The Essential Guide to Understanding OP Swabs
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?
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.
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.
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.
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.
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.
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.
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.
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.
Potential advantages of OP swabs:
Potential disadvantages:
Key unresolved questions:
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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