Abstract:
Methods and devices for conducting platelet aggregation analysis. A method for conducting platelet aggregation analysis by a cartridge device, including providing a blood sample in the cartridge device, stirring the blood sample within the cartridge device, measuring the electrical impedance between electrodes to obtain measured electrical impedance values, comparing measured electrical impedance values, discarding and repeating the measurements of the electrical impedance in case a variation of the measurements is outside a predetermined threshold range, or reporting the measured electrical impedance values in case the variation of the measurements is within the predetermined threshold range. Reported measured electrical impedance values indicate platelet aggregation in a blood sample.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application which claims the benefit of and priority to U.S. utility patent application Ser. No. 10/583,062 filed Jan. 31, 2007, now U.S. Pat. No. 7,901,629, issued on Mar. 8, 2011 which claims priority to PCT/EP2003/014329 filed Dec. 16, 2003, the entire contents of which are hereby incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a cartridge device for blood analysis and a method for testing platelet functions. 
     BACKGROUND ART 
     Though the present invention can be used in many fields of measuring fluids it will be described in regard to measuring the platelet function of blood in the following. 
     Blood consists of cells suspended in so called plasma, a protein rich fluid. The major groups of cells in the blood are red cells, white cells and platelets. The platelets are responsible for plugging gaps or holes in the blood vessel wall. This is achieved by a mechanism called aggregation-adhesion reaction. When the platelets aggregate, they become sticky and, as a result thereof, they stick to each another and to the damaged tissue. Usually this happens when the platelets come into contact with certain materials and chemicals, especially those related to damaged cells. 
     Platelet adhesion to injured blood vessels is an essential property in order to close wounds and thus to ensure survival of the organism after for example a trauma. However the adhesion and aggregation of platelets can also be extremely dangerous when the platelets mistake an aged or inflamed vessel for a vascular injury and thus impair blood flow in tissues of vital importance. Such processes take place during a myocardial infarction or a stroke, diseases which account for more deaths in the industrialised nations than infectious diseases or cancer. 
     An increasing number of patients who have suffered myocardial infarction or stroke as well as patients who are at high risk for these events is treated for a reduced tendency of their platelets to aggregate with substances called “anti-platelet agents”. Besides their beneficial effect—to reduce the incidence of platelets closing vitally important vessels—these drugs may also induce bleeding. However a larger danger is due to the fact that in some of the patients the drugs seem not to work properly. Current studies have shown that up to 25% of the patients treated do not adequately respond to this treatment. 
     It is thus not only of scientific interest, but also of high clinical importance to be able to test the function of the platelets and the individual response to drugs which interfere with their activation. Several techniques according to the prior art are used to analyse platelet functions or the action of anti-platelet drugs. 
     An early but still widely used development is the Born aggregometer which measures the change of light transmission of platelet-rich plasma (PRP) during the process of aggregation. Platelet rich-plasma is obtained by centrifugation of anticoagulated blood at a relatively low speed, which removes the heavy (hemoglobin-filled) red cells from the plasma, but leaves the much lighter platelets in the solution. Because of the platelet content the light transmission of PRP is relatively low. When the platelets aggregate the optical density is reduced, because the platelets form few large aggregates, which interfere much less with the light transmitted through the sample. 
     A disadvantage of this technique is the necessity of producing PRP, whose extraction is a complicated, time-consuming and thus expensive procedure. Furthermore, the aggregation of platelets is not measured in its natural environment, blood, thus the influence of red and white cells on the platelets is not measured. 
     Other methods disclosed in documents U.S. Pat. No. 4,604,894 of Kratzer and Born, U.S. Pat. No. 6,010,911 of Baugh et al. and U.S. Pat. No. 5,922,551 of Durbin et al. require relatively complex and expensive cartridges interfering with the use of these techniques for routine testing. 
