The Essential Guide to Understanding OP Swabs

What are OP Swabs and How Effective are They for COVID-19 Testing?

Oropharyngeal (OP) swabs refer to swabs taken from the back of the throat and tonsillar areas to collect respiratory secretions for detecting respiratory viruses like SARS-CoV-2, the virus that causes COVID-19. With the COVID-19 pandemic, OP swabs have been widely used for diagnosis along with nasopharyngeal (NP) swabs. But how effective are OP swabs compared to the standard NP swabs?

How does OP swab sample collection work?

OP swabs involve inserting a swab into the mouth and swabbing the posterior pharynx and tonsillar areas, avoiding the tongue. The swab tip is then placed into a sterile viral transport medium tube for testing. OP swabs aim to collect respiratory secretions from the throat that may contain virus if infection is present. Proper sample collection technique is important to ensure adequate sampling.

CDC provides instructions on how to correctly collect an OP swab sample, recommending tilting the patient's head back, ensuring good visibility of the throat, and avoiding contact with the tongue, teeth, and cheeks when inserting and removing the swab. The sample should be collected quickly before the patient gags.

What type of swabs can be used for OP sampling?

CDC recommends using synthetic fiber swabs with plastic shafts for optimal specimen collection and to avoid wood shafts that may contain substances that inactivate some viruses and inhibit PCR testing. Unflocked swabs are typically used.

Flocked swabs designed for nasopharyngeal sampling are not required, making OP swabs more readily accessible. However, some studies using flocked swabs for OP sampling showed performance comparable to flocked nasopharyngeal swabs. Further studies are needed on whether flocking improves OP swab sensitivity.

How does OP swab performance compare to nasopharyngeal swabs?

Several studies have compared the performance of OP swabs versus nasopharyngeal (NP) swabs for detecting SARS-CoV-2 by RT-PCR. Overall, OP swabs appear to have moderately high sensitivity, but lower than NP swabs.

A meta-analysis of 6 studies found OP swabs detected SARS-CoV-2 in 84% of positive cases, versus 88% by NP swabs. However, only 68% of cases were positive by both specimen types, indicating imperfect agreement. The percentage detected by NP swabs was unusually low compared to other studies, skewed by one study collecting samples late in hospitalization.

Other studies found 36-86% of cases were positive by OP swabs, versus 79-100% by NP swabs. Two studies using flocked OP and NP swabs showed 97% detected by both methods. However, most compared unflocked OP to flocked NP swabs, which may impact relative sensitivity.

Importantly, studies used different collection techniques, timing of sampling during illness, and patient populations, making direct comparisons difficult. But overall, OP swabs appear moderately sensitive but inferior to NP swabs. More standardized studies are needed.

Does self-collection affect OP swab performance?

Limited data exist on self-collected OP swabs. One study found 14/24 positives detected by self-collected OP swabs versus 22/24 by technician-collected OP swabs. Another study using observed self-collection found 12/12 positives detected by OP and NP swabs.

While promising, current data suggests self-collection may reduce sensitivity versus clinician-collected OP swabs. More studies are needed on self-sampling techniques and performance.

How does OP swab performance vary by timing of collection?

The concentration of virus in the throat likely changes over the course of infection, which could impact OP swab results. However, few studies reported data over multiple time points to assess this.

One study that collected OP and NP swabs near the end of hospitalization in COVID-19 patients found much lower NP swab sensitivity (41%) versus OP swabs (86%). This may indicate higher throat viral levels later in infection, but more studies across different illness stages are needed.

Does OP swab sensitivity differ in asymptomatic versus symptomatic patients?

No studies directly compared OP swab performance in asymptomatic versus symptomatic patients. However, a few studies included both asymptomatic and symptomatic individuals.

One study found 36-86% of symptomatic patients were positive by OP swabs versus 79-88% by NP swabs. Another study reported 52% of previously diagnosed asymptomatic individuals were positive by OP swabs versus 93% by saliva.

While limited, this data suggests OP swabs may have reduced sensitivity in asymptomatic individuals versus those with symptoms. More systematic comparisons are needed.

How does sample processing and PCR assay sensitivity impact results?

No studies assessed if OP swab sample processing impacts results. However, for other specimen types, extracting nucleic acids prior to PCR increased sensitivity versus direct PCR input.

