title: Thick blood film examination for Plasmodium falciparum malaria has reduced sensitivity and underestimates parasite density.
abstract: 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.
authors.affiliation: Centre for Clinical Vaccinology and Tropical Medicine, University of Oxford, Oxford, OX3 7LJ, UK. pbejon@well.ox.ac.uk
authors.forename: Philip
authors.initials: P
authors.lastname: Bejon
authors.forename: Laura
authors.initials: L
authors.lastname: Andrews
authors.forename: Angela
authors.initials: A
authors.lastname: Hunt-Cooke
authors.forename: Frances
authors.initials: F
authors.lastname: Sanderson
authors.forename: Sarah C
authors.initials: SC
authors.lastname: Gilbert
authors.forename: Adrian V S
authors.initials: AV
authors.lastname: Hill
mesh_terms: 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