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A range of bacterial species, most commonly Haemophilus influenzae , Streptococcus pneumoniae , Pseudomonas aeruginosa , Staphylococcus aureus and Moraxella catarrhalis , are frequently isolated in sputum from patients with chronic lung disease, 1 - 3 and it is generally accepted that samples must be cultured as soon as possible after collection to obtain representative results. 4 This is largely based on studies showing that the isolation rate of H influenzae and S pneumoniae is lower from "old" specimens compared with fresh samples. 5 The reduced isolation rate has been attributed largely to the action of lysozomal enzymes in sputum and the inability of some bacteria to remain viable outside their normal environment. 5 Sputum provides a good medium for growth of non-fastidious bacteria, and to maintain viability samples are often refrigerated prior to transport to the laboratory. P aeruginosa , however, still multiplies at or near refrigerator temperature (4°C), whereas H influenzae may be killed at low temperatures. 6
There have been limited studies to investigate the effects of storage on the recovery of bacteria from sputum, with varying results observed. Storage at 4°C for 48 h has been reported to produce no detectable change in the numbers of S aureus , P aeruginosa , H influenzae or S pneumoniae recovered, 7 whereas an earlier study of specimens kept at room temperature and transported by post showed a 50% decrease in the isolation rate of H influenzae and S pneumoniae compared with fresh samples from the same patients. 5
The Working Party on the Retention and Storage of Pathological Records and Archives 8 has recommended that all specimens should be retained for a period of 48 h following issue of final reports. Therefore, a prospective study was co-ordinated in three laboratories using individual standard operating procedures. 9 Following storage of samples at 4°C for 48 h, identical results to those obtained initially were obtained in up to 75% of specimens, but H influenzae , M catarrhalis and S pneumoniae failed to be recovered in 8.7% of samples. 9 That study did not, however, include samples from patients with bronchiectasis who are often colonised with several bacteria, and where overgrowth by organisms such as Pseudomonas spp. could be a significant problem.
Receiving samples from...