• RodeOktavija ĐakovićOĐhttp://orcid.org/0000-0001-8023-4314University Hospital for Infectious Diseases "Dr. Fran Mihaljević", Mirogojska 8, 10000, Zagreb, Croatia. orode@bfm.hr.School of Dental Medicine, University of Zagreb, Zagreb, Croatia. orode@bfm.hr., Ivan-Christian Kurolt, Ivan Puljiz, Rok Čivljak, Nataša Cetinić Balent, Renata Laškaj, TiljakMirjana KujundžićMKSchool of Medicine, University of Zagreb, Zagreb, Croatia.Andrija Štampar School of Public Health, Zagreb, Croatia., Radojka Mikulić, and Alemka Markotić.
• University Hospital for Infectious Diseases "Dr. Fran Mihaljević", Mirogojska 8, 10000, Zagreb, Croatia. orode@bfm.hr.
• Eur. J. Clin. Microbiol. Infect. Dis. 2021 Feb 1; 40 (2): 261-268.

AbstractAccording to anti-SARS-CoV-2 seroresponse in patients with COVID-19 from Croatia, we emphasised the issue of different serological tests and need for combining diagnostic methods for COVID-19 diagnosis. Anti-SARS-CoV-2 IgA and IgG ELISA and IgM/IgG immunochromatographic assay (ICA) were used for testing 60 sera from 21 patients (6 with severe, 10 moderate, and 5 with mild disease). The main clinical, demographic, and haemato-biochemical data were analysed. The most common symptoms were cough (95.2%), fever (90.5%), and fatigue and shortness of breath (42.9%). Pulmonary opacities showed 76.2% of patients. Within the first 7 days of illness, seropositivity for ELISA IgA and IgG was 42.9% and 7.1%, and for ICA IgM and IgG 25% and 10.7%, respectively. From day 8 after onset, ELISA IgA and IgG seropositivity was 90.6% and 68.8%, and for ICA IgM and IgG 84.4% and 75%, respectively. In general, sensitivity for ELISA IgA and IgG was 68.3% and 40%, and for ICA IgM and IgG 56.7% and 45.0%, respectively. The anti-SARS-CoV-2 antibody distributions by each method were statistically different (ICA IgM vs. IgG, p = 0.016; ELISA IgG vs. IgA, p < 0.001). Antibody response in COVID-19 varies and depends on the time the serum is taken, on the severity of disease, and on the type of test used. IgM and IgA antibodies as early-stage disease markers are comparable, although they cannot replace each other. Simultaneous IgM/IgG/IgA anti-SARS-CoV-2 antibody testing followed by the confirmation of positive findings with another test in a two-tier testing is recommended.

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