• Omar Rasheed Chughtai, Hijab Batool, Muhammad Dilawar Khan, and Shakeel Ashraf.
• Department of Histopathology, Chughtai Institute of Pathology, Lahore, Pakistan.
• J Coll Physicians Surg Pak. 2020 Aug 1; 30 (8): 801-804.

ObjectiveTo determine the percentage of seroconverted real time reverse transcriptase polymerase chain reaction (RT-PCR) positive coronavirus disease (COVID-19) cases at different days post-symptom onset; and also find the agreement of chemiluminescence assay used for total antibody detection using RT-PCR as a reference method.Study DesignCross-sectional study.Place And Duration Of StudyChughtai Institute of Pathology from April to May 2020.MethodologyFifty pre-pandemic samples (healthy population) and 75 COVID-19 patients were included in the study. RT-PCR confirmed COVID-19 patients were divided into 3 equal groups (25 each), according to the days of symptom onset. The samples were analysed using electro-chemiluminescence as assay principle. Positive and negative agreement of COVID-19 antibodies was calculated using EP evaluator to find out the sensitivity of chemiluminescence assay for total antibody detection. The results were analysed using SPSS version 23.0.ResultsAll the pre-pandemic samples tested were negative for antibodies with a negative agreement of 100%. Total agreement at day 7 post-symptom onset was 84%; whereas, it was 94% at day 14 and increased rapidly to 100% at day 21 post-symptom onset. At day 7 post-symptom onset, 68% of patients were seroconverted; and this percentage was 88% and 100% at day 14 and 21 post-symptom onset, respectively.ConclusionPre-pandemic samples were non-reactive for COVID-19 antibodies and seroconversion started within the first week post-virus exposure. There was 100% concordance between RT-PCR result and antibody positivity 21 days post-symptom onset. Key Words: COVID-19, SARS CoV-2, Seroconversion, Chemiluminescence.

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