INTRODUCTION/OBJECTIVE: Accurate SARS-CoV-2 results are crucial for patient management and infection prevention. Result confidence decreases with low viral load because near the assay’s limit of detection (LOD), test results may alternate between positive and negative, as characterized by Poisson distribution for target analytes at low density. Low positive results may indicate past infection, early infection, a vaccinated individual with low level viral shed, or a false-positive result. EUA methods provide guidance on test interpretation, but laboratories should assess clinical accuracy. The purpose of this study was to assess clinical accuracy of specimens with low positive results. METHODS/CASE REPORT: Respiratory specimens were tested by Cepheid Xpert® Xpress SARS-CoV-2 assay with positive results up to a Ct of 45. A low positive (defined as Ct ≥35), which could not be confirmed by Hologic Aptima® SARS-CoV-2 assay was reported as indeterminate and repeat testing recommended. Repeat testing occurred by Cepheid, Hologic, BioFire, Roche, or Quest assays. Retrospectively, final results were extracted from the LIS (Epic Beaker, Madison, WI, version May 2020) for 5-months (12/1/2020 to 5/31/2021), and chart review performed. RESULTS (IF A CASE STUDY ENTER NA): A total of 19,969 tests were performed; 10.4% (n=2,083) were positive, 89% (n=17,728) negative, and 0.79% (n=158) indeterminate. Previous infection (up to 3 months prior) was documented in 18% (n=28) of indeterminate results and defined as true positive. Of remaining indeterminate results, 43% (n=68) had repeat testing as recommended by laboratory; 26% (n=18) were positive and 74% (n=50) were negative. The average number of days between indeterminate and negative result was 7.25 (range 1-38). CONCLUSION: Result discordance occurred in < 1% of all samples, excellent agreement. For low positive samples, discordance was higher, as expected. It’s impossible to determine if negative results from the 50 repeat samples were false-positive by Cepheid or false-negative by other methods. In summary, 32% (50/158) of indeterminant samples did not repeat as positive. Overall concordance was high and results fluctuate when low virus is present. In absence of symptoms, we conclude repeat testing is not routinely recommended. Laboratories must recognize that normal variability occurs near assay LOD and must critically assess performance against other methods with similar LODs to fully assess performance of EUA methods.
Evaluation of indeterminate SARS-CoV-2 results with repeat testing on an alternative platform