Clinical Evaluation of a Loop-Mediated Amplification Kit for Diagnosis of Imported Malaria

(See the major article by Hopkins et al on pages 645–57.) Since the 1880s, the standard diagnostic test for malaria has been microscopic examination of peripheral blood smears [1–3]. The development of rapid antigen-detection tests (RDTs) in the early 1990s has improved diagnosis of Plasmodium infection in malaria-endemic countries and targeting of treatment [4, 5]. In countries where malaria is not endemic, where imported malaria cases in travelers can be mistaken for nonspecific viral illness [6, 7], causing delay in diagnosis and in some cases progression to severe illness and death [8–11], RDTs have improved the capacity for rapid malaria diagnosis by nonspecialist health workers [12–15]. While RDTs and expert microscopy are considered adequate for case management in malaria-endemic populations [16], there is growing interest in “improvements in point-of-care tests for case management, and the development of new tests capable of identifying very low parasite densities in asymptomatic individuals in field settings for mass screening and treatment” [17]. Polymerase chain reaction (PCR), in its most sensitive form, has a limit of detection as low as 50 parasites per milliliter of peripheral blood [18, 19], although results take 10–16 hours, whereas expert microscopy is less sensitive but routinely produces results in 60 minutes. Real-time quantitative PCR (qPCR) can reliably detect parasite DNA within 3–5 hours of sample receipt but is also less sensitive than nested PCR [20]. Recent studies show that loop-mediated amplification (LAMP) assays for malaria, which deploy isothermal molecular amplification in a closed system with visual readout, can deliver PCR-level diagnostic accuracy in a little more than 1 hour, with lower laboratory capacity requirements [21–23]. To date, malaria LAMP has not been available in a format suited to routine diagnosis in the clinic. A clinically validated, CE-marked assay would be an attractive alternative to RDT and microscopy in settings where malaria is or is not endemic. A malaria LAMP kit has been developed as a point-of-care diagnostic test, and after CE marking it was released commercially in mid-2012. This kit comprises a disposable extraction device and tubes containing vacuum-dried and temperature-stable reaction components. We investigated the diagnostic accuracy of this kit for case management in a study of sequential blood samples from suspected imported malaria cases received by a specialist parasitology laboratory in London, United Kingdom, over the first 7 months of 2011. Primary diagnosis was by expert microscopic examination of blood films. An established nested PCR assay was deployed as the reference standard [18]. Diagnostic accuracy of the new kit was superior to that of microscopy and similar to that of nested PCR but with the additional benefits of reduced assay time and ease of operation.