Chloroquine resistant Plasmodium falciparum: effect of rabbit serum and incubation time on the in vitro (microtechnique) prediction of in vivo resistance.

A study of chloroquine resistance of 54 isolates of Plasmodium falciparum is reported. Sixty-four percent of the isolates tested produced schizonts in vitro (micro-technique), whereas the remaining 36 percent did not. The accuracy of the in vitro test to predict in vivo resistance was increased when the primary parasite isolates were cultured in the presence of rabbit serum and when the cultures were allowed to incubate for more than 48 hours. Thirteen isolates of P. falciparum that showed in vitro resistance were confirmed in vivo resistant. Eleven of these cases were identified as R-I and two as R-II. Only one case of in vivo resistance (R-II) was observed among the 19 isolates that failed to produce schizonts in vitro.