Since 1990, our laboratory has prepared a set of 8 fresh whole blood samples for use in a countrywide quality assessment (QA) programme. The samples are intended as external controls for haemocytometry analysers. These samples are of 8 different haematocrit levels and each one is prepared from a single donor. About 210 laboratories participate in this QA programme. From the start of this programme large interlaboratory variations in platelet counts were encountered in some of the samples. This variability was much higher than would be expected and was independent of the platelet count of the samples. The main cause was thought to be formation of platelet aggregates. The aim of the present study was to find a parameter that can predict a high interlaboratory variation in the QA programme. Therefore we investigated the initial platelet activation status in the donors and the activation status of platelets in the prepared QA blood. As a marker for platelet activation P‐selectin expression on the platelets was measured using flow cytometry. During 5 rounds of the QA programme we found a good correlation of r = 0.53 (p < 0.001) between P‐selectin expression on platelets in the reconstituted QA blood and the interlaboratory platelet count variability. We conclude that P‐selectin expression in the prepared QA blood is an important parameter to exclude samples that lead to high CVs of platelet counts in the QA programme.