SwabExpress: An end-to-end protocol for extraction-free COVID-19 testing

Srivatsan S, Heidl S, Pfau B, Martin BK, Han PD, Zhong W, van Raay K, McDermot E, Opsahl J, Gamboa L, Smith N, Truong M, Cho S, Barrow KA, Rich LM, Stone J, Wolf CR, McCulloch DJ, Kim AE, Brandstetter E, Sohlberg SL, Ilcisin M, Geyer RE, Chen W, Gehring J, Seattle Flu Study Investigators, Kosuri S, Bedford T, Rieder MJ, Nickerson DA, Chu HY, Konnick EQ, Debley JS, Shendure J, Lockwood CM, Starita LM. 2021. bioRxiv: 2020.04.22.056283.


Background: The urgent need for massively scaled clinical testing for SARS-CoV-2, along with global shortages of critical reagents and supplies, has necessitated development of streamlined laboratory testing protocols. Conventional nucleic acid testing for SARS-CoV-2 involves collection of a clinical specimen with a nasopharyngeal swab in transport medium, nucleic acid extraction, and quantitative reverse transcription PCR (RT-qPCR). As testing has scaled across the world, the global supply chain has buckled, rendering testing reagents and materials scarce. To address shortages, we developed SwabExpress, an end-to-end protocol developed to employ mass produced anterior nares swabs and bypass the requirement for transport media and nucleic acid extraction.

Methods: We evaluated anterior nares swabs, transported dry and eluted in low-TE buffer as a direct-to-RT-qPCR alternative to extraction-dependent viral transport media. We validated our protocol of using heat treatment for viral activation and added a proteinase K digestion step to reduce amplification interference. We tested this protocol across archived and prospectively collected swab specimens to fine-tune test performance.

Results: After optimization, SwabExpress has a low limit of detection at 2-4 molecules/uL, 100% sensitivity, and 99.4% specificity when compared side-by-side with a traditional RT-qPCR protocol employing extraction. On real-world specimens, SwabExpress outperforms an automated extraction system while simultaneously reducing cost and hands-on time.

Conclusion: SwabExpress is a simplified workflow that facilitates scaled testing for COVID-19 without sacrificing test performance. It may serve as a template for the simplification of PCR-based clinical laboratory tests, particularly in times of critical shortages during pandemics.