SINGLE COLOR READING METHOD FOR DETERMINING FRUCTOSAMINE

Abstract: A single color determination method of quantization of the amount of fructosamine in a sample such as serum by removing other interfering reducing agents and developing a color with a coloring agent such as tetrazolium salt.

Description: TECHNICAL FIELD This invention relates to a single color reading method for determining fructosamine and thus glucose levels in samples such as blood to aid in detecting diabetes and monitoring diabetes status in patients. More particularly, this invention relates to an end point type method of determining fructosamine levels as distinguished from a kinetic method. BACKGROUND ART Colorimetric methods of determining frustosamine levels in samples such as blood serum using reagents that produce a color change due to the reducing action of fructosamine are well known as illustrated by John Richard Baker's European Patent No. 0 085 263. The Baker European Patent uses a basic treatment of blood serum with a dye selected from tetrazolium salts, e.g., tetranitro blue, tetrazolium, or preferably nitroblue tetrazolium which, when reduced, changes to a highly colored blue (purple) formazan dye having a broad absorbance peak at about 535nm. The Baker method uses two color readings with a definite time interval therebetween to develop a kinetic color change which is utilized to calculate from standards the fructosamine level or content. As is well known, kinetic methods are difficult to perform manually with precision, due to the exact timing required. DISCLOSURE OF INVENTION It is an aim of this invention to provide method for determining fructosamine levels that utilises a single color reading. According to the invention there is proposed method of determining fructosamine levels in a sample of blood or a sample derived from blood, said method comprising the steps of controlling the pH of the sample to between 10 and 14, adding a coloring agent to the sample and after a delay, taking at least one color measurement and comparing the resultant color with the color in a standard solution, the selected coloring agent and pH conditions being such that a change of color in the coloring agent is caused by the glucose in the sample while that glucose is still reacted or associated with an amine group of protein in that blood and thus is present in the form of fructosamine, said method being characterised by the step of selectively eliminating ascorbate and glutathione from the sample before adding the coloring agent to eliminate interfering reducing substances leaving fructosamine so as to enable the fructosamine content to be determined by a single color measurement. An advantage of this invention is that, once the interferences, i.e., the other reducing agents ascorbate and glutathione, are eliminated, any color formed is due only to the reaction between fructosamine and the tetrazolium salt. Therefore, the fructosamine reaction can be stopped, which makes the timing of the color determination less critical. Thus, the addition of a solution that halts the reaction between the fructosamine and the tetrazolium coloring agent allows the spectrophotometric reading to be done on a non-timed basis and thus the color measurements can be performed at the operator's convenience rather than at a particular time as required by the kinetic method so allowing greater freedom from the timed sequence required in the kinetic method. The primary advantage of a single color measurement is that it takes less time than the Baker method. BEST MODE FOR CARRYING OUT THE INVENTION In one aspect of this method a sample is taken such as a blood sample, and it is then treated to remove or destroy the reducing agents ascorbate and gluthathione to leave fructosamine. This treatment to eliminate the other reducing agents or interfering substances may consist of any one of the following methods (hereinafter referred to as the elimination of interfering substances step): 1. Incubation of samples for long periods of time at low temperatures either with or without a base, preferably a strong base. 2. Incubation of sample at high temperatures, for instance at a temperature of 37°C and higher either with or without a base, preferably strong. 3. Incubation of sample at high pH, at least 10 or higher. 4. Desalting the sample via dialysis or gel permeation chromatography. 5. Adding one or more oxidizing agents which would oxidize interferences but not fructosamine. 