White blood cell counts by automated and manual methods with backlighting.

White blood cell (WBC) counts were compared in the presence of "backlighting" using the Coulter S-Plus Jr and a manual system. Platelet aggregates and large platelets in specimens anticoagulated with ethylenediamine tetraacetic acid (EDTA) cause interference and prompt the WBC count on the Coulter S-Plus Jr to backlight. This means that the background behind the count on the data terminal display lights up, alerting the operator to a questionable result. A total of 29 automated backlighted WBC counts of blood samples collected in sodium and potassium salts of EDTA tubes were compared with the results by hemocytometer method. Values for WBC count by both methods showed good agreement. Smear examination detected platelet clumps and large platelets in 66% of the Na 2EDTA tubes. Only low WBC counts prompted backlighting in K 3EDTA. tubes. Liquid K 3EDTA is a preferred anticoagulant for whole blood analysis because of its rapid solubility, eliminating clumping of platelets and thus backlighting.