RESUMO
BackgroundDetection of SARS-CoV-2 antigens in blood has high sensitivity in adults with acute COVID-19, but sensitivity in pediatric patients is unclear. Recent data suggest that persistent SARS-CoV-2 spike antigenemia may contribute to multisystem inflammatory syndrome in children (MIS-C). We quantified SARS-CoV-2 nucleocapsid (N) and spike (S) antigens in blood of pediatric patients with either acute COVID-19 or MIS-C using ultrasensitive immunoassays (Meso Scale Discovery). MethodsPlasma was collected from inpatients (<21 years) enrolled across 15 hospitals in 15 US states. Acute COVID-19 patients (n=36) had a range of disease severity and positive nasopharyngeal SARS-CoV-2 RT-PCR within 24 hours of blood collection. Patients with MIS-C (n=53) met CDC criteria and tested positive for SARS-CoV-2 (RT-PCR or serology). Controls were patients pre-COVID-19 (n=67) or within 24h of negative RT-PCR (n=43). ResultsSpecificities of N and S assays were 95-97% and 100%, respectively. In acute COVID-19 patients, N/S plasma assays had 89%/64% sensitivity, respectively; sensitivity in patients with concurrent nasopharyngeal swab cycle threshold (Ct) [≤] 35 were 93%/63%. Antigen concentrations ranged from 1.28-3,844 pg/mL (N) and 1.65-1,071 pg/mL (S) and correlated with disease severity. In MIS-C, antigens were detected in 3/53 (5.7%) samples (3 N-positive: 1.7, 1.9, 121.1 pg/mL; 1 S-positive: 2.3 pg/mL); the patient with highest N had positive nasopharyngeal RT-PCR (Ct 22.3) concurrent with blood draw. ConclusionsUltrasensitive blood SARS-CoV-2 antigen measurement has high diagnostic yield in children with acute COVID-19. Antigens were undetectable in most MIS-C patients, suggesting that persistent antigenemia is not a common contributor to MIS-C pathogenesis. Key pointsIn a U.S. pediatric cohort tested with ultrasensitive immunoassays, SARS-CoV-2 nucleocapsid antigens were detectable in most patients with acute COVID-19, and spike antigens were commonly detectable. Both antigens were undetectable in almost all MIS-C patients.
RESUMO
BackgroundDiagnosis of COVID-19 by PCR offers high sensitivity, but the utility of detecting samples with high cycle threshold (Ct) values remains controversial. Currently available rapid diagnostic tests (RDTs) for SARS-CoV-2 nucleocapsid antigens (Ag) have sensitivity well below PCR. The correlation of Ag and RNA quantities in clinical nasopharyngeal (NP) samples is unknown. MethodsAn ultrasensitive, quantitative electrochemiluminescence immunoassay for SARS-CoV-2 nucleocapsid (the MSD(R) S-PLEX(R) CoV-2 N assay) was used to measure Ag in clinical NP samples from adults and children previously tested by PCR. ResultsThe S-PLEX Ag assay had a limit of detection (LOD) of 0.16 pg/mL and a cutoff of 0.32 pg/mL. Ag concentrations measured in clinical NP samples (collected in 3.0 mL media) ranged from less than 160 fg/mL to 2.7 ug/mL. Log-transformed Ag concentrations correlated tightly with Ct values. In 35 adult and 101 pediatric PCR-positive samples, sensitivity was 91% (95% CI, 77-98%) and 79% (70-87%), respectively. In samples with Ct [≤] 35, sensitivity was 100% (88-100%) and 96% (88-99%), respectively. In 50 adult and 40 pediatric PCR-negative specimens, specificity was 100% (93-100%) and 98% (87-100%), respectively. ConclusionsNucleocapsid concentrations in clinical NP samples span 8 orders of magnitude and correlate closely with RNA concentrations (Ct values). The S-PLEX Ag assay had 96-100% sensitivity in samples from children and adults with Ct values [≤] 35, and 98-100% specificity. These results clarify Ag concentration distributions in clinical samples, providing insight into the performance of Ag RDTs and offering a new approach to diagnosis of COVID-19. Key pointsSARS-CoV-2 nucleocapsid concentrations in clinical nasopharyngeal samples, measured with an ultrasensitive assay, spanned an 8-log range and correlated closely with PCR Ct values. The assay was 96-100% sensitive in pediatric/adult samples with Ct values [≤] 35, and 98-100% specific.