     Document U.S. Pat. No. 4,319,194 of Cardinal et al. discloses a platelet aggregation analysis typically performed in whole blood by measuring the electric impedance between two electrodes, being immersed in a sample. During initial contact with the blood or PRP, the electrodes are coated with a monolayer of platelets. When an aggregating agent is added, platelets gradually accumulate on the monolayer coating, increasing the impedance between the electrodes. The change in impedance is recorded as a function of time. It is preferred that the electrodes comprise precious metals since base metals drift in blood-saline mixtures. 
     One disadvantage of precious metal electrodes is high costs. Hence they are too expensive to be disposable. Therefore, the electrode assembly must be cleaned by hand between tests, exposing the operator to contact with the sample, and thus potentially exposing the operator to diseases transmitted through the fluids contained in the sample. Since diseases such as hepatitis and AIDS can be transmitted through handling of blood products, there is an understandable reluctance on the part of medical professionals to handle blood, blood products and objects contamined therewith. 
     A further disadvantage of this structure is due to the fact that the electrodes of the aggregometer have to be handled by the user during the cleaning procedure, potentially disturbing the adjustment of the distance between the electrodes, causing inconsistent results. Furthermore each electrode requires exact placement of the wires during fabrication, making the final product expensive. 
     Document U.S. Pat. No. 4,591,793 of Freilich describes a substitution of the wire electrodes by a conductive ink printed on a plastic non-reactive base. 
     However this device is detrimental due to the fact that the platelets have difficulties in adhering to the exposed conductive surface of the Freilich device. Sometimes the aggregated platelets break off the surface, causing a sudden change in impedance. Hence the measured results by the device are inconsistent and not reproducible. 
     A further measuring cell assembly according to the prior art is disclosed in Document U.S. Pat. No. 6,004,818 of Freilich et al., which is shown in  FIGS. 11A and 11B . 
       FIG. 11A  illustrates an explosive view of a part of a measuring cell device comprising an insulator  1 , which is sandwiched between two flag-shaped electrodes  2 . Each electrode  2  includes a connection tab  3  at one end and a tip  4  at the other end thereof, with a shaft  5  joining the tab  3  and the tip  4  respectively. After joining the two electrodes  2  and insulator  1  together a non-conductive coating is applied to the insulator  1  and to the electrode shafts  5 . 
       FIG. 11B  illustrates a perspective view of a measuring cell device according to the prior art. As shown in  FIG. 11B  the electrode assembly is fixed within a cuvette  56  using a positional clip  7 . Prior to and during measurement, a stir bar  8  is activated to generate a circular flow of sample within the cuvette  6 . 
     One drawback of the aforementioned measuring cell device is the use of punched sheet metal for the electrodes  2 . The outline of the electrodes can economically be produced by the process of punching or related methods. However the surface qualities produced by these methods are relatively poor and vary during the production of large quantities (because of the aging of the punching blades applied during the process). The quality of the surfaces, which strongly affect the measurement, are thus strongly varying resulting in high variations between different disposable electrodes. 
     A further disadvantage is due to the fact that the complete measuring cell device consists of six different pieces, namely the two electrodes  3 , the insulator  1 , the coating, a cuvette  6 , a positional clip  7  and a stir bar  8 . This results in an expensive, complicated production process, which is either labor intensive or requires high investments for an automated assembly line. 
     Additionally the measuring cell device of Freilich does not overcome the relatively high variation reported for whole blood aggregometry. In document U.S. Pat. No. 6,004,818 a variation of around 10-15% between multiple measurements is reported for said measuring cell assembly. 
     SUMMARY OF THE INVENTION 
     Hence it is an object of the present invention to provide a cartridge device which overcomes the above mentioned drawbacks, particularly to provide a cartridge device which is economical, reproducible, safe and easy to use. 
     It is another object of the present invention to eliminate the limited reproducibility of previous methods according to the prior art for the measurement of aggregation in whole blood. 
     It is still another object of the present invention to provide a cartridge device which is accurate and so economical to produce that it can be discarded after each test. 
