Thick blood film examination for Plasmodium falciparum malaria has reduced sensitivity and underestimates parasite density.
BACKGROUND
Thick blood films are routinely used to diagnose Plasmodium falciparum malaria. Here, they were used to diagnose volunteers exposed to experimental malaria challenge.


METHODS
The frequency with which blood films were positive at given parasite densities measured by PCR were analysed. The poisson distribution was used to calculate the theoretical likelihood of diagnosis. Further in vitro studies used serial dilutions to prepare thick films from malaria cultures at known parasitaemia.


RESULTS
Even in expert hands, thick blood films were considerably less sensitive than might have been expected from the parasite numbers measured by quantitative PCR. In vitro work showed that thick films prepared from malaria cultures at known parasitaemia consistently underestimated parasite densities.


CONCLUSIONS
It appears large numbers of parasites are lost during staining. This limits their sensitivity, and leads to erroneous estimates of parasite density.
Bejon
Philip
P
Centre for Clinical Vaccinology and Tropical Medicine, University of Oxford, Oxford, OX3 7LJ, UK. pbejon@well.ox.ac.uk
Andrews
Laura
L
Hunt-Cooke
Angela
A
Sanderson
Frances
F
Gilbert
Sarah C
SC
Hill
Adrian V S
AV
D000818:Animals; D001769:Blood / Q000469:parasitology*; D006801:Humans; D016778:Malaria, Falciparum / Q000097:blood / Q000175:diagnosis* / Q000469:parasitology*; D008853:Microscopy; D010963:Plasmodium falciparum / Q000302:isolation & purification*; D012680:Sensitivity and Specificity