ROX-T4 to cuvette and mix. Measure the absorbance UV-VIS, 400 nm to 800 nm. Absorbance maximum is measured to be 582 nm.   7. Using a rotating polarometer with a 75 watt Xenon short-arc lamp. The rotating polarizer has three channels: Polarization (P), Total Intensity (V+H), and V−H.   8. Add 2 μl STOCK ROX-T4 to 3000 μl (3 mL or 3 cc) BUFFER. Measure P, V+H and V−H after short mixing and 30 second equilibration. P=0.075; V+H=2291; V−H=178. Conclusions: Intensity has increased due to presence of more fluorophore; Polarization has remained constant because there is no binding.   9. Add 10 μl. Antibody solution (anti-thyroxine in PBS, dilution unknown, probably 5 mg in 1000 μl PBS and further diluted 1:10 in PBS). New volume is 3017 μl. Measure P, V+H and V−H after short mixing. Results are shown in Table 3 below:       

     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Time(s) 
                 P (mP) 
                 V + H 
                 V − H 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 0 
                 75 
                 2291 
                 178 
               
               
                   
                 10 
                   
                   
                 660 
               
               
                   
                 13 
                   
                   
                 860 
               
               
                   
                 18 
                   
                   
                 936 
               
               
                   
                 20 
                   
                   
                 1006 
               
               
                   
                 26 
                   
                   
                 1130 
               
               
                   
                 30 
                   
                   
                 1175 
               
               
                   
                 33 
                   
                   
                 1200 
               
               
                   
                 44 
                   
                   
                 1259 
               
               
                   
                 49 
                   
                   
                 1280 
               
               
                   
                 53 
                   
                   
                 1290 
               
               
                   
                 62 
                 392 
               
               
                   
                 70 
                 395 
               
               
                   
                 78 
                   
                   
                 1345 
               
               
                   
                 83 
                   
                 3375 
               
               
                   
                 95 
                 397 
               
               
                   
                 600 
                 398 
                 3320 
                 1340 
               
               
                   
                   
               
            
           
         
       
     
     These results are shown graphically in  FIG. 3 . The results indicate that an increase in the values of P, V−H and V+H occurs over the 600 second time course of the reaction. 
     Since the reaction between the fluorescent conjugate and its antibody is measurable, a study of the inhibition of the binding of the fluorescent conjugate by a sample thought to contain an analyte of interest is carried out. In particular, a change in the initial rate of polarized fluorescence intensity difference PFID (V−H) is monitored as a function of analyte in the sample. 
     The amount of analyte in the sample is determined by measuring the binding of the fluorescent conjugate to the specific binding component in the presence of the sample and determining the amount of analyte in the sample from a standard curve. Such a standard curve is constructed by first measuring the binding of the fluorescent conjugate to its specific binding partner (e.g., its specific antibody) and determining the extent of inhibition of this binding by different known quantities of unlabeled analyte. The standard curve is then constructed, which shows the degree of binding by the labeled analyte as a function of the quantity of the unlabeled analyte. 
     Example 6 
     Influence of Sodium Benzoate Combined with an Organic Solvent on the Rate of the Reaction Between F-Thyroxine and Anti-Thyroxine and on the Non-Specific Binding of Serum Components 
     The present example shows the influence of sodium benzoate combined with 10% 2-propanol on the velocity of the reaction between fluorescent hapten (F-thyroxine) and anti-thyroxine, and on the initial non-specific binding between F-thyroxine and serum components. The buffer and stock solutions described in Example 1 and the measurement methodology described in Example 2 were used. 
     Experimental Conditions: 
     
         
         
           
             Fluorescein-Thyroxine (1 nM) 
             Anti-Thyroxine (1:300 dilution) 
             PBS with 10% 2-propanol 
             Benzoate in different concentrations 
             At t=0 s antibody was pipetted into the cuvette. 
             A 3 ml cuvette was used, the total volume in the cuvette was 2 ml. A small magnetic stirrer was used for mixing 
           
