Evaluation of a New Rapid Antigen Detection Test for the Diagnosis of Influenza Virus Infection in a Tertiary Care Hospital of Southern India

Introduction: Rapid antigen testing can help in early diagnosis of Influenza in resource limited settings in patients presenting with Influenza Like Illness (ILI). Aim: To evaluate the usefulness of Rapid Influenza Direct antigen Testing (RIDT) in comparison to Real Time Reverse Transcriptase Polymerase Chain Reaction (RT-PCR) for diagnosis of Influenza virus infection in clinically suspected patients with ILI in a tertiary care hospital. Material and Methods: A prospective observational independent study was done to evaluate RIDT for Influenza virus infection, considering RT-PCR as gold standard. Throat swabs and nasal swabs from patients meeting Category 3 definition of ILI by Ministry Of Health and Family Welfare (MOHFW) were collected from July 20th, 2019 to October 20th, 2019 from hospitals in and around the study area. Onsite Rapid test (Biogenix, China) was used as per manufactures instructions for rapid detection of Influenza antigen. RNA extraction was done using Qiagen kit followed by RT-PCR using primer, probes recommended by National Institute of Virology (NIV, Pune) as per their protocols. Data was recorded on a predesigned proforma and managed using Microsoft Excel 2018 (Microsoft Corp, Redmond, WA). The Statistical software MedCalc version 9.1 for Windows was used for data analysis. Results: The overall sensitivity of the RIDT was 37.9% and specificity was 94.4%. The Positive Predictive Value (PPV) and Negative Predictive Value (NPV) were 73.3% and 78.8%, respectively. Kappa was calculated to be 0.37 that suggested a fair agreement between RIDT and RT-PCR. When the Cycle threshold (Ct) values were compared to the results of RIDT, there were more false positives at low Ct value and more false negatives at high Ct values. Conclusion: The RIDT is not useful in screening during an outbreak in acute care settings. The negative result of RIDT should be followed up with RT-PCR. With a strong clinical suspicion, it is better to start antivirals than wait for laboratory report in severely ill patients with ILI.