6. Adding enzymes which would convert interfering substance(s) to a form(s) that will not react with the coloring reagent. After the interfering substances have been removed from the sample, it is then mixed with the coloring reagent and incubated for a period of time. The amount of color formed is measured photometrically and is proportional to the amount of fructosamine in the sample. A serum blank may also be run in order to subtract out any background color due to the color of the serum. Of the physiologically relevant reducing substances present in blood serum, including glucose, fructose, glucosamine, creatinine, and urate, only ascorbate and glutathione show significant interferences with tetrazolium and related dye reductions. Thus, by eliminating or destroying ascorbate and glutathione in samples, it is possible to then determine fructosamine content with a single color determination. Interference from reducing substances in the sample, other than fructosamine, can be lowered and/or eliminated by one or more of the methods discussed previously. In the preferred method, serum is mixed with a small volume of base for instance sodium hydroxide solution to raise its pH to about 10. The sample is allowed to sit at room temperature for at least 30 minutes. This effectively eliminates the interference. The treated sample is then assayed for fructosamine content according to the Baker patent methodology, except that only one color reading is taken. It is important for all samples, standards, blanks, etc., to be assayed under identical conditions. The reaction of fructosamine with tetrazolium is very dependent on factors such as pH, time and temperature being the same. These factors must all be controlled in order to assure that the amount of color generated is proportional to the fructosamine concentration in all samples. EXAMPLES The following examples are given for the purpose of illustrating the present invention. Example 1 A serum sample was aliquoted and admixed with the following solutions on a microliter by volume basis at ambient temperature: Exactly 30 minutes after the four serum trials were mixed, 20 microliters of each trial was mixed with 1 mL of color reagent. The color was allowed to develop for 900 seconds (12 minutes) and the absorbance at 500 nm was immediately read for each trial on a suitable spectrophotonometer. The resultant absorbances were: Trial D which contained ascorbate and was treated with sodium hydroxide solution showed no interference from ascorbate. Trial B which contained ascorbate, but was not treated with sodium hydroxide solution showed interference. Example 2 A serum sample was aliquoted, admixed on a microliter by volume basis, and incubated as follows: The samples were allowed to incubate for 200 minutes and were then assayed as in Example 1. The resultant asorbances were: These trials demonstrate that elevated temperatures can be used to eliminate ascorbate interference. Example 3 A serum sample containing 1.3 mM 1-deoxy, 1-morpholino fructose (DMF) was treated as in Example 1. The resultant absorbances were: This shows that DMF, a synthetic fructosamine compound which is used to prepare standards for this test, is not effected by this sample preparation step. Example 4 A serum sample was treated as in Example 1, except 2.0 mM mercuric acetate an oxidizing agent was substituted for the sodium hydroxide solution. The resultant absorbances were: This demonstrates the ability of mercuric ion to reduce the interference caused by ascorbate. Example 5 A serum sample was treated as in Example 1, except reduced glutathione (obtained from Sigma Chemical) at a concentration of 10 mM was substituted for the ascorbic acid solution. The resultant absorbances were: These trials demonstrate that glutathione interference can also be removed by a 30 minute incubation with sodium hydroxide. Example 6 A serum sample was treated as in Example 1, except ascorbate oxidase spatulas (obtained from Boehringer, Mannheim Biochemicals, Indianapolis, Indiana, product #736619) were substituted for the sodium hydroxide solution in trials C and D. Also, the following volumes of water on a microliter basis were used to dilute the serum: A and C - 50, B and D - 0. The spatulas contain about 17 units of ascorbate oxidase enzyme (E.C. #1.10.3.3) which is used to remove ascorbic acid from aqueous solutions. The spatulas were used to mix the serum occasionally throughout the 30 minute incubation. The resultant ascorbances were: These trials demonstrate the ability of ascorbate oxidase to reduce the interference caused by ascorbate. Example 7 A serum sample was desalted using gel permeation chromatoraphy. The serum sample was split into two 500 microliter portions. Fifty microliters of water was added to the first portion and 50 microliters of 1.1 mM ascorbic acid was added to the second portion. Two desalting columns (obtained from Isolab, Inc., product #QS-2B) were equilibrated with isotonic saline. Five hundred microliters of each portion was added to a separate column and was allowed to elute. Next 300 microliters of isotonic saline was added to each column and was