PCR assay sensitivity also varied widely between studies, from 100 to over 1 million copies/mL limit of detection. Studies using more sensitive assays could detect virus missed by less sensitive assays, impacting performance comparisons between specimen types.

Do OP swabs perform differently across respiratory viruses?

Most COVID-19 studies focused on SARS-CoV-2 detection. However, one study compared OP and NP swabs for detecting 8 different respiratory viruses by PCR.

NP swabs were significantly more sensitive for adenovirus, influenza B, and parainfluenza viruses 1-3. OP swabs were more sensitive for pandemic H1N1 influenza. For RSV, hMPV, influenza A H3N2, and parainfluenza 1, there was no significant difference in sensitivity between sample types.

So OP swab sensitivity appears to vary across viruses. However, more studies are needed to clarify patterns.

What are the advantages and disadvantages of using OP swabs?

Potential advantages of OP swabs:

• - More comfortable and easier to tolerate than NP swabs
• - Reduced risk of vifirus aerosolization during sample collection
• - Use less specialized swabs and equipment than NP swabs

Potential disadvantages:

• - May have reduced sensitivity compared to NP swabs
• - Self-collection may reduce sensitivity further
• - Performance may vary by timing of collection during illness course
• - Unclear how collection technique impacts results

Key unresolved questions:

• - What is the impact of flocking, extraction, assay sensitivity, and self-collection on OP swab performance?
• - How does sensitivity compare in asymptomatic versus symptomatic patients at various illness stages?
• - How do OP swabs perform in children?
• - Can sensitivity be improved with technical advances?

Current evidence suggests OP swabs have moderately high sensitivity for SARS-CoV-2, but may be inferior to NP swabs. However, OP swabs are more tolerable and use less specialized equipment. Their accessibility promotes broader testing. More research on ways to maximize OP swab sensitivity is warranted to expand options for reliable diagnosis. Careful attention to collection technique, timing, processing methods, and PCR assay sensitivity is important to optimize performance.

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References

1. 1. Kim C, Ahmed JA, Eidex RB, Nyoka R, Waiboci LW, Erdman D, Tepo A, Mahamud AS, Kabura W, Nguhi M, Muthoka P, Burton W, Breiman RF, Njenga MK, Katz MA. Comparison of nasopharyngeal and oropharyngeal swabs for the diagnosis of eight respiratory viruses by real-time reverse transcription-PCR assays. PLoS One. 2011;6(6):e21610. doi: 10.1371/journal.pone.0021610. Epub 2011 Jun 30. PMID: 21738731; PMCID: PMC3128075.
2. 2. Monita R Patel, Darin Carroll, Emily Ussery, Hilary Whitham, Christopher A Elkins, Judith Noble-Wang, James Kamile Rasheed, Xiaoyan Lu, Stephen Lindstrom, Virginia Bowen, Jessica Waller, Gregory Armstrong, Susan Gerber, John T Brooks, Performance of Oropharyngeal Swab Testing Compared With Nasopharyngeal Swab Testing for Diagnosis of Coronavirus Disease 2019—United States, January 2020–February 2020, Clinical Infectious Diseases, Volume 72, Issue 3, 1 February 2021, Pages 482–485, https://doi.org/10.1093/cid/ciaa759
3. 3. Kim C, Ahmed JA, Eidex RB, Nyoka R, Waiboci LW, Erdman D, Tepo A, Mahamud AS, Kabura W, Nguhi M, Muthoka P, Burton W, Breiman RF, Njenga MK, Katz MA. Comparison of nasopharyngeal and oropharyngeal swabs for the diagnosis of eight respiratory viruses by real-time reverse transcription-PCR assays. PLoS One. 2011;6(6):e21610. doi: 10.1371/journal.pone.0021610. Epub 2011 Jun 30. PMID: 21738731; PMCID: PMC3128075.
4. 4. Desmet, T., Paepe, P.D., Boelens, J. et al. Combined oropharyngeal/nasal swab is equivalent to nasopharyngeal sampling for SARS-CoV-2 diagnostic PCR. BMC Microbiol 21, 31 (2021). https://doi.org/10.1186/s12866-021-02087-4