     It is still another object of the present invention to provide a cartridge device which can be provided ready to use, so the application of the technique is simple and contains only few user-related sources of error and variation. 
     The present invention provides a cartridge device for analysing blood comprising a cell having a receiving portion for receiving a blood sample and a jack portion for receiving a plug; a stirring device for circulating said blood sample within said receiving portion; and an electrode holder having at least one incorporated electrode wire pair; wherein the electrode holder is attachable to the cell such that one end of the at least one electrode wire pair forms a sensor unit for measuring the electrical impedance between the two electrode wires of the at least one electrode wire pair within the blood sample and that the opposite end of the at least one electrode wire pair forms a plug portion being connectable directly to the plug for an electrical connection of the sensor unit to an analyser. 
     Furthermore the present invention provides a cartridge device for analysing blood comprising a cell having a receiving portion for receiving a blood sample; a stirring device for circulating said blood sample within said receiving portion; and at least two electrodes for measuring the electrical impedance between the at least two electrodes within the blood sample; wherein the at least two electrodes consist of a metal comprising a first material with a high electrical conductivity, which is covered by a second material, which has a high electrical conductivity and which is resistant against oxidation. 
     Additionally the present invention provides a method for analysing blood by means of a cartridge device comprising at least three electrode wires or electrodes for measuring the electrical impedance between at least two of the at least three electrode wires or electrodes, comprising the following steps: measuring the electrical impedance between at least two different pairs of electrode wires or electrodes; comparing the measured electrical impedance values; discarding and repeating the measurements in case the variation is outside a predetermined threshold range; or indicating the measured electrical impedance values and/or the mean value thereof in case the variation is within the predetermined threshold range. 
     The dependent claims disclose further features and advantages of the present invention. 
     Hence the present invention provides a disposable device which overcomes the main problems of the aforementioned developments. The apparatus measures the change of impedance during the process of aggregation, whereas the quality of the measurement is ensured by a double-, triple- or multi-determination of the impedance. The cartridge can be manufactured at very low costs and is well suited for full automation of the production process. 
     The device for monitoring blood-platelet aggregation comprises a cartridge including electrodes; a means for stirring the sample; a power source for supplying electric current to the electrodes; and a data analysis device for receiving and analysing the change in electrical resistance or impedance between the electrodes. 
     A device is provided which is economical, accurate and quality-controlled. The test includes a disposable test cell having preferably at least two separate sensor elements. Each sensor element consists of preferably at least two separate conductive elements. During the analysis blood is placed in the test cell. By adequate means it is stirred. Optionally reagents activating or inhibiting platelet function are added. When blood platelets are activated they adhere on the conductive elements and alter electrical variables measured e.g. the resistance between the conductive elements or the electrical impedance. By comparing the values assessed on the different electrodes the result of the analysis is being controlled. When an adequate accordance between the individual results is attained, the result is accepted and the value or the mean (or median) value of the results is reported. If highly discrepant results are determined the analysis is rejected and a repeated analysis is requested. 
     The present invention further describes an economical and standardized production technique using injection moulding providing an improved analytical signal using suitable electrode compositions. The conductive elements are formed by wires which incorporate two functions. In their upper part they form a connector—or jack—by which the conductivity signal is detected. In the lower part they are contacted with the blood sample and serve as a sensor for the platelet aggregation. The conductive elements are formed from a material of high conductivity and precious surface. A silver-copper material coated by silver or other precious material is preferably applied. This leads to a strong signal, due to the higher difference in conductivity between the platelet-coated vs. the uncoated electrode. In addition also the detection of the conductivity over the connector—or jack—is improved. 
