African serum interference in the determination of chloroquine sensitivity in Plasmodium falciparum.

Isolates of Plasmodium falciparum from villagers in central Sudan were tested for chloroquine and mefloquine sensitivity using the WHO microtechnique procedure and a modified 48-h in vitro test for drug resistance. No drug-resistant strains were noted. In the WHO procedure, in which parasites were cultivated in the presence of the patient's plasma, 72% of the isolates failed to mature to the schizont stage, but when infected erythrocytes were washed free of the patient's plasma and cultivated in pooled nonimmune serum only 28.8% of the isolates failed to develop to the schizont stage. In subsequent experiments, sera from P. falciparum-infected patients or from noninfected "immune" adults were used to supplement standard in vitro test plates which contained parasites of known chloroquine sensitivities. Sera from malaria-infected patients or from immune adults retarded parasite development in the presence or absence of drug. The effect of these humoral factors and the antimalarial drugs was additive. The replacement of the patient's plasma with nonimmune serum in drug sensitivity tests performed with African isolates is recommended.