         
       
    
     In example 6, the reactions between F-Thyroxine and Anti-Thyroxine, which were carried out in the presence of serum under different reaction conditions, are compared ( FIG. 4 ). There are seven different amounts of sodium benzoate, between 0 and 1.0 Molar in the cuvette. The reaction was performed in a phosphate buffered saline buffer containing 10% 2-propanol. The table in  FIG. 4  lists the two important parameters. The first column shows the rate of change in polarization value after five seconds (t=5). The second column shows the polarization value before the addition of antibody (t=0). 
     In reaction 6-1, the buffer background fluorescence was measured and blanked. F-thyroxine was added and mixed. The polarization value was observed to be 100 mP. Antibody (anti-T4) was added, stirred, and at t=5 s the rate of change of P was observed to be 25 mP/s. 
     In reaction 6-2, the buffer contained an additional 5 μL serum. The background fluorescence was measured and blanked. F-thyroxine was added and mixed. The polarization value was observed to be 195 mP. Antibody (anti-T4) was added, stirred, and at t=5 s the rate of change of P was observed to be 9.7 mP/s. 
     The interaction between serum proteins and/or other serum components and F-thyroxine was quite strong, as the difference between the measured mP value before and after addition of serum almost doubled. The reaction rate between F-thyroxine and its antibody was also affected by the presence of serum, as it slowed to less than half its value. 
     In experiments 6-3 through 6-7, the effect of increasing sodium benzoate, was be observed to perform two tasks. The first was to slow the reaction rate down to about 30% its value from zero additive agent (9.7 to 3.2). The second was the elimination of non-specific binding, as expressed by the decrease of the mP value due to the interaction with serum. This was reduced from 195 down to 100, equivalent to the original mP value before the addition of serum as observed in experiment 6-1. 
     Example 7 
     Influence of Sodium Benzoate Combined with an Organic Solvent on the Velocity of the Reaction Between F-Thyroxine and Anti-Thyroxine in the Absence of Non-Specific Binding Components 
     The present example shows the influence of sodium benzoate in the absence of serum or other non-specific binding components, on the antibody reaction velocity between F-thyroxine and anti-T4 when the reaction was performed in a PBS buffer containing an additional 5% 2-propanol ( FIG. 5 ). The buffer and stock solutions described in Example 1 and the measurement methodology described in Example 2 were used. 
     Experimental Conditions: 
     
         
         
           
             Fluorescein-thyroxine, 1 nM 
             Anti-Thyroxine (1:300) 
             PBS with 5% 2-propanol 
             Sodium Benzoate in different concentrations 
           
         
       
    
     From this experiment, it was observed that the reaction rate between F-thyroxine and antibody slowed dramatically as the concentration of sodium benzoate increased to 1.0 molar. 
     Example 8 
     Influence of Sodium Benzoate Combined with an Organic Solvent on the Velocity of the Reaction Between F-Thyroxine and Anti-Thyroxine and on Non-Specific Binding of BSA 
     The present example shows the influence of sodium benzoate in the presence of the non-specific binding substance BSA, on the antibody reaction velocity between F-thyroxine and anti-T4 and on the non-specific binding between F-thyroxine and BSA ( FIG. 6 ) when the reaction was performed in a PBS buffer containing an additional 5% 2-propanol. The buffer and stock solutions described in Example 1 and the measurement methodology described in Example 2 were used. 
     Experimental Conditions: 
     
         
         
           
             Fluorescein-Thyroxine, 1 nM 
             Anti-Thyroxine (1:300) 
             PBS with 5% 2-propanol 
             BSA 0.1 μM 
             Sodium benzoate in different concentrations 
           
         
       
