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.