Background SARS-CoV-2 RNA quantities, measured by reverse transcription quantitative PCR (RT-qPCR), have been proposed to stratify clinical risk or determine analytical performance targets. We investigated reproducibility and how setting diagnostic cut-offs altered the clinical sensitivity of COVID-19 testing. Methods Quantitative SARS-CoV-2 RNA distributions (Cq and copies/mL) from more than 6000 patients from three clinical laboratories in UK, Belgium and the Republic of Korea were analyzed. Impact of Cq cut-offs on clinical sensitivity was assessed. The June/July 2020 INSTAND EQA scheme SARS-CoV-2 materials were used to estimate laboratory reported copies/mL and to estimate the variation in copies/mL for a given Cq. Results When the WHO suggested Cq cut-off of 25 was applied, the clinical sensitivity dropped to as little as about 16%. Clinical sensitivity also dropped to as little as about 27% when a simulated LOD of 106 copies/mL was applied. The inter-laboratory variation for a given Cq value was >1000 fold in copies/mL (99% CI). Conclusion While RT-qPCR has been instrumental in the response to COVID-19, we recommend Cq (Ct or Cp) values not be used to set clinical cut-offs, or diagnostic performance targets, due to poor inter-laboratory reproducibility; calibrated copy-based units (used elsewhere in virology) offer more reproducible alternatives. We also report a phenomenon where diagnostic performance may change relative to the effective reproduction number (R). Our findings indicate that the disparities between patient populations across time are an important consideration when evaluating or deploying diagnostic tests. This is especially relevant to the emergency situation of an evolving pandemic.

The material requirements for printing gel food with a fused deposition modeling 3D printer were determined based on fidelity, shape retention, and extrudability, as described by the rheological parameters of storage modulus (G’), yield stress (τ0), and phase angle (δ). The material requirements were determined for printing gel food using three formulations containing gelatin, gelatin and pectin, and gum mixture as the gelling agents. As compared with formulations based on gelatin alone, pectin-containing gelatin-based formulations yielded higher δ and lower G’ and τ0 values, while gum mixture-based formulations formed a gel with higher G’ and δ values and a wider range of τ0. Overall, this study presents quantitative material requirements for printing gel products containing gelatin, gelatin–pectin, and gum mixtures.

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