Detection of Dirofilaria immitis microfilariae in peripheral blood. A quantitative comparison of the efficiency and sensitivity of four techniques.

Four techniques for the detection and quantification of Dirofilaria immitis microfilariae in peripheral blood were compared, namely the conventional thick smear, thick films prepared by cytocentrifugation, filtration of blood through polycarbonate membranes, and density gradient centrifugation followed by membrane filtration. The efficiency of the methods and their sensitivity was assessed by determining quantitatively the recovery of known numbers of microfilariae from defined volumes of blood. Polycarbonate membrane filtration either alone or in combination with density gradient centrifugation showed an efficiency of 0.9 and 0.8, respectively, and reached 100% sensitivity with microfilariae densities of greater than or equal to 10 microfilariae per ml of blood. Conventional thick smears and cytocentrifugation were of considerably lower efficiency (0.02 and 0.03, respectively) and reached 100% sensitivity with more than 200 microfilariae per ml.