     Hence the present invention allows to asses the whole blood aggregation in a routine setting. Unlike previously applied methods for the assessment of whole blood aggregation it is easy to use and does not require the cleaning of blood-contaminated devices. Compared to the measurement of aggregation in platelet rich plasma the inventive method provides the advantage to eliminate the need to centrifuge blood to obtain PRP for use in measuring aggregation of platelets optically. Thus reduced labor costs, increased speed, and test of the platelets in their natural milieu are important advantages. The measurement in whole blood also allows studies to be performed in cases where optical aggregation is not reliable, such as giant platelets (Bernard-Soulier syndrome), where red cells have been lysed or where it is difficult to obtain enough blood to make PRP, such as with small animals or babies. 
     In addition whole blood aggregometry has the advantage that—like the Born aggregometry—it provides a continuous signal of the kinetics of platelet aggregation, unlike other methods, which provide only a measure for total aggregation, without revealing the kinetics of the process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be further understood by reference to the drawings, wherein: 
         FIG. 1  is a perspective view of a cell according to an embodiment of the present invention; 
         FIG. 2  is a perspective view of an electrode holder according to an embodiment of the present invention (with double determination technique); 
         FIG. 3  is a side view of an electrode holder according to certain embodiments of the present invention (double, linear triple or linear quadruple detection technique); 
         FIG. 4  is a side view of the cartridge according to certain embodiments of the present invention (double, linear triple or linear quadruple detection technique); 
         FIG. 5  is a top plan view of a cartridge according to a first embodiment of the present invention (with double determination technique); 
         FIG. 6  is a perspective view of the cartridge according to the first embodiment of the present invention (with double determination technique); 
         FIG. 7  is a sectional view of a cartridge device with attached plug and cable according to an embodiment of the present invention; 
         FIG. 8  is a perspective view of a cartridge device according to a second embodiment of the present invention (with triple determination technique; 
         FIG. 9  is a perspective view of a cartridge device according to a third embodiment of the present invention (with linear quadruple determination technique); 
         FIG. 10  is a top plan view of a cartridge device according to a fourth embodiment of the present invention (with circular triple determination technique; 
         FIG. 11A  is an explosive view of a part of a cartridge device according to the prior art; and 
         FIG. 11B  is an perspective view of a cartridge device according to the prior art. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In all figures like reference numerals are used to demote like elements and/or like functions of respective elements as long as nothing else is stated. It should be noticed that in the following the term cartridge describes the disposable structure applied in preferred embodiments of the invention. It consists of a cell  9  and an electrode holder  14 . 
     In a preferred embodiment the cartridge device  20  consists of two main parts  9  and  14  made by particularly injection moulding. The one part is a single pieced receiving means and subsequently called cell  9 . 
       FIG. 1  depicts a cell  9  in a perspective view according to a preferred embodiment of the present invention. The cell  9  comprises at least a preferably cylindrical receiving portion or cup portion  10 , which is open at one face side, holding a sample during the analysis. The sample is placed into the cup portion  10  of the cell  9  by means of a particularly conical funnel tube  11 . 
     The cell  9  further comprises a jack portion  12  adjacent to the cup portion  10  and separated therefrom by a stopping wall  27 . Together with an electrode holder  14 , described later in further details (see  FIG. 2 ), the jack portion  12  forms a jack  18  (see  FIG. 3 ), in which a plug  22  (described later in further details) is arranged and which allows the electrical connection of the cell  9  to an instrument, for example an analyser. 
     A guiding rail  13  is formed on both sides of the funnel tube  11  of the cell  9 , respectively, for guiding the electrode holder  14  into a secure and tight connection with the cell  9  as shown in  FIG. 4 . 
     Preferably and as shown in  FIG. 1  the whole cell  9  is formed in a way that it can be easily unmoulded with a two part injection moulding mould. This requires that the cup  9  does not form any undercuts and that the outer and inner surfaces are at least slightly conical. Although the cell  9  has three different functional portions, it can inexpensively be produced as one part using injection moulding. This minimises production costs on the one hand and the need for manual handling for the use on the other hand. 
     The cell  9  is in particular produced of blood compatible material such as polystyrene. Other usable materials are polymethyl methacrylate (PMMA) or polyethylene. The importance of using a blood compatible material is that the blood platelets will not get activated by the contact with the cell material. This allows to activate specifically the platelets as intended for the different test methods performed. 
