The role and diagnostic accuracy of serology for COVID-19

Adult patients with fever for  >5 days, admitted to a tertiary care teaching hospital in South India, were prospectively recruited between June and December 2020. Patients confirmed by RT-PCR of SARS- CoV-2 were classified as cases and patients with febrile illness with an alternative laboratory-confirmed diagnosis and healthy participants were controls. All participants were tested with SCoV-2 Detect™ IgM ELISA Kit and SCoV-2 Detect™ IgG ELISA Kit (InBios International, Seattle, USA) (Inbios), SARS-CoV-2 Total and SARS-CoV -2 IgG (Siemens Healthcare Diagnostics Inc., Tarrytown, USA) (Siemens)

Roche Elecsys® Anti-SARS-CoV-2 (Roche Diagnostics, Rotkreuz, Switzerland) (Roche), Abbott SARS-CoV-2 IgG ( Abbott Diagnostics, IL, USA) (Abbott) and Liaison® SARS-CoV-2 S1/S2 IgG (DiaSorinS.p.A., Saluggia, Italy) (Liaison). Sensitivities, specificities, positive predictive values ​​(PPV), negative predictive values ​​(NPV) and accuracies were compared.

There were 303 participants: 153 cases and 150 controls. ELISA detecting anti-protein S antibodies was more sensitive (88.9% for IgG and 86.3% for IgM) than CLIA (82.4% for total antibodies and 76.5 to 85.6 % for IgG). Among CLIA, Roche IgG was the most sensitive (85.6%), followed by Abbott (83%) and Liaison (83%). Abbot had the best PPV (88.8%) and was more specific (89.3%) than Liaison (82%) and Roche (82%). Siemens IgG was less sensitive (76.5%) than Siemens Total (82.4%).

The specificity of all serological tests was modest (75-90%). Antibody test positivity increased with disease duration reaching 90% after 10 days of illness. When cases were compared to pre-pandemic controls, IgG yielded excellent specificity (98-100%). For seroprevalence studies, InBios IgG had the best accuracy (90.8%) with a sensitivity of 88.9% and a specificity of 97.6%.

Serological tests are important adjuncts for the diagnosis of COVID-19 in patients with persistent symptoms, especially during the second week of illness. The value of serological diagnostic tests is limited in the first week of illness and they provide additional value in seroprevalence studies. The diagnostic accuracy of the ELISA and CLIA platforms was comparable.