Blood parasites: problems in diagnosis using automated differential instrumentation.

To examine potential problems inherent in using automated differential instruments, we have reviewed herein two cases where blood parasites, Plasmodium vivax and Plasmodium falciparum, were completely missed by use of this method. Diagnosis of these infections was made when blood was sent to the parasitology laboratory after having been missed prior to that time. The first problem involved the laboratory request slip; no indication was made concerning possible suspect organisms. Therefore, peripheral blood examinations were performed using automated equipment. The number of fields scanned by a technologist on these smears is quite low; thus failure to pick up a light parasitemia is almost guaranteed. In both cases, after diagnosis had been made on smears submitted to the parasitology division, all previous smears examined by the automated system were reviewed and found to be positive for parasites. Failure to make the diagnosis resulted in delayed therapy. Although these instruments are not designed to detect intracellular blood parasites, the inability of the automated systems to discriminate between uninfected red blood cells and those infected with parasites may pose serious diagnostic problems.