Converting to an international unit system improves harmonization of results for SARS-CoV-2 quantification: Results from multiple external quality assessments

BACKGROUND: The detection of SARS-CoV-2 vRNA in clinical samples has relied almost exclusively on RT-qPCR as the gold standard test. Published results from various external quality assessments (“ring trials”) worldwide have shown that there is still a large variability in results reported for the same samples. As reference standards of SARS-CoV-2 RNA are available, we tested whether using standard curves to convert Ct values into copies/mL (cp/mL) improved harmonization. METHODS: Nine laboratories using 23 test systems (15 of which were unique) prepared standard dilution curves to convert C(t) values of 13 SARS-CoV-2 positive samples to cp/mL (hereafter IU/mL). The samples were provided in three rounds of a virus genome detection external quality assessment (EQA) scheme. We tested the precision and accuracy of results reported in IU/mL, and attempted to identify the sources of variability. RESULTS: Reporting results as IU/mL improved the precision of the estimated concentrations of all samples compared to reporting Ct values, although some inaccuracy remained. Variance analysis showed that nearly all variability in data was explained by individual test systems within individual laboratories. When controlling for this effect, there was no significant difference between all other factors tested (test systems, EQA rounds, sample material). CONCLUSIONS: Converting results to copies/mL improved precision across laboratory test systems. However, it seems the results are still very specific to test systems within laboratories. Further efforts could be made to improve accuracy and achieve full harmonization across diagnostic laboratories.