S568 Pre-Operative SARS-CoV2 Screening With TMA Nasopharyngeal Swab in Asymptomatic Patients Undergoing Non-Urgent Outpatient Procedures

Abstract

Introduction: Pre-operative SARS-CoV2 nasopharyngeal swab screening is performed prior to non-urgent procedures in many institutions in the United States. It remains unknown whether a standardized screening algorithm is effective in capturing asymptomatic SARS-CoV2 carriers to limit exposure to other patients and staff. Our aim was to study the effectiveness of the screening approach used in our center to identify these patients. Methods: We performed a retrospective study at our center between 06/03/2020 and 11/18/2020. Firstly, patients were screened for typical symptoms of SARS-CoV2 by phone using a standard questionnaire. Those asymptomatic were then required to undergo transcription mediated amplification (TMA) testing within 72 hours of the procedure. Lastly, those with negative TMA were contacted again for another phone screening 24 hours prior to the procedure. All our patients were tested using the same SARS-CoV2 nasopharyngeal swab kit. Number of false negative tests were estimated by computing pre-test odds and negative likelihood ratio. The pre-test probability among asymptomatic patients was determined by dividing the number of positive TMA by all those tested in the same setting, assuming those who tested positive had the disease for 14 days. The negative likelihood ratio was calculated based on sensitivity (98.5%) and specificity (99%) of the TMA test that was used. Descriptive statistics were used to analyze the entire study period, including both the pre-surge (until 10/6/2020) and post-surge (after 10/6/2020) period. Results: A total of 8925 SARS-CoV2 TMA tests were performed as part of the pre-operative screening for non-urgent procedures during the study period. Of those, 67 patients tested positive. This represents a positivity rate of 0.75% and an estimated prevalence of 740 per 100,000. In the pre-surge period, 27 of 6355 patients tested positive (positivity rate 0.42%; prevalence 410 per 100,000), while in the post-surge period, 40 of 2570 patients tested positive (positivity rate 1.6%, prevalence 1540 per 100,000). Based on post-test odds, the number of false negatives during the entire study was estimated to be less than 1 (0.39 for pre-surge and 0.62 for post-surge period). The calculated number of false negative tests within the study period was 1.01. Conclusion: A three-tier pre-operative SARS-CoV2 screening approach including nasopharyngeal TMA testing among patients undergoing non-urgent procedures, effectively captures the overwhelming majority of asymptomatic cases.