Over 90% of clinical swabs used for SARS-CoV-2 diagnostics contain sufficient nucleic acid concentrations

During the coronavirus disease 2019 pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), reliable diagnostics are absolutely indispensable. Molecular SARS-CoV-2 diagnostics based on nucleic acids (NA) derived from oro- or nasopharyngeal swabs constitute the current gold standard. Given the importance of test results, it is crucial to assess the quality of the underlying swab samples and NA extraction procedures. We determined NA concentrations in clinical samples used for SARS-CoV-2 testing applying an NA-specific dye. In comparison to cut-offs defined by SARS-CoV-2-positive samples, internal positive controls, and references from a federal laboratory, 90.85% (923 of 1016) of swabs contained NA concentrations enabling SARS-CoV-2 recognition. Swabs collected by local health authorities and the central emergency department either had significantly higher NA concentrations or were less likely to exhibit insufficient quality, arguing in favor of sampling centers with routined personnel. Interestingly, samples taken from females had significantly higher NA concentrations than those from males. Among eight longitudinal patient sample sets with intermitting negative quantitative reverse transcription polymerase chain reaction results, two showed reduced NA concentrations in negative specimens. The herein described fluorescence-based NA quantification approach is immediately applicable to evaluate swab qualities, optimize sampling strategies, identify patient-specific differences, and explain some peculiar test results including intermittent negative samples with low NA concentrations.