Some Covid-19 infections could be missed by inefficient swabbing

Inaccurate swab collection could increase false negative rates, say authors of Beaumont study

 The GAA Handball alley at Croke Park is being used as a HSE Covid test centre for the public. Photograph: Alan Betson

The GAA Handball alley at Croke Park is being used as a HSE Covid test centre for the public. Photograph: Alan Betson


Some Covid-19 infections could be missed through poor technique when patients are being swabbed for a test, new Irish research suggests*.

With Covid-19 putting health systems under huge pressure, and public health measures to control the virus heavily reliant on testing, a team of researchers at Beaumont Hospital in Dublin decided to test the quality of swabbing, which is used to collect samples for analysis in the laboratory.

This is the first time the quality of nasopharyngeal swabbing technique has been tested in this way, they believe.

The study comes as the National Public Health Emergency Team (Nphet) announced three additional deaths from Covid-19, bringing the total to 1,830. There were also 811 confirmed cases of the virus announced on Tuesday evening.

More than 1.3 million people in Ireland have so far been tested for Covid-19, starting with a sample taken by a swab inserted inside the back of the throat and nose. The sample is then placed in a fluid and transported to a laboratory for processing using a technique known as PCR (Polymerase Chain Reaction), which looks for DNA fragments of the virus.

PCR is regarded as the “gold standard” of Covid-19 testing but is reliant on swabbing being carried out efficiently to pick up sputum from inside the patient’s nose and throat.

Hospital Report

Confirmed cases in hospital Confirmed cases in ICU
263 64

In the Beaumont study, the technique of 229 staff members was assessed using a swab navigated in a three-dimensional model head in the hospital over two days.

The model is normally used by ear, nose and throat (ENT) surgeons in the hospital to plan procedures, according to study co-author Dr Eoghan de Barra. The progress made by the “guinea pig” swabbers could be viewed and measured on a computer screen.

Swab angle and length

According to the study, technique was poor, with a success rate of swabbing the nasopharynx at 38.6 per cent. The angle and length of insertion of the swab varied significantly between those with successful and unsuccessful technique.

Doctors were significantly more accurate at swabbing than nurses and non-healthcare professionals, the study found.

In a properly executed test, the swab is inserted deep into the nasal passages and then rotated 360 degrees. According to Dr de Barra, the swabbing “shouldn’t hurt, but you’ll know when it’s done”.

The authors say inaccurate specimen collection from poor swab technique could contribute to a high false negative rate of testing for Covid-19. They suggest the provision of training in the anatomy of the nose and throat and in swab technique to improve accuracy levels.

Dr de Barra said it wasn’t yet known whether bad technique ultimately impacted on the positivity of tests, adding that researchers in other countries had found that swabbing the front of the throat could provide acceptable samples.

The study was presented by physician associate Kellie Nwaokorie last weekend at the academic meeting of the Irish Otorhinolaryngology/Head and Neck Society, where it won the prize for best oral presentation.

*Clarification: Dr de Barra added on Wednesday that the study was carried out in April at the height of pandemic. Since then, further training has been provided to testers which will have improved the quality of samples collected.

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