The Multiplate, a whole blood (WB) platelet function test, has shown promising results identifying patients on antiplatelet therapy at increased risk of rethrombosis. In the present study, the influence of low platelet count on platelet aggregation was analyzed and compared with aggregation results in an artificial matrix, platelet-rich plasma (PRP). Heparinized and citrated blood was diluted with autologous plasma to platelet concentrations 200 to 25 × 10(9)/L in WB samples (n = 10) and 200 to 100 × 10(9)/L in PRP samples (n = 7). The platelet aggregation was investigated by the ADP-, ASPI-, COL-, and TRAP-test. The WB responses decreased at platelet concentration of ≤100 × 10(9)/L (all P <.03), except for heparin-TRAP (50 × 10(9)/L, P = .008) and citrate-ASPI (150 × 10(9)/L, P = .03). In general, WB samples demonstrated higher aggregation than PRP samples at platelet concentrations 200 to 100 × 10(9)/L (P <.05). In conclusion, platelet concentration of