Objectives: The study aimed to evaluate the performance of rapid antigen test (RAT) and reverse transcription polymerase chain reaction (RT-PCR) in detecting COVID-19 and the impact of laboratory biomarkers on the severity of the disease. Material and Methods: A total of 150 nasopharyngeal swabs and blood samples were collected from symptomatic COVID-19 patients in Tehsil head-quarter Hospital, Dargai from August 2021 to March 2022. Results: RAT revealed a sensitivity of 86.67% and specificity of 100%, while RT-PCR revealed a sensitivity of 93.33% and specificity of 100%. The highest infection rate was found in males, and the average age of patients was 53.87 years. The most common symptoms were fever and sore throat. Patients were categorized into four groups based on cycle threshold values and blood biomarkers: mild, moderate, severe, and critical. White blood cell count, platelet count, and C-reactive protein were significantly different between the groups. Lactate dehydrogenase, D-dimer, and serum ferritin were significantly increased in critical patients. The receiver operating characteristic curve showed that inflammatory biomarkers had a comparative performance in predicting disease severity in COVID-19 patients. Conclusion: Blood biomarkers are associated with the disease severity in COVID-19 patients, and further studies, such as metabolomics, are recommended to explore the immunological mechanisms behind these biomarkers.