     Dual Electrode Cartridge Device 
     The other part of the cartridge device  20  is the electrode holder  14 , illustrated in  FIG. 2  in a perspective view according to a first embodiment of the present invention. The electrode holder  14  consists particularly of a plastic body  15 , in which two tiny electrode wire pairs  16 ,  24  are incorporated. Each wire of a wire pair  16 ,  24  has preferably a diameter of about 0.1-0.5 mm, most preferably 0.3 mm. 
     The electrode holder  14  comprises especially a L-formed body  15  with a long part  15   a  and a short part  15   b  perpendicular to the long part  15   a . At the face side of the long part  15   a  of the electrode holder  14  each electrode wire of the electrode pairs  16 ,  24  protrudes and forms a sensor portion  16   a ,  24   a . For example, two sensor portions  16   a  or  24   a  form together a sensor unit  17   a  or  17   b . During the analysis the sensor units  17   a  and  17   b  are completely immersed into the blood sample. The blood platelets adhere on the sensor portions  16   a ,  24   a  of the wire pairs  16 ,  24  and change the electrical impedance between said two wires of a respective wire pair  16  or  24 . These measured impedance values can be compared with each other and/or with a predetermined threshold. 
     At the face side of the short part  15   b  of the electrode holder  14  each electrode wire of the electrode pairs  16 ,  24  protrudes under a predetermined angle, for example 50°, and forms a connector portion  16   b ,  24   b , respectively. 
     The electrode holder  14  of this preferred embodiment comprises two linear independent sensor units  17   a ,  17   b , each formed by an electrode wire pair  16 ,  24 . The sensor units  17   a ,  17   b  are particularly placed symmetrically to each other at the face side of the long part  15   a  of the body  15  in order to ensure identical flow conditions around each wire pair and acceptable measurement results. Preferably the two wires of an electrode pair  16  ( 24 ) are positioned parallel to each other and parallel to the wires of another electrode pair  24  ( 16 ). 
     The electrode holder is preferably inexpensively produced by injection moulding. This requires to insert the wires of the electrode pairs  16 ,  24  into the mould and to extrusion-coat them by the resin. This poses the problem not to bend the thin wires by the high pressure in the injection mould. Also it is important to automate the process of placing the wires into the mould, thus allowing fully automated production and thus low costs. Preferably the wires are automatically placed into the mould from a roll, then extrusion coated and afterwards automatically taken out of the mould and then cut into the right dimensions. This also provides the advantage that cable on a roll is less expensive than pre-cut cable bars. It is advantageous to keep the wires under tension during the injection process in order to prevent bending of the thin wires. 
     It is also advantageous to keep the body  15  of the electrode holder  14  thin, as it is shown in  FIG. 2 . This leads to reduced pressure requirements in the injection process and thus also to a lower tendency to bend the wires of the electrode wire pairs  16 ,  24 . A thickness of 1-5 mm of the body  15  is preferred most. The plastic material used for the electrode holder is preferably a blood compatible material such as polystyrene, PMMA or polyethylene, most preferably polystyrene. 
     The material selected for the electrodes or wires needs to fulfil several requirements. It should provide a low electrical resistance. This provides a good electrical connection of the wires in the connection portions  16   b ,  24   b  to plugs  22  or equivalent devices and in addition it provides a stronger signal of the analysis. In the sensor portions  16   a ,  24   a  the wires are coated by the activated platelets which enhances the electrical resistance between them. When wires of a lower resistance are applied this leads to a stronger resistance change when the wires are coated by the platelets compared to a material with lower conductivity. Materials with a high conductivity include for example copper and copper alloys (copper-silver alloy, copper-magnesium alloy). 
