Impedance aggregometers, such as those manufactured by Chrono-log, determine platelet aggregation in whole blood. This methodology does not require the preparation of platelet-rich plasma since whole blood is used in the test. The instrument measures the change in resistance (impedance) between two electrodes immersed in the whole-blood sample as platelets adhere to the electrode wires and aggregate in response to agonists added to the sample.
A two-wire electrode is inserted in the sample cuvette containing the patient's whole blood sample. A small voltage is applied across the electrode, and the impedance caused by the platelets coating the wires is measured.
Before an agonist is added, the interactions between the platelets and the electrodes stabilize and the impedance between the two electrode wires becomes constant, creating a baseline reading. When an agonist is added, platelets in the specimen are activated and begin to aggregate. The platelet coating on the immersed wires thickens over the next several minutes, increasing the electrical impedance between the electrode wires. This change in impedance is directly proportional to the extent of platelet aggregation and is indicated on the digital display after a specified time. Platelet aggregation is reported as the change in electrical impedance and is expressed in ohms.
For aspirin response testing, arachidonic acid is used as an agonist at concentrations between 0.5 mmol/L to 1.0 mmol/L. A decrease in platelet aggregation for patients receiving aspirin therapy is considered a normal response. Platelet aggregation that is not halted in the presence of arachidonic acid may indicate a decreased response to antiplatelet medication.
*Image adapted from Rotem.de