An evaluation of the Coulter thrombocounter in counting platelets before and after contact with foreign surfaces and its use in tests of platelet retention.

The counting of platelets by electronic methods has become widely accepted by laboratories and the methods is considered to give better reproducibility than the conventional microscope count. Prompted by an impression that prior contact of whole blood with glass beads or cellulose-based dialysis membranes resulted in platelet counts showing a poor correlation between the electronic and microscopic methods of counting, an evaluation of the Coulter Thrombocounter was carried out with regard to its ability in counting platelets before and after contact with such foreign surfaces in two tests for the measurement of platelet retention. The effect of citrate and heparin anticoagulation with and without the addition of EDTA on the efficiency of the Thrombocounter was also studied. The results indicate that an excellent correlation is obtained between microscopic and electronically estimated platelet counts before foreign surface contact when EDTA is included in the anticoagulation, and that the Thrombocounter gives good reproducibility over a wide range of platelet counts. After contact with the foreign surfaces, the Thrombocounter significantly underestimated the platelet counts and thus overestimated the platelet retention values. Using the membrane test cell system with citrate plus EDTA anticoagulation, platelet retention results estimated by Thrombocounter platelet counts were significantly higher than those estimated by microscopic platelet counts but there was a good linear correlation between the two sets of results. Thus, the Thrombocounter may be used with this method. However, using the glass-bead column technique and the same anticoagulation, although the platelet retention results gave the same mean value with both methods of platelet counting, there was no correlation between the two sets of results indicating that microscopy alone should be used with this method. The other anticoagulant regimens rendered the Thrombocount inefficient for use in these tests of platelet retention.