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Template copy number and the sensitivity of quantitative PCR for Plasmodium falciparum in asymptomatic individuals

Authors
  • Thompson, Trevor A.1, 2
  • Touré, Mahamoudou B.1, 3
  • Sanogo, Daouda1, 3
  • Shaffer, Jeffrey G.1, 2
  • Doumbia, Seydou O.1, 3
  • Krogstad, Donald J.1, 2
  • 1 West African International Center of Excellence for Malaria Research, Bamako, Mali , Bamako (Mali)
  • 2 Tulane School of Public Health and Tropical Medicine, 1430 Tulane Avenue, #8317, J.B. Johnston Building, Room 510, New Orleans, LA, 70112-2699, USA , New Orleans (United States)
  • 3 University of the Sciences, Techniques and Technologies of Bamako, Bamako, Mali , Bamako (Mali)
Type
Published Article
Journal
Malaria Journal
Publisher
Springer (Biomed Central Ltd.)
Publication Date
Aug 18, 2020
Volume
19
Issue
1
Identifiers
DOI: 10.1186/s12936-020-03365-8
Source
Springer Nature
Keywords
License
Green

Abstract

BackgroundThe identification of asymptomatic individuals with Plasmodium falciparum infection is difficult because they do not seek medical treatment and often have too few asexual parasites detectable using microscopy or rapid diagnostic tests (≤ 200 parasites per μl). Quantitative PCR (qPCR) may provide greater sensitivity and permits estimation of the initial template DNA concentration. This study examined the hypothesis that qPCR assays using templates with higher copy numbers may be more sensitive for P. falciparum than assays based on templates with lower copy numbers.MethodsTo test this hypothesis, ten qPCR assays for DNA sequences with template copy numbers from 1 to 160 were compared using parasite DNA standards (n = 2) and smear-positive filter paper blots from asymptomatic smear-positive subjects (n = 96).ResultsBased on the testing of P. falciparum parasite DNA standards and filter paper blots, cycle threshold values decreased as the concentrations of template DNA and template copy numbers increased (p < 0.001). Likewise, the analytical and clinical sensitivities of qPCR assays for P. falciparum DNA (based on DNA standards and filter paper blots, respectively) increased with template copy number. Despite the gains in clinical sensitivity from increased template copy numbers, qPCR assays failed to detect more than half of the filter paper blots with low parasite densities (≤ 200 asexual parasites per μl).ConclusionsThese results confirm the hypothesis that the sensitivity of qPCR for P. falciparum in the blood of individuals with asymptomatic infection increases with template copy number. However, because even the most sensitive qPCR assays (with template copy numbers from 32 to 160) detected fewer than 50% of infections with ≤ 200 asexual parasites per μl, the sensitivity of qPCR must be increased further to identify all smear-positive, asymptomatic individuals in order to interrupt transmission.

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