Internationally standardized respiratory viral load testing with limited resources: A derivative-of-care calibration strategy for SARS-CoV-2

Introduction: SARS-CoV-2 has demonstrated that, in targeted circumstances, viral quantification within respiratory specimens can valuably inform patient management, as well as research. Nevertheless, the pandemic has illustrated concomitant challenges for obtaining high-quality (and broadly comparable) respiratory viral loads. This includes a critical need for standardization and calibration, even though the necessary resources may not always be available for emergent pathogens and non-bloodstream specimens.

Methods: To these ends, we describe a novel strategy for implementing quantitative SARS-CoV-2 testing with International Unit-based calibration. Earlier in the course of the pandemic-when analytic resources were far more limited-select residual SARS-CoV-2 positive specimens from routine care in our diagnostic laboratory were pooled to formulate a clinically realistic secondary standard of high volume and analyte concentration, which was cross-calibrated to the primary SARS-CoV-2 standard of the World Health Organization.

Results: The resultant calibrators were integrated into the original CDC RT-qPCR assay for SARS-CoV-2, whose (now broadened) performance characteristics were defined to generate a test appropriate for both clinical and research use. This test allowed for the quantification of virus in respiratory specimens down to a validated lower limit of quantification of 10^3.4 IU/ml.

Conclusions: By self-formulating calibrators from this derivative-of-care secondary standard, we successfully validated respiratory viral loads without the commercial availability (at that time) of quantitative assays or calibrators. As the SARS-CoV-2 pandemic continues to decline-and even beyond this pathogen-this strategy may be applicable for laboratories seeking to implement viral load testing for nontraditional specimen types despite limited resources.