Abstract
BackgroundMalaria rapid diagnostic tests (RDTs) are kits that generally include 20 to 25 test strips or cassettes, but only a single buffer vial. In field settings, laboratory staff occasionally uses saline, distilled water (liquids for parenteral drugs dilution) or tap water as substitutes for the RDT kit's buffer to compensate for the loss of a diluent bottle. The present study assessed the effect of buffer substitution on the RDT results.MethodsTwenty-seven RDT brands were run with EDTA-blood samples of five malaria-free subjects, who were negative for rheumatoid factor and antinuclear antibodies. Saline, distilled water and tap water were used as substitute liquids. RDTs were also run with distilled water, without adding blood. Results were compared to those obtained with the RDT kit's buffer and Plasmodium positive samples.ResultsOnly eight cassettes (in four RDT brands) showed no control line and were considered invalid. Visible test lines occurred for at least one malaria-free sample and one of the substitutes in 20/27 (74%) RDT brands (saline: n = 16; distilled water: n = 17; and tap water: n = 20), and in 15 RDTs which were run with distilled water only. They occurred for all Plasmodium antigens and RDT formats (two-, three- and four-band RDTs). Clearance of the background of the strip was excellent except for saline. The aspects (colour, intensity and crispness) of the control and the false-positive test lines were similar to those obtained with the RDT kits' buffer and Plasmodium positive samples.ConclusionReplacement of the RDT kit's dedicated buffer by saline, distilled water and tap water can cause false-positive test results.
Highlights
Malaria rapid diagnostic tests (RDTs) are kits that generally include 20 to 25 test strips or cassettes, but only a single buffer vial
Eleven (40.7%) of them had CE-mark compliance, one was approved by the U.S Food and Drug Administration, 22 (81.5%) were included in the World Health Organization (WHO) list of RDTs adequate evidence of good manufacturing practice (GMP), 22 (81.5%) were evaluated by the WHO/FIND [11,12], and 9 (30%) are included in the list of malaria RDTs eligible for procurement by WHO [16]
Results for the RDTs when run with samples of malaria free subjects and substitute liquids When run with their kit’s buffer, none of the RDT brands tested positive with any of the samples of the five malaria free subjects
Summary
Malaria rapid diagnostic tests (RDTs) are kits that generally include 20 to 25 test strips or cassettes, but only a single buffer vial. Two-band RDTs are mostly designed to detect Plasmodium falciparum; they display a control line and a test line, which targets either histidine-rich protein-2 (HRP-2) or P. falciparum-specific parasite lactate dehydrogenase (Pf-pLDH). Three- and four-band RDTs display a control line and two or three test lines, one targeting P. falciparum specific antigen, another line targeting antigens common to the four species, such as pan-Plasmodium-specific lactate parasite dehydrogenase (pan-pLDH) or aldolase, and, in case of the four band. During the last few years, RDTs have technically improved and so-called one-step tests have replaced the older multistep tests. Despite their apparent simplicity, they are not completely fail-proof [3,4]
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