engleski

Molecular testing for severe acute respiratory syndrome Coronavirus 2 (SARS-CoV-2)

Introduction. In accordance with the prescribed legislation in Republic of Croatia all patients prior to elective surgery, transplantation or hospital admission as well as healthcare professionals who have been in contact with a positive person, must undergo molecular testing for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). In some cases the interpretation of molecular test (positive or negative for viral ribonucleic acid) in timely manner is challenging, especially in asymptomatic individuals or patients with unknown epidemiological background and/or medical history. The aim of this study was to present previous experiences of Department of Medical Biochemistry and Laboratory Medicine Merkur University Hospital in molecular test decision making. Materials and methods., Respiratory samples (6500 nasopharyngeal and oropharyngeal swabs) of asymptomatic individuals and symptomatic patients were obtained in Laboratory for Molecular Diagnostics from June to December 2020. Diagnostic detection of SARS-CoV-2 was done by real- time reverse transcription polymerase chain reaction (RT-PCR) according to the Berlin Protocol (Corman et al.) by using two sets of primer and probe sequences (Applied Biosystems, USA) for two (E and RdRp) viral genes. In all assays the same cycle threshold (Ct) value of 34 as verified platform limit of detection (LoD) was used. Results. Of a total 6500 samples 711were considered positive (Ct value < 34) and 5789 negative (Ct value > 34). 28 out of 6500 samples (0, 4%) had Ct value at or near the limit of detection (Ct value 33 or 34). After retesting (24 or 48 hours after) 16 were considered positive (57%) with Ct value < 20, while 12 with higher or the same Ct value as at the initial testing (43%) were considered negative for SARS-CoV-2. Conclusions. Ct value represents the number of amplification cycles that have passed before the target genes are detected. Therefore it can distinguish the lower from higher values of viral RNA in the sample. If a Ct value at or near the limit of detection (Ct value 33 or 34) appears, at first reisolation of viral RNA and repeated RT-PCR from the same sample should be considered to reduce the impact of poor sampling or possible contamination during viral RNA isolation. In case of obtaining identical result, retesting from a new sample after 24 or 48 hours could give reliably result and proper interpretation of a molecular test. Our molecular protocol showed that Ct value of 34 as verified platform LoD gives a very small number of retested samples, reliably distinguishes between positive and negative results and is good decision making in interpretation of a molecular test.

molecular diagnosis ; SARS-CoV-2 infection

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