Comparison of the Quantitative DiaSorin Liaison Antigen Test to Reverse Transcription-PCR for the Diagnosis of COVID-19 in Symptomatic and Asymptomatic Outpatients

Abstract

We evaluated the quantitative DiaSorin Liaison severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antigen test in symptomatic and asymptomatic individuals consulting their general practitioners (GPs) during a period of stable intense virus circulation (213/100,000 habitants per day). Leftover reverse transcription-PCR (RT-PCR) positive (n = 204) and negative (n = 210) nasopharyngeal samples were randomly selected among fresh routine samples collected from patients consulting their GPs. Samples were tested on Liaison XL according to the manufacturer's instructions. Equivocal results were considered negative. The overall sensitivity and specificity of the Liaison antigen test compared to RT-PCR were 65.7% (95% confidence interval [CI], 58.9% to 71.9%) and 100% (CI, 97.8% to 100%). Sensitivity in samples with viral loads of ≥10⁵, ≥10⁴, and ≥10³ copies/ml were 100% (CI, 96.3% to 100.0%), 96.5% (CI, 91.8% to 98.7%), and 87.4% (CI, 81.3% to 91.5%), respectively. All samples with ≤10³ copies/ml were antigen negative. The ratio of antigen concentration to viral load in samples with ≥10³ copies/ml was comparable in symptomatic and asymptomatic individuals (P = 0.58). The proportion of RT-PCR-positive participants with a high viral load (≥10⁵ copies/ml) was not significantly higher in symptomatic than in asymptomatic participants (63.9% [CI, 54.9% to 72.0%] versus 51.9% [CI, 41.1% to 62.6%]; P = 0.11), but the proportion of participants with a low viral load (<10³ copies/ml) was significantly higher in asymptomatic than in symptomatic RT-PCR-positive participants (35.4% [CI, 25.8% to 46.4%] versus 14.3% [CI, 9.0% to 21.8%]; P < 0.01). Sensitivity and specificity in samples with a viral load of ≥10⁴ copies/ml were 96.5% and 100%. The correlation of antigen concentration with viral load was comparable in symptomatic and asymptomatic individuals.

Keywords: COVID-19 testing; SARS-CoV-2; ambulatory care; antigen; diagnosis; sensitivity and specificity.

FIG 1

RT-PCR results and viral load according to Liaison antigen test results. (A to C) RT-PCR results for the N1 (A) and N2 (B) gene regions and viral loads (C) in samples that were positive and negative in the Liaison antigen test. (D) Correlation of the C_T values for the N1 and N2 gene regions in RT-PCR-positive samples.

FIG 2

Quantitative Liaison antigen test results in symptomatic and asymptomatic patients compared to viral load (A) and lowest C_T value of the N1 or N2 gene region (B). Vertical lines represent the limit of detection (22 TCID₅₀/ml), cutoff for equivocal result (100 TCID₅₀/ml), cutoff for positive result (200 TCID₅₀/ml), and upper limit of quantification (100,000 TCID₅₀/ml).

FIG 3

Distribution of results for viral load (A) and quantitative Liaison antigen test results (B) in 119 symptomatic and 79 asymptomatic RT-PCR positive patients. Results are ordered from high to low. Horizontal lines represent the limit of detection (22 TCID₅₀/ml), cutoff for equivocal result (100 TCID₅₀/ml), cutoff for positive result (200 TCID₅₀/ml), and upper limit of quantification (100,000 TCID₅₀/ml).