Duration of respiratory sample stability at -80ºC for SARS-CoV-2 PCR

Objective: To determine the stability of respiratory samples for SARS-CoV-2 PCR at standard laboratory ultra-freezer temperatures (-80°C).

Methods: Five hundred and sixty-five archived, SARS-CoV-2 PCR positive patient specimens received at the Pathology Department of the Indus Hospital & Health Network between January 2021 and June 2021 were retested in June 2021. Samples had been stored at -70°C or below throughout this duration. Sample integrity following storage was assessed as the percentage of samples with reproducible results, and as consistency of cycle threshold (Ct) values between the original testing and the repeat testing.

Results: Of the 565 samples evaluated in this study, 86% gave reproducible results upon retesting. However, there was no correlation between the duration of storage and result reproducibility, though the majority (69% for PCR Target-I and 78% for PCR Target-II respectively) of non-reproducible results had Ct values above 30. Similarly, there was a consistent increase of Ct values upon storage at ultra-freezer temperatures, though the effect again was more contingent upon freezing the sample in the ultra-freezer rather than the duration of storage.

Conclusion: SARS-CoV-2 positive respiratory specimens for PCR can be stored for up to six months at -70°C or below without loss of sample integrity, though there is some loss of PCR-detected viral targets as evidenced by an immediate increased in the PCR-generated Ct values. In addition, samples with initial Ct values above 30 are more likely to give non-reproducible results.