Detection of SARS-CoV-2 infection in gargle, spit and sputum specimens

Mäkelä, H.; Poukka, E.; Hagberg, L.; Vo, T.; Nohynek, H.; Ikonen, N.; Liitsola, K.; Helve, O.; Savolainen-Kopra, C.; Dub, T.

The gold standard for SARS-CoV-2 infection diagnosis is RT-PCR from nasopharyngeal specimen (NPS). Its collection involves a close contact between patients and healthcare workers requiring a significant amount of workforce and putting them at risk of infection. We evaluated self-collection of alternative specimens and compared their sensitivity and Ct values to NPS. We visited acute COVID-19 outpatients to collect concomitant nasopharyngeal and gargle specimens and had patients self-collect a gargle and either sputum or spit specimens on the next morning. We included 40 patients and collected 40 concomitant nasopharyngeal and gargle specimens, as well as 40 gargle, 22 spit and 16 sputum specimens on the next day, as 2 patients could not produce sputum. All specimens were as sensitive as NPS. Gargle specimens had a sensitivity of 0.97 (CI 95% 0.92-1,00), whether collected concomitantly to NPS or on the next morning. Next morning spit and sputum specimens showed a sensitivity of 1.00 CI (95% 1.00-1.00) and 0.94 (CI 95% 0.87-1.00), respectively. The gargle specimens had a significantly higher mean cycle threshold (Ct) values, 29.89 (SD 4.63) (p-value <0.001) and 29.25 (SD 3.99) (p-value <0.001) when collected concomitantly and on the next morning compared to NPS (22.07, SD 4.63). Ct value obtained with spit (23.51, SD 4.57, p-value 0.11) and sputum (25.82, SD 9.21, p-value 0.28) specimens were close to NPS. All alternative specimen collection methods were as sensitive as NPS, but spit collection appeared more promising, with a low Ct value and ease of collection. Our findings warrant further investigation.