ct2vl: Converting Ct Values to Viral Loads for SARS-CoV-2 RT-qPCR Test Results

ABSTRACT RT-qPCR is the de facto reference method for detecting the presence of SARS-CoV-2 genomic material in infected individuals (1). Although RT-qPCR is inherently quantitative and despite SARS-CoV-2 viral loads varying by 10 orders of magnitude and therefore being potentially highly clinically informative, in practice SARS-CoV-2 RT-qPCR results are usually reported qualitatively as simply positive or negative. This is both because of the mathematical complexity of converting from C t values to viral loads and because the same C t value can correspond to orders-of-magnitude differences in viral load depending on the testing platform (2, 3, 4). To address this problem, here we present ct2vl , a Python package designed to help individual clinical laboratories, investigators, and test developers convert from C t values to viral loads on their own platforms, using only the data generated during validation of those platforms. It allows any user to convert C t values to viral loads and is readily applicable to other RT-qPCR tests. ct2vl is open source, has 100% code coverage, and is freely available via the Python Package Index ( PyPI ). IMPORTANCE Up to now, COVID-19 test results have been reported as positive vs. negative, even though “positive” can mean anywhere from 1 copy of SARS-CoV-2 virus per milliliter of transport media to over 1 billion copies/mL, with attendant clinical consequences. Democratizing access to this quantitative data is the first step toward its eventual incorporation into test development, the research literature, and clinical care.