    
     It was observed that the non-specific binding due to the presence of BSA was reduced by the addition of the additive, as seen by the lowering of the initial P value with respect to the measurement without the additive. It was also observed that the rate of the antibody reaction was substantially slowed by the addition increasing concentrations of sodium benzoate. 
     Example 9 
     Influence of Guandine Hydrochloride Combined with an Organic Solvent on the Velocity of the Reaction Between F-Thyroxine and Anti-Thyroxine and on Non-Specific Binding of BSA 
     The present example shows the influence of guanidine hydrochloride on the antibody reaction velocity between F-thyroxine and anti-thyroxine in the presence of the non-specific binding substance BSA and on the non-specific binding between F-thyroxine and BSA ( FIG. 7 ) when the reaction was performed in a PBS buffer containing an additional 5% 2-propanol. The buffer and stock solutions described in Example 1 and the measurement methodology described in Example 2 were used. 
     Experimental Conditions: 
     
         
         
           
             Fluorescein-Thyroxine, 1 nM 
             Anti-Thyroxine (1:300) 
             PBS with 5% 2-propanol 
             BSA 0.1 μM 
             Guanidine hydrochloride in different concentrations 
           
         
       
    
     It was observed that the non-specific binding due to the presence of BSA was reduced by the addition of the additive, as seen by the lowering of the initial P valve with respect to the measurement without the additive. It was also observed that the rate of the reaction was substantially slowed by the addition of increasing concentrations of guanidine hydrochloride. 
     Example 10 
     Influence of Sodium P-Tolulene Sulfonate Combined with an Organic Solvent on the Velocity of the Reaction Between F-Thyroxine and Anti-Thyroxine and on Non-Specific Binding of BSA 
     The present example shows the influence of sodium p-toluene sulfonate on the antibody reaction velocity between F-thyroxine and anti-thyroxine in the presence of the non-specific binding substance BSA and on the non-specific binding between F-thyroxine and BSA ( FIG. 8 ) when the reaction was performed in a PBS buffer containing an additional 5% 2-propanol. The buffer and stock solutions described in Example 1 and the measurement methodology described in Example 2 were used. 
     Experimental Conditions: 
     
         
         
           
             Fluorescein-Thyroxine, 1 nM 
             Anti-Thyroxine (1:300) 
             PBS with 5% 2-propanol 
             BSA 0.1 μM 
             Sodium p-toluene sulfonate in different concentrations 
           
         
       
    
     It was observed that the non-specific binding due to the presence of BSA was reduced by the addition of the additive, as seen by the lowering of the initial P value with respect to the measurement without the additive. It was also observed that the rate of the reaction was substantially slowed by the addition of increasing concentrations of sodium p-tolulene sulfonate. 
     Example 11 
     Influence of Potassium Benzoate Combined with an Organic Solvent on the Velocity of the Reaction Between F-Thyroxine and Anti-Thyroxine and on Non-Specific Binding of BSA 
     The present example shows the influence of potassium benzoate on the antibody reaction velocity between F-thyroxine and anti-thyroxine in the presence of the non-specific binding substance BSA and on the non-specific binding between F-thyroxine and BSA ( FIG. 9 ) when the reaction was performed in a PBS buffer containing an additional 4.75% 2-propanol. The buffer and stock solutions described in Example 1 and the measurement methodology described in Example 2 were used. 
     Experimental Conditions: 
     
         
         
           
             Fluorescein-Thyroxine, 1 nM 
             Anti-Thyroxine (1:300) 
             PBS with 4.75% 2-propanol 
             BSA 0.1 μM 
             Potassium benzoate in different concentrations 
           
         
       
    
     It was observed that the non-specific binding due to the presence of the BSA was reduced by the addition of the additive, as seen by the lowering of the initial P value with respect to the measurement without the additive. It was also observed that the rate of the reaction was substantially slowed by the addition of increasing concentrations of potassium benzoate. 
     Example 12 
     Influence of Sodium Chloroacetate Combined with an Organic Solvent on the Velocity of the Reaction Between F-Thyroxine and Anti-Thyroxine and on Non-Specific Binding of BSA 
     The present example shows the influence of sodium chloroacetate on the antibody reaction velocity between F-thyroxine and anti-thyroxine in the presence of the non-specific binding substance BSA and on the non-specific binding between F-thyroxine and BSA ( FIG. 10 ) when the reaction was performed in a PBS buffer containing an additional 4.75% 2-propanol. The buffer and stock solutions described in Example 1 and the measurement methodology described in Example 2 were used. 
     Experimental Conditions: 
     
