Laboratory Methods Blood will be drawn into sodium citrate for all tests. Three agonists will be used in the whole blood aggregometry tests: thrombin, collagen and ADP(3). The output of this instrument (Chronolog Whole Blood Aggregometer, model 504) is in ohms, with higher impedance values indicating greater platelet aggregation. Because this test is conducted in whole blood (i.e., erythrocytes present), it is thought to be the most physiologically-relevant method for testing platelet function.