A study published by Tsang and others in The Lancet April 12th 2021 looked carefully into this important issue. 

They argued that the comparative performance of different clinical sampling methods for the diagnosis of SARS-CoV-2 infection by PCR among populations with suspected infection remained unclear.

They systematically searched databases and included original clinical studies that examined the performance of nasopharyngeal swabs and any additional respiratory specimens for the diagnosis of SARS-CoV-2 infection amongst individuals presenting in ambulatory care.  

Diagnostic performance, including sensitivity, specificity, positive predictive value and negative predictive value, were examined using random effects models and double arcsine transformation. 

Of the 5577 studies identified in their research, 23 studies including 7973 participants with 16,762 respiratory samples were included.

Respiratory specimens examined in these studies included 7973 nasopharyngeal swabs, 1622 nasal swabs, 6110 saliva samples, 338 throat swabs and 719 pooled nasal and throat swabs. 

Using nasopharyngeal swabs as the gold standard, pooled nasal and throat swabs gave the highest sensitivity of 97% whereas lower sensitivities were achieved by saliva at 85% and nasal swabs at 86% and a much lower sensitivity by throat swabs at 68%. 

Comparably high positive predictive value was obtained by pooled nasal and throat and nasal swabs and a slightly lower positive predictive value by saliva.  Throat swabs had the lowest positive predictive value at 75%. 

The review suggested that compared with a gold standard of nasopharyngeal swabs, pooled nasal and throat swabs offered the best diagnostic performance of the alternative sampling approaches for diagnosis of SARS-CoV-2 infection in ambulatory care.

Saliva and nasal swabs gave comparable and very good diagnostic performance. 

Throat swabs gave a much lower sensitivity and positive predictive value and were not recommended.  

Self-collections of pooled nasal and throat swabs and nasal swabs were not associated with any significant impairment of diagnostic accuracy.

The London General Practice always use both nose and throat swabs in the identification of SARS-CoV-2 by PCR. 
The London General Practice is approved by the government for all forms of coronavirus testing by PCR.  

It undertakes fit to fly, test to release and day two and day eight COVID testing.  

The London General Practice has a close relationship with its laboratory and samples received before 12 noon are able to achieve results the same day.

Dr Paul Ettlinger

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