Evaluation of rapid SARS-CoV-2 antigen tests, AFIAS COVID-19 Ag and ichroma COVID-19 Ag, with serial nasopharyngeal specimens from COVID-19 patients

Abstract

We evaluated the diagnostic accuracy of two newly developed, point-of-care, rapid antigen tests (RATs) for detecting SARS-CoV-2, the AFIAS COVID-19 Ag and the ichromaTM COVID-19 Ag, and investigated antigen kinetics. A total of 200 serially collected nasopharyngeal (NP) specimens from 38 COVID-19 patients and 122 specimens from negative controls were analyzed. Diagnostic sensitivity and specificity were assessed in comparison to molecular test results and subdivided according to targeted genes (E, RdRP, and N) and days post-symptom onset (PSO). For the kinetics evaluation, cut-off-indices from serial NP specimens were used according to the number of days PSO. Both RATs showed sensitivity of 91.3‒100% for specimens with cycle threshold (Ct) < 25. The specificity of AFIAS was 98.7‒98.9% and that of ichromaTM was 100.0%. The kappa values of AFIAS and ichromaTM for the molecular testing of specimens with Ct < 25 (RdRP) were 0.97 and 1.00, respectively. The sensitivity of AFIAS and ichromaTM for all genes was lower for specimens collected at 8‒14 PSO than for those collected before 7-days PSO. The kinetics profiles showed that antigen levels gradually decreased from ≤ 7-days PSO to > 22-days PSO. Both RATs showed excellent specificity and acceptable sensitivity for NP specimens with higher viral loads and for specimens collected within 7-days PSO. Hence, they have the potential to become useful tools for the early detection of SARS-CoV-2. However, because of concerns about false negativity, RATs should be used in conjunction with molecular tests.

Fig 1

Cycle threshold value distribution of E, RdRP, and N genes according to the positivity of (A) the AFIAS COVID-19 Ag and (B) the ichroma^TM COVID-19 Ag assays. Abbreviations: Ag, antigen; PSO, Pos.; positive; Neg., negative.

Fig 2. Antigen kinetics determined for specimens from 38 COVID-19 patients at different post-symptom-onset periods assessed by two antigen assays according to cut-off index values, the AFIAS COVID-19 Ag and the ichroma^TM COVID-19 Ag.

The smoothing splines of both assays showed gradual decrement from the days 1‒7 to days >22, while it was shown that their RdRP gene cyclic threshold values gradually increased or their viral loads decreased. Abbreviations: Ag, antigen; COI, cut-off index; AFIAS, AFIAS COVID-19 Ag (Boditech Med., Chuncheon-si, Gang-won-do, Republic of Korea); ichroma, ichroma^TM COVID-19 Ag (Boditech Med.); Ct, cycle threshold.