Performance Evaluation of the STANDARD i-Q COVID-19 Ag Test with Nasal and Oral Swab Specimens from Symptomatic Patients

Abstract

We evaluated the diagnostic performance of the STANDARD i-Q COVID-19 Ag Test, which was developed to detect viral antigens, using nasal and oral swabs. Sixty positive and 100 negative samples were analyzed. We determined the distribution of the Ct values according to the day of sample collection after symptom onset, the diagnostic performance of the total samples and subgroups separated by Ct value or time of sample collection, and the Ct value at which maximal accuracy was expected. No differences were observed in Ct values, except for the samples obtained on the day of symptom onset. The diagnostic sensitivity and specificity of the oral swabs were 75.0 and 100.0%, respectively, whereas those of the nasal swabs were 85.0 and 98.0%, respectively. The sensitivity was higher in samples with a high viral load collected earlier than those collected later, although the difference was not significant. False-negative results were confirmed in all samples with a Ct value ≥ 30.0. These results indicate that tests using oral and nasal swabs are helpful for diagnosing acute symptomatic cases with suspected high viral loads. Our tests exhibited relatively low sensitivity but high specificity rates, indicating the need to assess negative antigen test results.

Keywords: COVID-19; SARS-CoV-2 rapid antigen test; nasal swab; oral swab; performance evaluation; point-of-care testing.

Figure 1

Distribution of C_t values for (A) RtRp and (B) E genes in the polymerase chain reaction (PCR)-positive samples compared using sample collection days after symptom onset and using an analysis of variance (ANOVA) with a Bonferroni post hoc test. The p-values from the Bonferroni test are presented. The circles represent the C_t values of each subject, triangle represent extreme outliers confirmed by Tukey’s methods, and asterisk represent the subgroups that showed significant difference with p-value < 0.05. Abbreviation: DASO, days after symptom onset.

Figure 1

Distribution of C_t values for (A) RtRp and (B) E genes in the polymerase chain reaction (PCR)-positive samples compared using sample collection days after symptom onset and using an analysis of variance (ANOVA) with a Bonferroni post hoc test. The p-values from the Bonferroni test are presented. The circles represent the C_t values of each subject, triangle represent extreme outliers confirmed by Tukey’s methods, and asterisk represent the subgroups that showed significant difference with p-value < 0.05. Abbreviation: DASO, days after symptom onset.

Figure 2

Distribution of C_t values and maximal antigen test sensitivity limits for (A) RtRp, (B) E in the nasal swab samples, (C) RtRp, and (D) E in the oral swab samples. The circles represent the C_t values of each subject.

Figure 2

Distribution of C_t values and maximal antigen test sensitivity limits for (A) RtRp, (B) E in the nasal swab samples, (C) RtRp, and (D) E in the oral swab samples. The circles represent the C_t values of each subject.