An explanation of the discrepancy between direct and indirect platelet counts.

By HARRY J. FITCH T HERE ARE t-iso standard nuethods for counting platelets, the direct and the indirect. For the direct nuethod the blood is accurately diluted with au anst-icoagulamut sohutiomu, usually in a hemocytomet-er pipet, and the platelets are enumsuei’ated ims a counting chanuber.1 For the indirect nuethod the blood is diluted amid a drop is placed on an ordinary microscope slide. It may then be spread amid dried3 or, as is more usual, a cos’ership is placed on the drop to make a wet preparat-ion.2 Platelets and red cells are then coumut-cd field by field and the ratio is established. The red cells are also counted directly in a counting chamber to deternsimse their concentrations in the blood. The platelet concentration is thems derised by referring the platelet-red cell ratio to this direct red cell counut. Whems platelets ins the sanue sample of blood are coumsted by these tsso methods the resuit-s are msot the same. A isormal count by the direct nuet-hod is about 250,000 per cu.mnuu. ; by the indirect method about 500,000. This discrepancy has beemu kmsosvn simsce the begimsning of the century.6 It has also been kmsosvns that si-hens the pt’oportioms of platelets to red cells is established in a counting chamber the rat-jo is about 1 :20, while betis-eeis a slide amid coverslip it. is about 1 : 10. When an imidirect count. is carried out its a counstimug chambers or on a dried smear3, the platelet count of nornsal humams blood is about 250,000, sihich is no differemst fronu that obtaimsed by the direct nsethod. The diffet’emuee between results usually obtailsed by the tsvo nucthods, si-hihe oftems a subject for speculatioms or debate, has not beems explained.7 The purpose of this repon’t is to demonstrate the reasoms for the differemuce ins these couist.s.