Recognized in 2019 in Wuhan, China, the new SARS-CoV-2 coronavirus is responsible for the occurrence of a global pandemic disease called COVID-19. So far, confirmation of infection is based on the detection of virus RNA in a sample taken from a person meeting the suspected case definition. However, in the laboratory diagnosis of SARS-CoV-2 infections, in addition to genetic tests, serological methods can also be used to detect specific antibodies of the IgM, IgG and IgA class produced after contact with antigens or to detect viral antigen. Currently, a number of rapid immunochromatographic, chemiluminescent and ELISA immunoassay tests developed by different manufacturers for the diagnosis of COVID-19 are available on the market. Despite this fact, so far there is no WHO or ECDC recommendations or even reliable research regarding the usefulness of serological investigations in the laboratory diagnosis of infections caused by SARS-CoV-2.

Significance and Impact of the Study: This study showed that serological analysis greatly complements bacterial isolation and helps in the diagnosis and confirmation of Shiga toxin (verotoxin)-producing Escherichia coli (STEC) infections. Serological tests can be performed to qualify the patient for the typical haemolytic uraemic syndrome (STEC-HUS). In Poland, STEC-HUS in children is mostly caused by the E. coli serotype O26, which indicates that there is an increasing number of non-O157 STEC infections associated with human diseases in Europe. AbstractTypical haemolytic uraemic syndrome (STEC-HUS), caused by Shiga toxin (Stx)-producing Escherichia coli (STEC), is a serious, life-threating disease that mainly affects children. Bacteriological and genetic tests are commonly used in the routine laboratory diagnosis of STEC-HUS; however, serological methods have emerged as useful and reliable diagnostic tools, especially when bacterial isolation fails. In this study, we present the results of the serological investigation of 72 paediatric patients suspected for HUS, hospitalized during 2011-2019 at the Department of Pediatrics and Nephrology of Children's Hospitals in Poland. During the routine laboratory investigation STEC strains were isolated only from nine stool samples. However, serological investigations confirmed 45 cases of STEC infections in children with HUS. In this study, 22 (48Á9%) paediatric patients were infected by E. coli serotype O26, 11 (24Á4%) by serotype O145, 9 (20Á0%) by serotype O157, and 3 (6Á7%) by E. coli serotype O111. In the majority of these patients, in addition to a high level of IgA, IgG and IgM antibodies to lipopolysaccharide of particular E. coli serotypes, antibodies to recombinant proteins Tir, Stx2b and intimin were detected. Our results confirm that serological tests are useful in the diagnosis of STEC-HUS.

Introduction: Tularemia is a highly infectious zoonotic disease caused by Gram-negative bacterium Francisella tularensis. The microbiological diagnosis of tularemia is based mainly on serological investigations. The present study was undertaken to determine the avidity of IgG class antibodies to Francisella tularensis in the course of tularemia in humans and to evaluate its value for estimation of the phase of diseases. Methods: Fifty two serum samples obtained from 40 patients with tularemia were tested by in-house ELISA in duplicate in the same plate, without and after the 0.5 h incubation with 8M urea. The age of the subjects was between 6 and 77 years. From one patient, a 9-years-old girl with oculoglandular form of tularemia, five serum samples were taken, respectively after 0.5, 1.5, 3, 6 and 12 months from the beginning of the first clinical symptoms. Results: The results of the study showed higher values of the avidity index (AI) of IgG antibodies for F. tularensis, often exceeding the value of 0.9, in children and adolescents than in adults. The examination of serum samples obtained 2-3 times in the course of tularemia from few patients did not show significant differences in the level of avidity index depending on the period of the disease. However, in five serum samples obtained from a 9-years-old girl in the different phases of tularemia the avidity index showed increasing values (0.51, 0.80, 0.92, 0.90 and 0.94, respectively). Conclusions: The avidity index of IgG may be helpful in excluding recent infection, but its usefulness in detecting an active phase of invasion requires further research.

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