Polyester nasal swabs collected in a dry tube are a robust and inexpensive, minimal self-collection kit for SARS-CoV-2 testing

Abstract

Background: In order to identify an inexpensive yet highly stable SARS-CoV-2 collection device as an alternative to foam swabs stored in transport media, both contrived ("surrogate") CoV-positive and patient-collected spun polyester swabs stored in dry tubes were evaluated for time- and temperature-stability using qPCR.

Methods: Surrogate specimens were prepared by combining multiple, residual SARS-CoV-2-positive clinical specimens and diluting to near-LOD levels in either porcine or human mucus ("matrix"), inoculating foam or polyester nasal swabs, and sealing in dry tubes. Swabs were then subjected to one of three temperature excursions: (1) 4°C for up to 72 hours; (2) 40°C for 12 hours, followed by 32°C for up to 60 hours; or (3) multiple freeze-thaw cycles (-20°C). The stability of extracted SARS-CoV-2 RNA for each condition was evaluated by qPCR. Separate usability studies for the dry polyester swab-based HealthPulse@home COVID-19 Specimen Collection Kit were later conducted in both adult and pediatric populations.

Results: Polyester swabs stored dry demonstrated equivalent performance to foam swabs for detection of low and moderate SARS-CoV-2 viral loads. Mimicking warm- and cold- climate shipment, surrogate specimens were stable following either 72 hours of a high-temperature excursion or two freeze-thaw cycles. In addition, usability studies comprised of self-collected patient specimens yielded sufficient material for molecular testing, as demonstrated by RNase P detection.

Conclusions: Polyester nasal swabs stored in dry collection tubes offer a robust and inexpensive self-collection method for SARS-CoV-2 viral load testing, as viral RNA remains stable under conditions required for home collection and shipment to the laboratory.

Fig 1. Absorption and recovery of specimens from dry polyester swabs.

(A) Dry polyester SARS-CoV-2 surrogate specimens were created across a range of viral loads (1x to 64x LoD). (B) The absorption and recovery of SARS-CoV-2 from dry polyester surrogate specimens was evaluated. N gene mean Ct values (n = 3 replicates) from hot pool serial dilutions tested directly (minus swab) were compared to Ct values from swabs spiked (plus swab) with hot pool serial dilutions.

Fig 2. Dry swab elution by vortex is superior to swab swirling and passive release.

Elution by vortex was compared to swab swirling (A) and passive release (B). Mean Ct values for the SARS-CoV-2 targets and RNase P are shown from paired polyester swabs collected from human volunteers (n = 5) and spiked with the low-positive hot pool (2x LoD; 20 GCE/PCR reaction). Ct values for swab swirling and passive release were all statistically higher (p<0.05) than Ct values from paired vortexed swabs.

Fig 3. Surrogate nasal swabs are stable at elevated temperatures mimicking warm-climate transport conditions.

Dry polyester (blue), dry foam (red), and polyester swabs stored in saline (gray) were stored refrigerated (A, 4°C) or at elevated temperatures (B, 40°C 12 hours, 32°C) through 72 hours. No PCR amplification was assigned a Ct value of 40. *Statistically different, p<0.05.

Fig 4. Clinical and surrogate dry SARS-CoV-2 specimens are stable at temperatures that mimic warm- and cold-climate shipment.

(A) Paired collections from SARS-CoV-2 positive (n = 5; green) and negative (n = 5; black) patients were stored refrigerated and at elevated temperatures though 72 hours prior to RT-qPCR analysis for SARS-CoV-2 targets and RNase P. (B) Paired polyester swabs collected from human volunteers (n = 10) were spiked with the high-positive pool (10x LoD; 100 GCE/PCR reaction) and stored refrigerated or cycled through two freeze thaws.

Fig 5. Usability studies demonstrate feasibility of at-home nasal specimen collection.

Researcher’s assessment of observed difficulty for each step during (A) adult and (B) pediatric usability studies for at-home, nasal specimen self-collection using the HealthPulse@home COVID-19 Specimen Collection Kit.