Erythrocyte Fy antigen phenotyping helps differentiate so-called benign tertian malarias.

Isolated cases of malaria are increasing in frequency in nonendemic countries. Blood film examination remains a mainstay of diagnosis of these sporadic cases because immunologic and molecular methods are unavailable, expensive, and problematic. Two tertian malarial species, Plasmodium vivax and Plasmodium ovale, may appear to be similar morphologically. Plasmodium ovale infection is infrequent, and misdiagnosis of this species is common. Plasmodium vivax infection can be ruled out, however, if a patient's erythrocytes phenotype as Fy(a-b-), because these cells completely resist entry by the latter species.