     However the wire material must also not oxidate when contacted to blood, even when different oxidating drugs are present. This requires the surface of the wires  15  to have a low tendency to oxidate. Such materials are for example precious metals such as platinum, gold, silver. The named materials with a high conductivity (copper and copper alloys) have a high tendency to oxidate. However the precious metals are too expensive to be used for the production of a single use cartridge. According to the present invention it was found that by coating a low cost wire material with a high conductivity (preferably a silver-copper alloy comprising 0.2-2% silver, most preferably 0.9% silver) with a precious material such as silver (preferably using a coating of 0.5-20 g/kg, most preferably 2 g/kg) a wire is obtained which is economical, which provides a good electrical impedance and which is also sufficiently resistant to oxidation during the analysis. Other coatings made of gold, platinum or other precious metals can be also applied. 
     The electrodes are preferably wires having a circular cross-section. The inventors have tested diameters between 0.1 mm and 0.50 mm. The signal turns out to become weaker with increasing diameter, so that the electrodes should be thin. The electrode holder is manufactured by injection moulding which requires tearproof rods, i.e. the rods should not be to thin. The optimal value for diameter of the electrodes which encompasses these two aspects turned out to be 0.3 mm. 
     The length of the ends of the electrodes which stand out of the electrode holder  14  is preferably 4 mm. The inventors have tested lengths between 2 mm and 6 mm. It turned out, that the longer the ends are, the weaker the signal becomes, so that short ends are preferred. However, when the ends become too short, the production of the electrode holder becomes more complicated, so that an optimal length is about 4 mm. 
     A spacing of 0.5-1 mm between the electrode wires was found to provide optimal signal and reproducibility. 
     The electrode wires are preferably bended two times as is best seen in  FIGS. 3 and 4 . This bending ensures that the ends of the electrodes form together with the jack portion  12  of the cell  9  a jack. In the present case the inventors use preferably a geometry consistent with the normed jack RJ12. This allows to use the standard RJ12 plug to connect the cartridge device  20  to an analyser just by putting the plug into the jack portion  18 . 
     After the injection moulding process the wires  16 ,  24  are bended for example under an angle of at least 90° as shown in  FIG. 3 . 
     Afterwards the electrode holder  14  is connected to the cell  9  by guiding a guiding part  28  of the holder  14  in said respective guiding rails  13  of the funnel tube  11  of the cell  9  until it contacts the upper edge of the stopping wall  27 . 
       FIGS. 4 to 6  illustrate an assembled cartridge device  20  in a sectional side view, a top plan view and a perspective view according to a preferred first embodiment of the present invention. 
     A magnetic or paramagnetic stir bar  19  for stirring the blood sample is placed into the cup portion  10 . Then the whole device is packed into appropriate means for storage and shipping to the customer. 
     Before the analysis the user takes the cartridge device  20  out of the packaging and places it into an appropriate receptacle of the analyser. The receptacle is preferably heated to 37° to ensure that the analysis takes place under standardized and physiological temperature conditions. 
     The filling amount of the blood sample is preferably large enough to ensure that the ends of the electrode wires projecting from the face side of the long part  15   a  of the body  15  are completely covered. 
     Appropriate means are placed under the receptacle that induce a stirring motion of the stir bar  19 . The preferred means for stirring the sample is the use of electromagnets that are alternately turned on and off and therefore induce a rotation of the stir bar  19 . The stir bar  19  can comprise a polytetrafluoroethylene (e.g., Teflon) coated stir bar, steel or siliconized steel. Siliconized stainless steel is the preferred material, as it is less expensive than polytetrafluoroethylene (e.g., Teflon) coated stir bars. Non-coated stir bars can alter the platelet activation due to the contact and adhesion of platelets to the thrombogenic steel material. Also permanent magnets that are rotated by adequate means or other means for inducing rotation of the sample (such as ultrasound, orbital movements of the cup) can be applied as obvious to a person skilled in the art. 