         
         
           
             Fluorescein-Thyroxine, 1 nM 
             Anti-Thyroxine (1:300) 
             PBS with 4.75% 2-propanol 
             BSA 0.1 μM 
             Sodium chloroacetate in different concentrations 
           
         
       
    
     It was observed that the non-specific binding due to the presence of BSA was reduced by the addition of the additive. It was also observed that the rate of the reaction was substantially slowed by the addition of increasing concentrations of sodium chloroacetate. 
     Example 13 
     Influence of Sodium Dichloroacetate Combined with an Organic Solvent on the Velocity of the Reaction Between F-Thyroxine and Anti-Thyroxine and on Non-Specific Binding of BSA 
     The present example shows the influence of sodium dichloroacetate on the antibody reaction velocity between F-thyroxine and anti-thyroxine in the presence of the non-specific binding between F-thyroxine and BSA ( FIG. 11 ) when the reaction was performed in a PBS buffer containing an additional 4.75% 2-propanol. The buffer and stock solutions described in Example 1 and the measurement methodology described in Example 2 were used. 
     Experimental Conditions: 
     
         
         
           
             Fluorescein-Thyroxine, 1 nM 
             Anti-Thyroxine (1:300) 
             PBS with 4.75% 2-propanol 
             BSA 0.1 μM 
             Sodium dichloroacetate in different concentrations 
           
         
       
    
     It was observed that the non-specific binding due to the presence of BSA was reduced by the addition of the additive, as seen by the lowering of the initial P valve with respect to the measurement without the additive. It was also observed that the rate of the reaction was substantially slowed by the addition of increasing concentrations of sodium dichloroacetate. 
     Example 14 
     Influence of Sodium Salicylate Combined with an Organic Solvent on the Velocity of the Reaction Between F-Thyroxine and Anti-Thyroxine and on Non-Specific Binding of BSA 
     The present example shows the influence of sodium salicylate on the antibody reaction velocity between F-thyroxine and anti-thyroxine in the presence of the non-specific binding substance BSA and on the non-specific binding between F-thyroxine and BSA ( FIG. 12 ) when the reaction was performed in a PBS buffer containing an additional 4.75% 2-propanol. The buffer and stock solutions described in Example 1 and the measurement methodology described in Example 2 were used. 
     Experimental Conditions: 
     
         
         
           
             Fluorescein-Thyroxine, 1.5 nM 
             Anti-Thyroxine (1:300) 
             PBS with 4.75% 2-propanol 
             BSA 0.1 μM 
             Sodium Salicylate in different concentrations 
           
         
       
    
     It was observed that the non-specific binding due to the presence of BSA was reduced by the addition of the additive, as seen by the lowering of the initial P valve with respect to the measurement without the additive. It was also observed that the rate of the reaction was substantially slowed by the addition of increasing concentrations of sodium salicylate. 
     Example 15 
     Influence of N-hydroxysuccinimide Combined with an Organic Solvent on the Velocity of the Reaction Between F-Thyroxine and Anti-Thyroxine and on Non-Specific Binding of BSA 
     The present example shows the influence of N-hydroxysuccinimide on the antibody reaction velocity between F-thyroxine and anti-thyroxine in the presence of the non-specific binding substance BSA and on the non-binding specific binding between F-thyroxine and BSA ( FIG. 13 ) when the reaction was performed in a PBS buffer containing an additional 4.75% 2-propanol and unspecific binding of fluorophore and BSA ( FIG. 13 ). The buffer and stock solutions described in Example 1 and the measurement methodology described in Example 2 were used. 
     Experimental Conditions: 
     
