To adjust or not to adjust the platelet count in light transmission aggregometry in patients receiving dual aspirin/clopidogrel treatment.

We evaluated whether the results of light transmittance aggregometry (LTA) differ when "native" platelet-rich plasma (PRP) or adjusted (to a standard platelet count of 250.000/microL) PRP is used in patients on dual antiplatelet therapy with aspirin and clopidogrel. LTA has been performed on the blood of 142 stable angina pectoris patients who were adequately pretreated with aspirin and clopidogrel. Platelet aggregation was significant higher in native PRP as compared to platelet count adjusted PRP (P<0.0001) for all four concentrations of adenosine-5'-diphosphate (ADP) (2, 5, 10 and 20 micromol/L). The interindividual variability was significantly higher in platelet count adjusted PRP as compared to native PRP when stimulated with 10 and 20 micromol/L of ADP. The absolute magnitude of aggregation in non-adjusted PRP is clearly dependent on platelet number. These observations are important since several studies have used empirically defined cut-off levels to segregate non-responders from responders to clopidogrel therapy.