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Cough plates for microbiological surveillance in cystic fibrosis
  1. Richard J Chavasse,
  2. Rebecca Cordle,
  3. Hawabibee Petkar
  1. Queen Mary’s Hospital for Children, Carshalton, London, UK
  1. Correspondence to:
    Dr R J Chavasse
    Queen Mary’s Hospital for Children, Wrythe Lane, Carshalton, London, UK; richard.chavasse{at}epsom-sthelier.nhs.uk

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Regular surveillance of respiratory tract flora is integral to the routine management of children with cystic fibrosis. In children who are not regular sputum producers, cough swabs are often used as a substitute.1 Maiya et al2 suggest that cough plates are more sensitive than cough swabs, and preferred by children.

We attempted to validate the technique for use in our cystic fibrosis clinic. In total, 52 consecutive children attending the clinic had both cough swab and cough plate specimens taken, as well as sputum if produced. Three different plates were used (chocolate agar; cystine, lactose, electrolyte-deficient plate; and cepacia-selective agar) to cover major pathogens. Cough swabs were routinely inoculated onto seven plates in the laboratory. The children were asked to cough twice on to each plate while it was held 2 inches from the mouth. Cough swabs were obtained using a standard technique.1 All specimens were obtained by the same physiotherapist. The tests were performed in random order (sealed envelope), including the order of use of the different cough plates.

In all, 50 paired specimens were obtained for cough plates and cough swabs (two were improperly labelled and were excluded). A total of 28 children produced sputum, of which 17 specimens cultured recognisable bacterial pathogens; 7 (41%) cough swabs and 7 (41%) cough plates had identical positive information. Two further sets of cough plates and cough swab identified one organism isolated from sputum (Staphylococcus aureus) but missed a second important pathogen (Pseudomonas aeruginosa).

In total, 22 specimens were taken from non-sputum-producing children. Seven positive cultures were present on cough swabs but none on cough plates. These included four isolates of P aeruginosa and three of S aureus.

Cough swabs had a higher sensitivity than previous reports (41% vs 34%–35%),1,2 but cough plates performed less well than the previous report (41% vs 80%).2 In only two instances was the cough plate positive when the cough swab was negative (both in sputum producers). In non-sputum producers we missed seven clinically relevant isolates. Although the children preferred to use cough plates, we will continue to use cough swabs.

References

Footnotes

  • Competing interests: None declared.

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