Effect of storage and postage on recovery and quantitation of bacteria in sputum samples

Abstract

Aims: The aims of the study were to compare bacterial recovery following storage of sputum samples at 20°C room temperature and 4°C (refrigerated) for 24 h, and to determine the effect of postage on viable bacterial numbers.

Methods: A total of 38 individual sputum samples from clinically stable patients with bronchiectasis were split into three equal aliquots and quantitative bacterial culture was performed (i) immediately, (ii) following storage at 4°C for 24 h or (iii) following storage at 20°C for 24 h. A further 42 sputum samples were split into two equal aliquots and quantitative bacterial culture was performed either immediately or following postage back to the laboratory by first-class mail from an outside location.

Results: The predominant organism could still be recovered following storage at 4°C and 20°C, but viable numbers were significantly reduced following storage at 4°C (p<0.004) by at least an order of magnitude (10-fold) in 24% of samples stored at 4°C compared with only 8% stored at 20°C. Posting samples back to the laboratory did not affect the recovery of bacterial species and there was no difference in viable numbers isolated.

Conclusions: The results suggest that storage at room temperature is preferable to refrigeration as it retains the species isolated and the viable number. The data also confirm that sputum samples can be posted to the laboratory from patients in the community without affecting qualitative or quantitative results.