Laboratory evaluation of the miniature direct-on-blood PCR nucleic acid lateral flow immunoassay (mini-dbPCR-NALFIA), a simplified molecular diagnostic test for Plasmodium

Abstract

Background: Point-of-care diagnosis of malaria is currently based on microscopy and rapid diagnostic tests. However, both techniques have their constraints, including poor sensitivity for low parasitaemias. Hence, more accurate diagnostic tests for field use and routine clinical settings are warranted. The miniature direct-on-blood PCR nucleic acid lateral flow immunoassay (mini-dbPCR-NALFIA) is an innovative, easy-to-use molecular assay for diagnosis of malaria in resource-limited settings. Unlike traditional molecular methods, mini-dbPCR-NALFIA does not require DNA extraction and makes use of a handheld, portable thermal cycler that can run on a solar-charged power pack. Result read-out is done using a rapid lateral flow strip enabling differentiation of Plasmodium falciparum and non-falciparum malaria infections. A laboratory evaluation was performed to assess the performance of the mini-dbPCR-NALFIA for diagnosis of pan-Plasmodium and P. falciparum infections in whole blood.

Methods: Diagnostic accuracy of the mini-dbPCR-NALFIA was determined by testing a set of Plasmodium-positive blood samples from returned travellers (n = 29), and Plasmodium-negative blood samples from travellers with suspected malaria (n = 23), the Dutch Blood Bank (n = 19) and intensive care patients at the Amsterdam University Medical Centers (n = 16). Alethia Malaria (LAMP) with microscopy for species differentiation were used as reference. Limit of detection for P. falciparum was determined by 23 measurements of a dilution series of a P. falciparum culture. A fixed sample set was tested three times by the same operator to evaluate the repeatability, and once by five different operators to assess the reproducibility.

Results: Overall sensitivity and specificity of the mini-dbPCR-NALFIA were 96.6% (95% CI, 82.2%-99.9%) and 98.3% (95% CI, 90.8%-100%). Limit of detection for P. falciparum was 10 parasites per microlitre of blood. The repeatability of the assay was 93.7% (95% CI, 89.5%-97.8%) and reproducibility was 84.6% (95% CI, 79.5%-89.6%).

Conclusions: Mini-dbPCR-NALFIA is a sensitive, specific and robust method for molecular diagnosis of Plasmodium infections in whole blood and differentiation of P. falciparum. Incorporation of a miniature thermal cycler makes the assay well-adapted to resource-limited settings. A phase-3 field trial is currently being conducted to evaluate the potential implementation of this tool in different malaria transmission areas.

Keywords: Laboratory evaluation; Malaria; Nucleic Acid Lateral Flow Immunoassay; PCR; Plasmodium; Simplified molecular diagnostics; miniPCR.

Fig. 1

Workflow of the mini-dbPCR-NALFIA

Fig. 2

Possible NALFIA results after dbPCR for pan-Plasmodium and P. falciparum. The strip has 3 potential lines that can appear: a pan-Plasmodium test line (1), a P. falciparum test line (2) and a flow control line (FC). A Positive pan-Plasmodium and P. falciparum test lines, thus a P. falciparum infection or a mixed infection including P. falciparum; B Pan-Plasmodium positive test line, but negative for the P. falciparum test line. This is a Plasmodium infection with a non-falciparum species (e.g. P. vivax); C Pan-Plasmodium negative, but a positive P. falciparum line. This result is also considered positive for P. falciparum; D The NALFIA has a flow control line but the pan-Plasmodium and P. falciparum test lines are absent. This is a Plasmodium-negative test result; E Pan-Plasmodium and P. falciparum negative, but also negative for the flow control line. This NALFIA test is invalid