Enumeration of platelets using a model B counter.

an engraved counting chamber and a light microscope, have many possibilities of error. The introduction of phase-contrast microscopy by Brecher, Schneiderman, and Cronkite (1953) has greatly facilitated the counting of platelets and improved the accuracy of the results. However, despite this improvement in accuracy the visual method is lacking in one basic essential, namely, speed in producing results. Because of this, a number of workers, in recent years, have sought for a more accurate and less laborious method for the enumeration of blood platelets. As the electronic counting of both erythrocytes and leucocytes has become established practice in many laboratories, similar methods have been investigated for counting blood platelets. Using a Coulter counter, Eggleton and Sharp in 1963 described an accurate method for counting platelets in dilutions of platelet-rich plasma. Similarly Eastham (1963 and 1965), using the same instrument, devised a method for counting the platelets in whole blood. In this technique dilutions of whole blood were made in saline and an appropriate platelet suspension was obtained by differential centrifugation. Aliquots of the supematant were then counted at two different threshold settings in order to distinguish between red cells and platelets. After a correction for coincidence had been made, the platelet count was obtained by subtraction. In 1965, Bull, Schneiderman, and Brecher introduced a different method designed to eliminate the errors resulting from loss of platelets during centrifugation. The sample of whole blood was allowed to undergo sedimentation in a narrow-bore plastic tube for 10 to 15 minutes. An aliquot of the resulting supematant platelet-rich plasma was then counted electronically. Because platelet-poor plasma was trapped within the red cell mass during sedimentation two corrections were made, one for the haematocrit and one for the excess platelets in the supernatant. This technique avoided some of the errors of the previous method but preparation was time consuming. Williamson in 1966 further modified Eastham's original method by altering the dilutions and centrifugation speeds but retained the differential counting method for separation of red cells from platelets. However, it has been found that this technique could be adapted for use with a model B Coulter counter, by using instrument settings designed to count only particles of platelet size, thereby enabling whole blood platelet counts to be read directly off the instrument.