Association between viral load and positivization time of a SARS-CoV-2 rapid antigen test in routine nasopharyngeal specimens

Abstract

Background: Rapid SARS-CoV-2 antigen tests are potentially useful tools for screening carriers with high viral load. This study was aimed to assess the potential association between viral load and positivization time of a manual SARS-CoV-2 commercial antigen test in routine nasopharyngeal specimens.
 Methods: In a sample of subjects undergoing routine diagnostic testing, SARS-CoV-2 positivity of nasopharyngeal samples was assayed with both molecular (Altona Diagnostics RealStar SARS-CoV-2 RT-PCR Kit) and antigenic (Roche SARS-CoV-2 Rapid Antigen Test) tests. Positivization time of rapid antigen test was correlated and compared with viral load expressed as mean of SARS-CoV-2 E/S genes cycle threshold (Ct) values.
 Results: The study sample consisted of 106 patients (median age 48 years, 55 women) with positive results of rapid SARS-CoV-2 antigen testing. A highly significant Spearman’s correlation was found between mean SARS-CoV-2 E/S genes Ct values and positivization time of manual antigen test (r= 0.70; p<0.001). The positivization time of rapid SARS-CoV-2 antigen test displayed an area under the curve of 0.82 (95%CI, 0.74-0.89) for predicting nasopharyngeal samples with high viral load (i.e., mean Ct <20). A positivization time cut-off of 32 sec had 94.9% sensitivity and 58.2% specificity for detecting specimens with high viral load. The overall agreement between mean Ct value <20 and positivization time <32 sec was 70.8%.
 Conclusions: Positivization time of rapid SARS-CoV-2 antigen tests may provide easy and rapid information on viral load, thus making this type of manual assay potentially suitable for quick and reliable detection and isolation of super-carries.

Quantification of SARS‐CoV‐2 viral load in patients using time‐to‐results in antigen lateral flow tests

“…In this case, the high concentration of labeled antibody present with a high viral load leads to early visualization of the sandwich at the test line site. Between our results with two home tests and those of others, who measured time‐to‐positivity in eight other visual read LFTs versus actual or surrogates of viral load, it is likely that most, if not all, of the COVID‐19 LFTs will behave similarly 7–10,19 …”

“…Between our results with two home tests and those of others, who measured time-to-positivity in eight other visual read LFTs versus actual or surrogates of viral load, it is likely that most, if not all, of the COVID-19 LFTs will behave similarly. [7][8][9][10]19 Accurate quantification requires first establishing a standard curve with known concentrations of the virus with each LFT before using it to estimate the viral load in a sample. Ideally this would be performed using artificial mucus spiked with the protein or virus rather than UTM as used in this study.…”

“…Additionally, two groups reported that the time-to-positive SARS-CoV-2 antigen result was inversely correlated with the Ct values. [8][9][10] In order to determine whether the utilization of timeto-positivity after application of nasal swab samples in two popular home SARS-CoV-2 antigen tests could be used to quantify actual viral load, we investigated the dose-response relationship between time-to-positive result by visual assessment in the antigen tests with purified nucleocapsid protein (NP), chemically inactivated WA1 and Omicron XBB.1.5 viral strains, and Ct values with dilutions of the viruses and in clinical samples. We also examined the feasibility of using time-to-positive result to predict the Ct values in clinical samples from patients with COVID-19.…”

“…demonstrated that measuring time‐to‐positive test results in six LFTs was shorter for a high viral concentration (2.5 × 10 5 TCID 50 /mL) than with a lower viral concentration (2.5 × 10 4 TCID 50 /mL). Additionally, two groups reported that the time‐to‐positive SARS‐CoV‐2 antigen result was inversely correlated with the Ct values 8–10 …”

“…Additionally, two groups reported that the time‐to‐positive SARS‐CoV‐2 antigen result was inversely correlated with the Ct values. 8 , 9 , 10 …”