Abstract

Rapid diagnostic tests (RDTs) are the current complement to microscopy for ensuring prompt malaria treatment. However, non-estimation of the parasite density is one of the limits of these RDTs. This study aimed to estimate the parasite density based on the time of positivity of two RDTs, SD Bioline Malaria Ag-Pf/Pan and Paracheck™-Pf. A highly parasitized blood sample underwent successive dilutions. On each resulting dilution, a thick blood film and two RDTs were performed. The time of onset of the positivity of the diluted blood samples was recorded from each RDT. A linear regression model was determined to estimate a range of parasite density from the time of positivity. The model was then assessed using known parasites density of additional blood samples from symptomatic malaria patients. For each RDT, the curve representing the time of positivity showed three stages. The latency stage, before the test line of the RDT appears, lasted 2 min 40 s for SD Bioline and 3 min 14 s for Paracheck. The optimal stage during which the line of positivity appears lasted from 2 min 40 s to 6 min (SD Bioline), and from 3 min14 s to 7 min 2 s (Paracheck). A phase of pre-negativity spanned from 6 min to 14 min 20 s (SD Bioline) and 7 min 2 s to 14 min 40 s (Paracheck). Two linear regressions equations were drawn to link the mean time of positivity appearance to the mean parasite density (Dm), that is, Dm = - 119.5tm + 81943.3 (SD Bioline) and Dm = - 142.0tm + 99779.1 (Paracheck). Based on the statistical models, RDTs time of positivity ranged from 2 min 40 s to 11 min 25 s, and from 3 min 14 s to 11 min 33 s for SD Bioline and Paracheck respectively. The linear regression models obtained from the time of positivity of RDTs allows the prediction of malaria parasite density. There was no significant difference between the theoretical malaria parasite density and the one observed with microscopy.

Key words: Malaria parasite density, linear regression model, rapid diagnostic tests (RDTs), Côte d'Ivoire.