     The user then connects a plug  22 , preferably a standard RJ12 plug, to the jack  18  as illustrated in  FIG. 7 , which shows a sectional view of an assembled cartridge device with a connected plug  22  according to a preferred embodiment of the present invention. The plug  22  is preferably a standard plug, which comprises particularly a conductive element  22  contacting the contact regions  16   b ,  24   b , i.e. the plug portions  21   a  and  21   b  of the electrode wire pairs  16  and  24 . The plug  22  is connected to an analyser (not shown) by means of a connection cable  23  and allows the signal of the analysis to be transferred from the cartridge device  20  to the analyser. 
     During the analysis the analyser continuously records the impedance change on both sensor units  17   a  and  17   b . Under optimal conditions the impedance change of both sensor units  17   a  and  17   b  will be identical or nearly identical. However when due to material variations, dirt, damage of the wires  16 ,  24  during transport or other errors one sensor unit is defect, a strong variation is determined between the two sensor units  17   a  and  17   b  and the measurement is discarded. An automatic algorithm in the software distinguishes acceptable from non-acceptable variation between the results of the sensor units  17   a  and  17   b  and automatically stops the analysis if required. When an acceptable variation is recorded, for example the mean value of the two determinations is reported to the user. This leads to significantly improved precision. The rate of wrong diagnosis is significantly decreased compared to single analysis units. 
     After the analysis the user disconnects the plug  22  from the cartridge device  20  and discards it. 
     Linear Triple Cartridge Device 
     A second preferred embodiment of the cartridge device  20  is shown in  FIG. 8 , which is a perspective view. The cell  9  according to the second embodiment is identical to the previous embodiment and therefore it is referred to the above evaluations. 
     However contrary to the first embodiment the electrode holder  14  according to the second embodiment comprises three sensor units  17   a ,  17   b ,  17   c , which are linearly arranged to each other. Each sensor unit consists of a pair of electrode wires  16 ,  24  and  25 , respectively. Due to the linear arrangement the electrode wire pair  24  is in the middle of the two remaining electrode wire pairs  16  and  25  and is exposed to a significantly lower blood flow when compared to the outer electrode wire pairs  16  and  25 . Hence according to the second embodiment it is possible to compare platelet adhesion and aggregation under varying flow conditions. It is obvious to a person skilled in the art that by variations of the number and geometric arrangement of the test units the flow conditions under which the platelets are analysed can be varied and also differentiated software algorithms can be applied. 
     Linear Quadruple Cartridge Device 
     According to a third preferred embodiment as shown in a perspective view in  FIG. 9 , the cell  9  is identical to the previous embodiments and therefore it is referred to the above evaluations. 
     However the electrode holder  14  according to the third embodiment comprises four sensor units  17   a ,  17   b ,  17   c ,  17   d . These sensor units are linearly arranged to each other. Each sensor unit  17   a ,  17   b ,  17   c  and  17   d  consists of a pair of electrode wires  16 ,  24 ,  25  and  26 , respectively. Due to the linear arrangement the middle two electrode wire pairs  24  and  25  are exposed to a lower blood flow compared to the outer electrode wires  16  and  26 . Hence according to the third embodiment it is possible to make a double-determination of the platelet aggregation under low and high blood flow conditions, referring to the flow conditions of blood in vessels with small and large diameter. 
     Circular Triple Cartridge Device 
       FIG. 10  illustrates a top plan sectional view of a cartridge device  20  according to a fourth embodiment of the present invention. The cell  9  is identical to the aforementioned embodiments and therefore it is referred to the above evaluations. 
     However the electrode holder  14  is formed to allow a circular arrangement ( FIG. 8 ) of several sensor units. According to the present arrangement shown in  FIG. 10  three sensor units  17   a ,  17   b  and  17   c  are arranged at the same radial location in the blood sample, i.e. in the receiving portion  10 , and exposed to the identical or nearly identical blood flow, which allows to directly compare the signals. 
     According to the fourth embodiment the analyser independently records the changes in conductivity between the wire pairs of the three sensor units  17   a ,  17   b  and  17   c . Thus three independent results are obtained. For example the mean or median value is reported to the user, resulting in an enhanced precision of the analysis when compared to the prior art and to inventive embodiments containing only two independent sensor units. 
