Performance of a qualitative rapid chromatographic immunoassay to diagnose COVID-19 in inpatients in a middle-income country

Costa, Silvia Figueiredo; Buss, Lewis; Espinoza, Evelyn Patrícia Sanchez; Vieira, Jose Mauro; de Oliveira- da Silva, Lea Campos; de Souza, Regina Maia; Neto, Lauro Perdigão; Porto, Ana Paula Matos; Lazari, Carolina; dos Santos, Vera Aparecida; Duarte, Alberto; Nastri, Ana Catharina; da Costa Leite, Gabriel Fialkovitz; Manuli, Erika; de Oliveira, Maura Salaroli; Zampelli, Daniella Bosco; Pastore, Laerte; Segurado, Aluísio Cotrim; Levin, Anna S.; Sabino, Ester

Abstract Objectives We evaluated a rapid chromatographic immunoassay (IgG/IgM antibodies) and an ELISA assay to diagnose COVID-19 in inpatients at two Brazilian hospitals. Methods A total of 122 subjects with COVID-19 were included: 106 SARS-COV-2 RT-PCR-positive patients and 16 RT-PCR-negative patients with symptoms and chest computed tomography (CT) consistent with COVID-19. Ninety-six historical blood donation samples were used as controls. Demographic and clinical characteristics were retrieved from electronic records. Sensitivity and specificity were calculated, as were their 95% binomial confidence intervals using the Clopper-Pearson method. All analyses were performed in R version 3.6.3. Results The sensitivity of the chromatographic immunoassay in all RT-PCR-positive inpatients, irrespective of the timing of symptom onset, was 85.8% (95% binomial CI 77.7% to 91.9%). This increased with time after symptom onset, and at >14 days was 94.9% (85.9% to 98.9%). The specificity was 100% (96.4% to 100%). 15/16 (94%) RT- PCR-negative cases tested positive. The most frequent comorbidities were hypertension and diabetes mellitus and the most frequent symptoms were fever, cough, and dyspnea. All RT-PCR-negative patients had pneumonia. The most frequent thoracic CT findings were ground glass changes (n = 11, 68%), which were bilateral in 9 (56%) patients, and diffuse reticulonodular infiltrates (n = 5, 31%). Conclusions The COVID-19 rapid chromatographic immunoassay evaluated in this study had a high sensitivity and specificity using plasma, particularly after 14 days from symptom onset. ELISA and qualitative rapid chromatographic immunoassays can be used for the diagnosis of RT-PCR-negative patients.