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Abstract
Background
SARS-CoV-2 nucleocapsid antigen can be detected in plasma, but little is known about its performance as a diagnostic test for acute SARS-CoV-2 infection or infectious viral shedding among nonhospitalized individuals.
Methods
We used data generated from anterior nasal and blood samples collected in a longitudinal household cohort of SARS-CoV-2 cases and contacts. Participants were classified as true positives if polymerase chain reaction (PCR) positive for SARS-CoV-2 and as true negatives if PCR negative and seronegative. Infectious viral shedding was determined by the cytopathic effect from viral culture. Stratified by 7 days after symptom onset, we constructed receiver operating characteristic (ROC) curves to describe optimized accuracy (Youden index), optimized sensitivity, and specificity.
Results
Of 80 participants, 58 (73%) were true positives while 22 (27%) were true negatives. Using the manufacturer's cutoff of 1.25 pg/mL for evaluating infection, sensitivity was higher from 0 to 7 days (77.6% [95% confidence interval {CI}, 64%–88.2%]) than from 8 to 14 days (43.2% [95% CI, 31.1%–54.5%]) after symptom onset; specificity was unchanged at 100% (95% CI, 88.1%–100%). This test had higher sensitivity (100% [95% CI, 88.4%–100%]) and lower specificity (65% [95% CI, 40.8%–84.6%]) for infectious viral shedding as compared with infection, particularly within the first week of symptom onset. Although the presence of N-antigen correlated with infectious viral shedding (r = 0.63; P < .01), sensitivity still declined over time. Additional cutoffs from ROC curves were identified to optimize sensitivity and specificity.
Conclusions
We found that this SARS-CoV-2 N-antigen test was highly sensitive for detecting early but not late infectious viral shedding, making it a viable screening test for community-dwelling individuals to inform isolation practices.
Details
1 Department of Epidemiology and Biostatistics, University of California , San Francisco, California , USA
2 Francis I. Proctor Foundation, University of California , San Francisco, California , USA
3 Institute for Global Health Sciences, University of California , San Francisco, California , USA
4 Department of Microbiology and Immunology, University of California , San Francisco, California , USA
5 School of Medicine, University of California , San Francisco, California , USA
6 San Francisco Veterans Affairs Medical Center , San Francisco, California , USA
7 Labcorp-Monogram Biosciences , South San Francisco, California , USA
8 Quanterix Laboratories , Billerica, Massachusetts , USA
9 Division of HIV, Infectious Disease, and Global Medicine, Department of Medicine, University of California , San Francisco, California , USA