     Surface-Coated Punched Electrodes 
     According to another preferred embodiment a cartridge device is provided, wherein the electrodes are formed by punched metal (as shown in  FIG. 11A ), especially by using a highly electrically conductive material such as copper or copper alloys. The surface is coated using a precious material with a high conductivity, such as silver, gold or platinum. This allows to correct the aforementioned poor surface quality of the punched material by the coating with the precious material. In addition the high conductivity of the used metal material (e.g. copper) improves the signal of the analysis by the higher conductivity reduction when the electrode surface is coated by the activated platelets. 
     Multiple Punched Electrodes 
     According to another preferred embodiment a cartridge device is provided, wherein at least three electrodes made of punched metal are in contact with the blood. The shape of the electrodes can be similar to the shape of the electrodes shown in  FIG. 11A . At least two separate measurements of conductivity are parallel performed. In case three electrodes are used, the conductivity between electrode  1  and electrode  2  is measured as signal  1 . The conductivity between electrode  2  and  3  is used as signal  2 . And the conductivity between electrode  3  and electrode  1  is used as signal  3 . In case the variation of a determined conductivity signal is higher than an acceptable threshold (preferably 20%) of the mean value of the three signals, this signal is rejected and only the two other signals are analysed. In case also the conductivity of the remaining two signals shows a variation being too high, the whole analysis is discarded and the user is instructed to repeat the analysis. In case the signal variation is acceptable, the mean value or median value of the signals is reported to the user. This allows a significantly reduced probability of wrong determinations and also a reduced variation of the results. 
     Hence the present invention provides a cartridge device which solves several problems. On the one hand the limited reproducibility of previous methods according to the prior art for the measurement of aggregation in whole blood is eliminated. On the other hand a cartridge is provided which is accurate and so economical to produce that it can be discarded after each test. 
     Another advantage of the present invention is due to the fact that a cartridge which can be manufactured ready to use is provided, so that the application of the technique is simple and contains only few user-related sources of error and variation. 
     While the forgoing description and examples set forth specific illustrations of the practice of the present invention, those illustrations are intended to be representative only. Thus it will be apparent that numerous modifications and variations upon the designs and processes particularly described herein may be resorted to by those skilled in the art within the scope of the appended claims. 
     For example instead of the electrode wire pairs of the first to third embodiments several single electrodes or electrode wires can be used to receive several independent and separate measurement results. 
     It is obvious to a person skilled in the art that the wire sections  16   b ,  24   b ,  25   b  and  26   b  can serve as a plug or a jack in order to allow a connection to an analyser. 
     REFERENCE NUMERAL LIST 
     
         
           1  insulator 
           2  electrode 
           3  connection tap 
           4  tip 
           5  shaft 
           6  cuvette 
           7  clip 
           8  stir bar 
           9  cell 
           10  receiving portion/cup portion 
           11  tunnel tube 
           12  jack portion 
           13  guiding rail 
           14  electrode holder 
           15  plastic body 
           15   a  long part of body 
           15   b  short part of body 
           16  first electrode wire pair 
           16   a  sensor portion 
           16   b  connection portion 
           17   a  first sensor unit 
           17   b  second sensor unit 
           17   c  third sensor unit 
           17   d  fourth sensor unit 
           18  jack 
           19  stir bar 
           20  cartridge device 
           21   a  plug portion 
           21   b  plug portion 
           21   c  plug portion 
           21   d  plug portion 
           22  plug 
           23  connection cable 
           24  second electrode wire pair 
           24   a  sensor portion 
           24   b  connection portion 
           25  third electrode wire pair 
           25   a  sensor portion 
           25   b  connection portion 
           26  fourth electrode wire pair 
           26   a  sensor portion 
           26   b  connection portion 
           27  stopping wall 
           28  guiding part