         
         
           
             Fluorescein-Thyroxine, 1.5 nM 
             Anti-Thyroxine (1:300) 
             PBS with 4.75% 2-propanol 
             BSA 0.1 μM 
             N-Hydroxysuccinimide in different concentrations 
           
         
       
    
     The ΔP diagram shows the difference of the P value of the free fluorescence labelled compound and its P value during the reaction, where ΔP=P−P t=0 . This shows the increase of the P value due only to the binding of analyte and antibody. It was observed that the non-specific binding due to the presence of BSA was slightly reduced by the addition of the additive. It was also observed that the rate of the reaction was substantially slowed by the addition of increasing concentrations of N-hydroxysuccinimide. 
     Example 16 
     Influence of Sodium Chloride Combined with an Organic Solvent on the Velocity of the Reaction Between F-Thyroxine and Anti-Thyroxine and on Non-Specific Binding of BSA 
     The present example shows the influence of sodium chloride on the antibody reaction velocity between F-thyroxine and anti-thyroxine in the presence of the non-specific binding substance BSA and on the non-specific binding between F-thyroxine and BSA ( FIG. 14 ) when the reaction was performed in a PBS buffer containing an additional 4.75% 2-propanol. The buffer and stock solutions described in Example 1 and the measurement methodology described in Example 2 were used. 
     Experimental Conditions: 
     
         
         
           
             Fluorescein-Thyroxine, 1.5 nM 
             Anti-Thyroxine (1:300) 
             PBS with 4.75% 2-propanol 
             BSA 0.1 μM 
             Sodium Chloride in different concentrations 
           
         
       
    
     It was observed that the non-specific binding due to the presence of BSA was reduced by the addition of the additive. However, it was also observed that the rate of the reaction was not substantially slowed by the addition of increasing concentrations of sodium chloride thereby making the antibody reaction more difficult to study in the presence of sodium chloride relative to in the presence of other salt agents. 
     Example 17 
     Effects of Sodium Benzoate Combined with an Organic Solvent on the Velocity of the Reaction Between Rhodamine-Thyroxine and Anti-Thyroxine and on Non-Specific Binding of Serum Components 
     The present example shows the effects in a Rhodamine-Thyroxine/Anti Thyroxine system of different amounts of sodium benzoate in serum and at a higher concentration of alcohol ( FIG. 15 ). The serum was diluted 1:500 in PBS/10% 2-propanol The buffer and stock solutions, as well as the measurement methodology described in Example 5 were used. 
     Experimental Conditions: 
     
         
         
           
             Rox-Thyroxine, 1.5 nM 
             Anti-Thyroxine (1:300) 
             PBS with 10% 2-propanol 
             Serum 1:500 
             Sodium benzoate in different concentrations 
           
         
       
    
     It was observed that the non-specific binding due to the presence of serum components was substantially reduced by the addition of this combination of additives. It was also observed that the rate of the reaction was substantially slowed by the addition of increasing concentrations of sodium benzoate. 
     Example 18 
     Standard Curve Constructed by Monitoring for a Change in the Initial Rate of Polarization as a Function of Different Known Quantities of the Analyte Thyroxine 
     The present example is directed to an experiment, such as in Example 17, performed in serum, and shows that the antibody reaction between Rox-thyroxine and anti-thyroxine can be performed in the presence of additive ( FIG. 16 ). In particular,  FIG. 16  shows a calibration curve, measured in the following conditions:
         1500 μl Serum diluted 1:500 with (PBS buffer containing 10% 2-propanol and 0.7 M sodium benzoate)   1.5 nM Rox-Thyroxine and 20 μl anti-Thyroxine (1:300 dilution)
 
The x-axis of the graph in  FIG. 16  is the concentration (nM) of analyte thyroxine in the cuvette. From the standard curve, it is possible to measure unknown quantities of the analyte.