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BinaxNOW Malaria Rapid Diagnostic Test
Abstract & Commentary
By Lin H. Chen, MD, Assistant Clinical Professor, Harvard Medical School; Director Travel Resource Center, Mount Auburn Hospital, Cambridge
Dr. Lin H. Chen reports no financial relationship relevant to this field of study. This article originally appeared in the October 2007 issue of Travel Medicine Advisor. It was edited by Frank Bia, MD, MPH, and peer reviewed by Phillip Fischer, MD, DTM&H. Dr. Bia is Professor of Medicine and Laboratory Medicine; Co-Director, Tropical Medicine and International Travelers' Clinic, Yale University School of Medicine, and Dr. Fischer is Professor of Pediatrics, Department of Pediatric and Adolescent Medicine, Mayo Clinic, Rochester, MN. Dr. Bia is a consultant for Pfizer and Sanofi Pasteur, and receives funds from Johnson & Johnson, and Dr. Fischer reports no financial relationships relevant to this field of study.
Synopsis: The US FDA approved the BinaxNow Malaria Test, making it the first rapid diagnostic test for malaria available in the United States.
The diagnosis of malaria has traditionally relied upon microscopy. However, microscopic diagnosis is labor intensive and somewhat subjective, and assurance of quality standards can be difficult at best. The BinaxNow Malaria Test is an immunochromatographic assay that uses monoclonal antibodies to detect malaria antigens, including both the aldolase enzyme present in all Plasmodium species and, more specifically, the histidine-rich protein 2 (HRP-2) of Plasmodium falciparum. The test requires whole blood obtained by either venipuncture or fingerstick, and must be performed immediately following sample collection, although blood can be refrigerated for up to 3 days. The test kit contains a dipstick card to which the blood sample is applied (about 15µl), followed by the addition of a reagent, and it is read after 15 minutes. The appearance of lines indicates the adequacy of test, presence of P. falciparum, or presence of other plasmodial species.
The manufacturer reports overall test sensitivity to be 99.7% for P. falciparum and 93.5% for P. vivax, and specificity to be 94.2% for P. falciparum and 99.8% for P. vivax. In an endemic population, a comparison of the BinaxNow Malaria Test against microscopy has demonstrated sensitivities of 99%-100%: 99.7% for P. falciparum and 81-82% for P. vivax, and specificities of 90%-95% for P. falciparum and 99%-100% for P. vivax. In a non-endemic population, the test demonstrated a 100% sensitivity and specificity for P .falciparum and 99% sensitivity and specificity for P. vivax. The test shows cross-reactivity with rheumatoid factor and human anti-mouse antibody (HAMA).
Rapid diagnostic tests (RDTs) for malaria have been advocated for use as field tests,1 and some tests have been available for use in hospitals and laboratories in Europe. A number of RDTs based on assays for HRP-2 and/or parasite LDH have been evaluated in the diagnosis of febrile returning travelers.2-6 One study that tested the NOW Malaria Test demonstrated sensitivity and specificity of 96.4% and 97%, respectively, in returned travelers in France.6 Other investigators found that the test had better sensitivity when used in a microbiology laboratory than at bedside.7 A study on non-European immigrants and travelers found that the NOW Malaria rapid test was both sensitivity (100%) and specificity (100%) in detecting P. falciparum infections; however, it was less specific (93.1%) and sensitive (90.7%) for identifying the other Plasmodium species.8
One significant drawback of the RDTs is that the parasite HRP-2 persists in the circulation for some time.9 Following effective treatment for malaria, a significant proportion of RDTs were false-positive at day 14 (98.2%), day 21 (94.6%), day 28 (92.0%), and day 35 (73.5%).10 The persistence of these positive results would limit its use in evaluating patients who have had recent malaria infections. Some test results cannot be interpreted because of interfering antibodies, which are known to occur in the presence of rheumatoid arthritis and human anti-mouse antibodies (HAMA), and possibly with other, yet unidentified, inhibitors.11 Additionally, there is concern that RDTs may need to be stored in cool conditions, and storage in the tropics may affect test reliability.12 The manufacturer states that BinaxNow Malaria Tests stored at 2°-37° Celsius remain stable until their date of expiration.
Some investigators have also evaluated RDTs for self-diagnosis of malaria by travelers.13-15 One study found 20.6% of tests were incorrectly interpreted with 14.1% false negative interpretations.13 Other investigators found high false-negative interpretations on samples with high parasitemia (> 2% parasitemia) tested with two different RDTs (MalaQuick and ParaSight), whereas 96.8% and 33.8% of tests, respectively, were correct interpretations.14 Both tests were associated with high levels of false-negative interpretations. One study had promising data to support self-diagnosis, but still showed 9% failure to perform test accurately.15 Such tests are not currently indicated for use by travelers. For now, RDTs can be a useful diagnostic tool for field work, as well as in practices that do not have reliable microscopy.