Lesson learned: Retrospective analysis of 'missed out' dengue NS1 positives among IgM negative population

ABSTRACT

To determine the proportion of dengue non-structural protein 1 (NS1) positives among laboratory confirmed dengue IgM negative patients. Methods: Data for 1 732 samples received from January to October 2017 at the Virus Research and Diagnostic Laboratory (VRDL) for dengue diagnosis were downloaded from the National Institute of Epidemiology server. Samples that were previously reported as IgM negative for dengue diagnosis were identified and their NS1 status was determined using ELISA. Thus, 'missed out' NS1 positives were correlated with the duration of illness. Furthermore, an epidemic curve for the study period was constructed. The increase in positivity rate within and between the months was compared by McNemar's and Pearson's chi-square test, respectively. Results: The reported IgM-negatives were 813, of which, 22.5% (183) were retrospectively positive for NS1 antigen. The addition of NS1 positives revealed by this study has raised the reported positivity across the months that ranged from 8.1% to 29.6%. By analyzing the dengue positives per month and the epidemic curve, the period between January and September, 2017 was identified as non-epidemic while the epidemic started from the month of October, 2017. Conclusions: Acute dengue infection is widely confirmed by detecting NS1 antigen in serum. Missing out of NS1 positives happen due to shortened window period and such cases act as reservoir for further viral transmission. Hence, this study highly emphasizes performing all three tests for dengue diagnosis that warrants the accurate dengue proportion in India.