A side-by-side evaluation of four platelet-counting instruments.

The performances of four instruments for counting platelets were evaluated in a side-by-side study: the Haema-Count MK-4/HC, an electronic impedance instrument that counts platelets in platelet-rich plasma; the Ultra-Flo 100, and the Coulter Counter Model S-Plus, electronic impedance instruments that count platelets in the presence of intact erythrocytes; and the AutoCounter, an optical instrument that counts platelets in the presence of lysed erythrocytes. The Ultra-Flo 100 and the S-Plus showed the best within-run precision, and all four instruments were considerably more precise than manual platelet counting, especially at low levels of platelet count. The four instruments were all linear in the ranges tested (5 to 650 x 10(9)/or greater), and sample carry-over was less than 0.7% for each. A noteworthy finding was that the erythrocyte concentration of the blood samples affected the displayed platelet count of the S-Plus and, to a lesser extent, that of the AutoCounter, in a predictable way, whereas it did not greatly affect the displayed count of the Ultra-Flo 100. In addition to differences in quality of performances, the four instruments differed considerably in speed and ease of operation and in cost.