New ICMR study: Rapid Antigen Test can detect COVID-19 patients within the first 5 days of illness
Rapid Antigen Test (RAT) showed an excellent specificity to 'rule-in' COVID-19 patients within the first 5 days of illness and had a moderate sensitivity. Therefore, patients showing positive results need to be immediately triaged and those with negative tests should be reconfirmed by an rRT-PCR.
The problem with the real-time reverse transcription-polymerase chain reaction (rRT-PCR) testing is that it requires a sophisticated Biosafety level (BSL)-2/BSL3 laboratory setup, and also trained technicians to run the test and interpret results. It takes a minimum of 8-10 h for the rRT-PCR from swab collection until the results are reported.
This duration can increase further in resource-limited and high-burden settings, eg. in small towns and cities, where molecular diagnostic laboratories are non-existent, and where they find it difficult to procure the reagents/ viral transport medium (VTM) and other resources. Therefore, the need of the hour is to rapidly detect and isolate positive cases to contain the disease spread, to quickly triage patients with severe acute respiratory illness (SARI) in emergency departments (EDs) and to ramp up testing facilities.
Many Rapid Antigen Test (RAT) - kits and devices are available/ being developed to facilitate point-of-care testing. However, there is very limited data on the performance and potential diagnostic utility of a rapid chromatographic immunoassay-based antigen test for SARS-CoV-2 in suspected patients.
A cross-sectional, single-blinded study was conducted at a tertiary care teaching hospital in north India. Paired samples were taken for RAT and rRT-PCR (reference standard) from consecutive participants screened for COVID-19 to calculate the sensitivity and specificity of the RAT. Further sub-group analysis was done based on the duration of illness and cycle threshold values.
- Of the 330 participants, 77 were rRT-PCR positive for SARS-CoV-2. 64 out of these 77 patients
also tested positive for SARS-CoV-2 by RAT. The overall sensitivity and specificity were 81.8 and 99.6
per cent, respectively.
- Overall, among the positive test results, with respect to the reference standard, the RAT detected 63 true-positives (19%) and gave one false-positive result. Among the negative test results, 252 (76.3%) were true- negatives and 14 (4.2%) were false-negatives.
What is significant in this study?
- The overall sensitivity of the test was 81.8% and specificity was 99.6 %. The test accuracy was 95.4 %. But the sensitivity of RAT was higher (85.9%) in participants with a duration of illness ≤5 days.
- The RAT in the present study showed a high positive Likelihood Ratio (LR) of 207.0, which indicates a 207-fold increase in the odds of having infection with SARS-CoV-2 in participants with positive RDT results.
- Likewise, the negative LR for the RAT was 0.18, which means that the odds of having SARS-CoV-2 infection had decreased by 5.5-fold after a negative RDT result.
With an excellent specificity and moderate sensitivity, this RAT may be used to 'rule in' COVID-19 in patients with a duration of illness ≤5 days. Large-scale testing based on this RAT across the country would result in quick detection, isolation and treatment of COVID-19 patients.
Gupta A, Khurana S, Das R, Srigyan D, Singh A, Mittal A, Singh P, Soneja M, Kumar A, Singh AK, Soni KD, Meena S, Aggarwal R, Sharad N, Aggarwal A, Kadnur H, George N, Singh K, Desai D, Trilangi P, Khan AR, Kiro VV, Naik S, Arunan B, Goel S, Patidar D, Lathwal A, Dar L, Trikha A, Pandey RM, Malhotra R, Guleria R, Mathur P, Wig N.
Rapid chromatographic immunoassay-based evaluation of COVID-19: A cross-sectional, diagnostic test accuracy study & its implications for COVID-19 management in India.
Indian J Med Res. 2020 Oct 31.
Epub ahead of print. PMID: 33146157.