Comparative evaluation of validity and cost-benefit analysis of rapid diagnostic test (RDT) kits in diagnosis of dengue infection using composite reference criteria: A cross-sectional study from south India.

BACKGROUND & OBJECTIVES: Rapid diagnostic test (RDT) kits are widely used in India for the diagnosis of dengue infection. It is important to evaluate the validity and reliability of these RDTs. The study was aimed to determine the sensitivity, specificity and predictive value of four commercially available RDTs [Panbio Dengue Duo cassette, Standard Diagnostics (SD) Bioline Dengue Duo, J. Mitra Dengue Day-1 test and Reckon Dengue IgG/IgM] against composite reference criteria (CRC), and compare the cost of the tests. METHODS: In this prospective observational study for diagnostic accuracy, we tested stored blood samples from 132 cases of dengue and 149 controls of other infections as classified based on CRC, with all the four RDTs. The CRC was based on the epidemiological considerations, common clinical features and laboratory abnormalities. The non-dengue controls were the cases of proven alternative diagnosis. The diagnostic performances of the tests were compared in terms of sensitivity, specificity and predictive value along with the cost involved per test. RESULTS: The sensitivity of the Panbio and SD RDT kits was found to be 97.7 and 64.3% respectively, and the specificities were 87.8 and 96.6% respectively. The sensitivity of the NS1 antigen capture by SD Duo, Reckon, J. Mitra RDTs was 20.9, 18.6 and 27.1% respectively. The prevalence of dengue specific IgG antibody with Panbio RDT kits was 49.3%. The cost per test for Panbio, SD, Reckon and J. Mitra is US$ 6.90, 4.27, 3.29 and 3.61 respectively. CONCLUSION: It was concluded that in dengue outbreak, Panbio IgM capture RDT alone is reliable and easily available test which can be used in acute phase of dengue infection in any resource limited set up. NS1 capture rates by any of the other three RDTs might not be reliable for the diagnosis of acute dengue infection.

Evaluation of tests for serological diagnosis of scrub typhus.

Two specific serological tests, a Dot enzyme immunoassay (EIA) and an immunoglobulin (Ig)M enzyme-linked immunosorbent assay (ELISA) using the 56 kDa antigen and the Weil-Felix test were evaluated for diagnosis of scrub typhus. Sensitivity of 100, 86.5 and 43.5% were observed with Dot EIA, IgM ELISA and Weil-Felix test, respectively. False-positive reactions were observed in patients with falciparum malaria, pulmonary tuberculosis, S. viridans septicemia and typhoid fever using Dot EIA and IgM ELISA. Therefore, although Dot EIA and IgM ELISA are useful in the serodiagnosis of scrub typhus, efforts should be made to rule out other febrile illnesses.

Nested polymerase chain reaction on blood clots for gene encoding 56 kDa antigen and serology for the diagnosis of scrub typhus.

PURPOSE: Scrub typhus is a zoonotic illness endemic in the Asia-Pacific region. Early diagnosis and appropriate management contribute significantly to preventing adverse outcomes including mortality. Serology is widely used for diagnosing scrub typhus. Recent reports suggest that polymerase chain reaction (PCR) could be a rapid and reliable alternative. This study assessed the utility of these tests for scrub typhus diagnosis. MATERIALS AND METHODS: Nested PCR to detect the 56 kDa antigen gene of O. tsutsugamushi was performed on blood clots from 87 individuals with clinically suspected scrub typhus. Weil-Felix test and scrub typhus IgM ELISA were performed on serum samples from the same patients. As a gold standard reference test was not available, latent class analysis (LCA) was used to assess the performance of the three tests. RESULTS: The LCA analysis showed the sensitivity of Weil-Felix test, IgM ELISA and PCR to be 59%, 100% and 58% respectively. The specificity of ELISA was only 73%, whereas those of the Weil-Felix test and PCR were 94% and 100% respectively. CONCLUSION: Nested PCR using blood clots while specific, lacked sensitivity as compared to IgM ELISA. In resource-poor settings Weil-Felix test still remains valuable despite its moderate sensitivity.