title: Prozone in malaria rapid diagnostics tests: how many cases are missed?
abstract: BACKGROUND
  Prozone means false-negative or false-low results in antigen-antibody reactions, due to an excess of either antigen or antibody. The present study prospectively assessed its frequency for malaria rapid diagnostic tests (RDTs) and Plasmodium falciparum samples in an endemic field setting.
  
  
  METHODS
  From January to April 2010, blood samples with P. falciparum high parasitaemia (≥ 4% red blood cells infected) were obtained from patients presenting at the Provincial Hospital of Tete (Mozambique). Samples were tested undiluted and 10-fold diluted in saline with a panel of RDTs and results were scored for line intensity (no line visible, faint, weak, medium and strong). Prozone was defined as a sample which showed no visible test line or a faint or weak test line when tested undiluted, and a visible test line of higher intensity when tested 10-fold diluted, as observed by two blinded observers and upon duplicate testing.
  
  
  RESULTS
  A total of 873/7,543 (11.6%) samples showed P. falciparum, 92 (10.5%) had high parasitaemia and 76 were available for prozone testing. None of the two Pf-pLDH RDTs, but all six HRP-2 RDTs showed prozone, at frequencies between 6.7% and 38.2%. Negative and faint HRP-2 lines accounted for four (3.8%) and 15 (14.4%) of the 104 prozone results in two RDT brands. For the most affected brand, the proportions of prozone with no visible or faint HRP-2 lines were 10.9% (CI: 5.34-19.08), 1.2% (CI: 0.55-2.10) and 0.1% (CI: 0.06-0.24) among samples with high parasitaemia, all positive samples and all submitted samples respectively. Prozone occurred mainly, but not exclusively, among young children.
  
  
  CONCLUSIONS
  Prozone occurs at different frequency and intensity in HRP-2 RDTs and may decrease diagnostic accuracy in the most affected RDTs.
authors.affiliation: Department of Clinical Sciences, Unit of Tropical Laboratory Medicine, Institute of Tropical Medicine, Nationalestraat 155, B 2000 Antwerp, Belgium. pgillet@itg.be
authors.forename: Philippe
authors.initials: P
authors.lastname: Gillet
authors.forename: Annelies
authors.initials: A
authors.lastname: Scheirlinck
authors.forename: Jocelijn
authors.initials: J
authors.lastname: Stokx
authors.forename: Anja
authors.initials: A
authors.lastname: De Weggheleire
authors.forename: Hélder S
authors.initials: HS
authors.lastname: Chaúque
authors.forename: Oreana D J V
authors.initials: OD
authors.lastname: Canhanga
authors.forename: Benvindo T
authors.initials: BT
authors.lastname: Tadeu
authors.forename: Carla D  D
authors.initials: CD
authors.lastname: Mosse
authors.forename: Armindo
authors.initials: A
authors.lastname: Tiago
authors.forename: Samuel
authors.initials: S
authors.lastname: Mabunda
authors.forename: Cathrien
authors.initials: C
authors.lastname: Bruggeman
authors.forename: Emmanuel
authors.initials: E
authors.lastname: Bottieau
authors.forename: Jan
authors.initials: J
authors.lastname: Jacobs
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D019411:Clinical Laboratory Techniques / Q000379:methods*; D003951:Diagnostic Errors / Q000706:statistics & numerical data*; D006801:Humans; D007118:Immunoassay / Q000379:methods; D007223:Infant; D016778:Malaria, Falciparum / Q000175:diagnosis*; D008875:Middle Aged; D009073:Mozambique; D010963:Plasmodium falciparum / Q000276:immunology*; D055815:Young Adult