Accuracy of Roche SARS-CoV-2 Rapid Antigen Test in Nasopharyngeal Swab: Clinical Impression Matters

Background: In COVID-19 pandemic, the diagnosis and treatment must be as early as possible to save the life of each patient. Moreover, screening of asymptomatic carriers, close contacts or healthy subjects must not be delay to prevent transmission to publics. For confirmation of diagnosis of SARS-CoV-2 infection, nasopharyngeal swab must be tested either by real-time Reverse Transcription Polymerase Chain Reaction (RT-PCR) tests or Rapid Antigen Test (RAT). RAT is faster, easier and cheaper; thus, it is suitable for health service in developing country. Objectives: The aim of this study was to assess the diagnostic accuracy of Roche SARS-CoV-2 Rapid Antigen Test (RAT) in diagnosing SARS-CoV-2 infection. Methods: Hospital based exploratory study was done in out-patient department and fever clinic, and molecular laboratory of No. (1) Defence Services General Hospital. Nasopharyngeal swabs were taken, and the Roche SARS- CoV-2 RAT was conducted in parallel with RT-PCR test (reference standard). Results: Among the 932 patients/subjects recruited, RT-PCR was positive in 468 individuals, corresponding to a prevalence of 50.2%. The RAT was positive in 363 patients (60.4%), false positive in 120 patients; it was negative in 569 individuals (39.6%), false negative in 225 patients. The overall sensitivity of the RAT was 51.9% (95% Confidence Interval [CI] 47.29-56.53) and, the specificity was 74.1% (95% CI 69.9-78.07); positive predictive value was 66.9% and negative predictive value was 60.5%. The sensitivity varied with Ct value; 78% in clinical samples with Ct values < 20, 57.5% in those with Ct values between 21 and 25, 41.8% in samples with Ct values between 26 and 30, and, 36.4% in samples with Ct value > 30. Conclusion: The accuracy of the SARS-CoV-2 Roche RAT in diagnosing SARS-CoV-2 infections was inferior to RT-PCR and manufacturer’s data. The sensitivity was with low Cycle threshold values < 20 which were inversely related to the viral load. RAT test should be used in association with clinical impression of physicians. In hospital setting especially in emergency department, the role of RAT should be reconsidered in those patients presenting with anosmia and some cases of dyspnoea, late symptoms in the course of disease, as the RAT results would be false negative. Other errors may arise if the operator for RAT has to handle more than recommended tests per hour especially in the peak of epidemics.