Real-life evaluation of a rapid antigen test (DPP SARS-CoV-2 Antigen) for COVID-19 diagnosis of primary healthcare patients, in the context of the Omicron-dominant wave in Brazil

Abstract

Objectives: We aimed to investigate the real-life performance of the rapid antigen test in the context of a primary healthcare setting, including symptomatic and asymptomatic individuals that sought diagnosis during an Omicron infection wave.

Methods: We prospectively accessed the performance of the DPP SARS-CoV-2 Antigen test in the context of an Omicron-dominant real-life setting. We evaluated 347 unselected individuals (all-comers) from a public testing centre in Brazil, performing the rapid antigen test diagnosis at point-of-care with fresh samples. The combinatory result from two distinct real-time quantitative PCR (RT-qPCR) methods was employed as a reference and 13 samples with discordant PCR results were excluded.

Results: The assessment of the rapid test in 67 PCR-positive and 265 negative samples revealed an overall sensitivity of 80.5% (CI 95% = 69.1%-89.2%), specificity of 99.2% (CI 95% = 97.3%-99.1%) and positive/negative predictive values higher than 95%. However, we observed that the sensitivity was dependent on the viral load (sensitivity in Ct < 31 = 93.7%, CI = 82.8%-98.7%; Ct > 31 = 47.4%, CI = 24.4%-71.1%). The positive samples evaluated in the study were Omicron (BA.1/BA.1.1) by whole-genome sequencing (n = 40) and multiplex RT-qPCR (n = 17).

Conclusions: Altogether, the data obtained from a real-life prospective cohort supports that the rapid antigen test sensitivity for Omicron remains high and underscores the reliability of the test for COVID-19 diagnosis in settings with high disease prevalence and limited PCR testing capability.

Keywords: COVID-19 diagnosis; Evidence-based; Omicron; Rapid antigen test; Viral control.

Graphical abstract

Fig. 1

Study design and DPP SARS-CoV-2 Antigen performance rates. (A) The organogram displays the number of nasopharyngeal samples evaluated and their distribution according to inclusion criteria, diagnostic test results and sequencing. Fifteen percent of the positive samples (n = 10) could not be assessed for the SARS-CoV-2 variant due to the low viral load (B) Performance rates observed in the study. The bars represent the 95% confidence intervals.

Fig. 2

Comparison of the PCR and the rapid antigen test results. (A) Correlation between the Ct values from the two PCR methods that were used combined as the reference method. The colourless datapoint represents the outlier sample (B) Distribution of Ct values and the proportion of positive samples in the rapid test. The N1 target (CDC protocol) is more skewed towards lower Ct values whereas the E target (EP/RP kit) is more skewed towards higher Ct Values. The dashed line represents the average values of the Ct values from positive samples. (C) Ct values grouped according to the rapid test result: Ag-positive samples had lower Ct values (higher viral load) than the Ag-negative samples. (D) Probability of positive Ag test according to the Ct values (N1 target) in groups of 3 (e.g. 16–18, 19–21). The curve was calculated by the non-linear asymmetric sigmoidal curve fit.

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