Dr Francis Gilchrist

Cough swabs commonly used in children with chronic wet cough miss most infections detected in the lungs, researchers report, raising questions about the test’s value in guiding treatment for protracted bacterial bronchitis.

In a multicentre study published in Thorax [link here] comparing airway sampling strategies, UK researchers found swabs detected only a small fraction of the pathogens identified through bronchoscopy, the current gold standard for sampling the lower airways.

They say cough swabs should not be relied upon to guide treatment decisions in children with suspected protracted bacterial bronchitis (PBB).

“The extremely low pathogen yield of cough swab and its high false negative rate mean that as well as not being a viable alternative to FB-BAL, it also has no clear role in the early assessment of children with PBB,” wrote the authors, led by Dr Francis Gilchrist of University Hospitals of North Midlands, UK.

Identifying the bacteria responsible for protracted bacterial bronchitis could help clinicians tailor antibiotic therapy, but that bronchoalveolar lavage performed during flexible bronchoscopy was invasive, required general anaesthesia and was typically reserved for children with recurrent or treatment-resistant disease, they said.

As a result, many children were treated empirically, often with broad-spectrum antibiotics, without microbiological confirmation of the causative organism, said the team who set out to test whether simpler airway samples could provide reliable microbiological information earlier in the disease course. 

“If a less invasive strategy could successfully sample the lower airways of children with PBB, it could be performed in the outpatient clinic providing microbiology data for more children and earlier in their patient journey,” they argued.

Comparing airway sampling methods

Researchers prospectively enrolled 137 children aged one to 10 years with protracted bacterial bronchitis who had been referred for bronchoscopy at four UK tertiary paediatric respiratory centres.

On the day of the procedure, each child provided three airway samples:

• a cough swab
• an induced sputum sample
• bronchoalveolar lavage (BAL) collected during bronchoscopy.

To better understand how bacteria were distributed across the lungs, investigators compared the non-invasive samples with single-lobe, two-lobe and six-lobe BAL results, treating six-lobe BAL as the reference standard.

The bacteria most commonly identified were the organisms typically associated with protracted bacterial bronchitis: Haemophilus influenzae, Moraxella catarrhalis, Streptococcus pneumoniae and Staphylococcus aureus.

Cough swabs frequently miss infection

Only 21% of cough swabs identified a pathogen, compared with 60% of induced sputum samples and 78% of six-lobe BAL samples.

When the results were compared directly with bronchoscopy findings, cough swabs and BAL disagreed in nearly three-quarters of cases.

The mismatch was largely explained by cough swabs failing to detect organisms that were present deeper in the lungs.

“Both cough swab and induced sputum had high levels of discordance with BAL6, but the cause of this differed between the two sampling strategies,” the investigators wrote.

“For cough swab, the vast majority of the discordance was caused by the cough swab being falsely negative as reflected by its high discordance OR.”

Overall, cough swabs detected just 8% of the pathogens identified on bronchoscopy, suggesting the test has limited value in identifying lower airway infection, investigators said.

Induced sputum offers more information

Induced sputum samples identified pathogens far more frequently than cough swabs, detecting bacteria in around 60% of cases.

However, they still failed to capture many infections detected through bronchoscopy and, unlike cough swabs, the mismatch between sputum and bronchoscopy was not driven only by missed infections. In some cases, sputum samples identified pathogens that were not detected on BAL.

Across children who provided all three sample types, 22 pathogens were identified only in sputum samples.

“Although BAL6 is usually assumed to be the gold standard when sampling the lower airway, 22 pathogens were identified on induced sputum that were not isolated on the paired BAL6 samples,” the authors wrote.

“This suggests that in some cases, IS samples can identify a causative PBB pathogen missed on FB-BAL.”

Although sputum samples did not perform as well as multi-lobe bronchoscopy, the authors said the findings highlighted a potential clinical role.

“The percentage of pathogen positive induced sputum samples was almost three times higher than that for cough swab and slightly higher than BAL1,” they wrote.

“While it did not perform as well as BAL2 and BAL6, this demonstrates its potential role in PBB.”

Feasibility in young children

Induced sputum has rarely been used in children with protracted bacterial bronchitis, partly because clinicians have questioned whether younger children can reliably produce sputum samples.

In the CLASSIC-PBB study, however, the technique proved feasible in most cases.

“In this cohort, the success rate of induced sputum at producing a mucoid sample was 85%, with 6% not tolerating the procedure and 9% failing to produce a sample,” investigators said, stressing the need to counsel parents and children ‘carefully’ before an induced sputum in attempted. 

Younger children were more likely to require suction during the procedure, although age did not influence whether a sample could ultimately be obtained.

They concluded that, while imperfect, induced sputum may provide useful microbiological information earlier in the diagnostic pathway in children who do not require bronchoscopy.

The authors also suggest that incorporating induced sputum into early assessment could reduce the number of bronchoscopies performed in children with protracted bacterial bronchitis, although bronchoscopy remains necessary in recurrent disease or when sputum results are negative despite ongoing symptoms.