allowed to elute. Receivers were placed beneath the columns and 500 microliters of isotonic saline was added to each column and the eluate collected. This eluate contained the desalted serum. The desalted eluates were aliquoted and admixed on a microliter basis with the following solutions: Next, 50 microliters of each trial was admixed with 1 mL of the color reagent in Example 1. The color was allowed to develop as in Example 1. The resultant absorbances were: These trials demonstrate that desalting the sample prior to the fructosamine assay eliminates ascorbate interference. Example 8 Three serum samples were treated as in Example 1, Trial B, stored at 4°C, and assayed as in Trial 1 on days 0, 1, and 4. DMF standards were run on each day to calibrate the relationship between absorbance and DMF content analogous to the method in the Baker patent. Also, the sample blanks were subtracted. The resultant concentrations of fructosamine in the simple are shown in the following table: This demonstrates that ascorbate interference can be reduced, just by storing the sample at 4°C. Example 9 A continuous flow autoanalyzer was used to perform the fructosamine assay. As in Example 2 of the Baker patent, a Technicon Autoanalyzer (Technicon Instruments Corp., Tarrytown, N.Y.) was used. The analyzer was programmed to allow one part sample to mix with three parts of 15 mM sodium hydroxide solution. This mixture was heated at 37°C for five minutes. The analyzer was then programmed to mix in three parts (for a total of seven parts) of the tetrazolium color reagent of Example 1 with the above mixture. This mixture was incubated for two minutes at 37°C. The analyzer was programmed to then read the absorbance at 500 nm. One normal and one elevated serum sample were mixed as in Example 1, Trials A and B, and analyzed immediately. The resultant absorbances were: Example 10 One normal and one elevated serum sample was treated as in Example 1, Trial D. Exactly 30 minutes after the serum trials were mixed, 20 microliters of each trial was mixed with 1mL of color reagent. The color was allowed to develop for 900 seconds at which time 2.0 mL of 1 N-hydrochloric acid was added and mixed in order to stop the reaction. The absorbance of each tube was measured at 5, 10, and 30 minutes after the acid was added. The resultant absorbances were: This demonstrates that the reaction can be stopped by lowering the pH to below 5 and that the generated color remains stable. This allows the color measurement to be performed at a convenient time.

Claims: A method of determining fructosamine levels in a sample of blood or a sample derived from blood, said method comprising the steps of controlling the pH of the sample to between 10 and 14, adding a coloring agent to the sample and after a delay, taking at least one color measurement and comparing the resultant color with the color in a standard solution, the selected coloring agent and pH conditions being such that a change of color in the coloring agent is caused by the glucose in the sample while that glucose is still reacted or associated with an amine group of protein in that blood and thus is present in the form of fructosamine, said method being characterised by the step of selectively eliminating ascorbate and glutathione from the sample before adding the colouring agent to eliminate interfering reducing substances leaving fructosamine so as to enable the fructosamine content to be determined by a single color measurement. A method according to claim 1, characterised in that the interfering substances are eliminated by treating the sample with a strong base, an oxidizing agent, an enzyme, or by desalting. A method according to claim 1, characterised in that the interfering substances are eliminated by holding the sample at a temperature of 37°C or higher. A method according to any preceding claim, characterised in that said pH level lies between 10.5 and 10.8. A method according to any preceding claim, characterised in that said pH level is adjusted by adding a buffer to the sample. A method according to claim 5, characterised in that said buffer comprises sodium carbonate and sodium bicarbonate in suitable proportions. A method according to any preceding claim, characterised in that the coloring agent is tetrazolium salt. A method according to claim 7, characterised in that the salt is INT. A method according to any preceding claim, characterised in that the reaction between the coloring agent and the fructosamine is stopped by the addition of an acid. A method according to claim 9, characterised in that the pH is lowered to below 5 to stop the reaction between the coloring agent and the fructosamine.

Inventors: ROSENTHAL, MURRAY A.
Applicants: ISOLAB, INC.
Application year: 1985
Main classification: G01N33
IPC codes: G01N33