The ongoing COVID-19 pandemic has highlighted the central role of diagnostic tests in pandemic control. Although reverse transcriptase quantitative real-time PCR (RT-qPCR) is the gold standard for the diagnosis of COVID-19, several rapid antigen tests (RAT) have been commercialized as rapid point-of-care diagnostics. To the best of our knowledge, there are limited data on the effect of patient’s clinical and laboratory parameters on RAT performance and no studies exist that tested the importance of combining laboratory measurements in patient’s blood in enhancing the performance of RAT. Here we tried to fill these gaps by evaluating the diagnostic performance of the RAT “Standard ™ Q COVID-19 Ag” in participant’s subgroups studying the influence of viral load, sampling time-post symptoms, clinical and laboratory features on test performance. Eighty-three nasopharyngeal and oropharyngeal swabs were tested for sever acute respiratory syndrome-coronavirus 2 (SARS-CoV-2) by both RT-qPCR and RAT. Diagnostic accuracy of the RAT was evaluated for participant’s subgroups that have various features. Support vector machine model was then used to investigate whether laboratory measurements in subject’s blood would enhance the predictive accuracy of this RAT. The sensitivity, specificity and accuracy of the RAT were 78.2, 64.2 and 75.9%, respectively. Samples with high viral load and those that were collected within one week post-symptom showed the highest sensitivity and accuracy. Measuring Laboratory indices did not enhance the predictive accuracy of this RAT. It is concluded that “Standard ™ Q COVID-19 Ag” should not be used alone for COVID-19 diagnosis due to its low diagnostic performance. This RAT is best used at early disease stage and in patients with high viral load.