Inoculum effect leads to overestimation of in vitro resistance for artemisinin derivatives and standard antimalarials: a Gambian field study

Artemisinin (QHS) and its derivatives are new antimalarials which are effec tive against Plasmodium falciparum parasites resistant to chloroquine (CQ). As these drugs are introduced it is imperative that resistance is monitore d. In this paper we demonstrate that the inoculum size used in in vitro tes ting influences the measured in vitro susceptibility to QHS and its derivat ive dihydroartemisinin (DHA) and to mefloquine (MEF) and CQ over the range of parasitaemias routinely used in testing with the WHO in vitro microtest. An increase in parasitaemia and/or haematocrit was accompanied by a decrea se in the measured sensitivity of 2 laboratory lines. In the context of a f ield study testing in vitro susceptibility of parasite isolates from patien ts with uncomplicated malaria in Fajara, The Gambia we demonstrate that fai lure to control for inoculum size significantly overestimates the level of resistance to QHS and DHA as well as MEF, halofantrine (HAL) and quinine (Q UIN). When controlling for the inoculum effect, cross-resistance mas observ ed between QHS, MEF and HAL suggesting the presence of a multidrug resistan ce-like mechanism. These studies underline the importance of inoculum size in in vitro susceptibility testing.