Prolonged incubation improves the micro-scale in-vitro test for drug sensitivity of Plasmodium falciparum

In the standard micro-scale in-vitro test for detecting resistance of Plasmodium falciparum to chloroquine, isolates are incubated for 24-28 h in medium that does not contain serum. The effect of prolonging incubation to 48 h and of adding human AB serum to the culture medium of P falciparum from Irian Jaya, Indonesia was studied. Addition of serum was not associated with any advantages or disadvantages. Production of schizonts was increased when incubation was prolonged. This facilitated reading of the malaria smears and interpretation of the results. The percentage of successful tests (greater than or equal to 20 schizonts/200 asexual parasites in control wells) increased from 31% at 28 h incubation to 94% at 48 h. Among the 7 (44%) isolates which were chloroquine-resistant in vivo, only 1 met the criteria for a positive test at 28 h incubation, but at 48 h all 7 did. To achieve optimum results from the micro-scale in-vitro test in Jayapura, Irian Jaya, isolates should be incubated for 48 h.