Quantitative sputum culture as a means of excluding false positive reports in the routine microbiology laboratory

Abstract

The bacteriology of human excretions, particularly sputum and urine, has given rise to considerable difficulties in interpretation. Kass (1957) contributed much to the better understanding of the significance of non-catheter urine culture when he introduced the concept of colony counts, in an effort to distinguish between infected urine and urine from normal persons contaminated by urethral flora. Lower respiratory tract infections may likewise show a heavy growth of commensal organisms, because of the necessity to traverse a heavily contaminated oropharynx. In respect to sputum sampling there are additional difficulties in view of the lack of homogeneity within the specimen. Results of sputum culture have been improved following the development of digestion techniques, but in using these combined with direct culture there is still difficulty in deciding whether the bacteria grown are significant or not, and no reliable measurement of bacterial numbers is possible. Dixon and Miller (1965) investigated the value of diluting sputum before culture. They concluded that a routine dilution of 10-4 excluded the great majority of contaminating organisms and allowed a more meaningful report to be made. Monroe, Muchmore, Felton, and Pirtle (1969) quantitated microorganisms in the sputum of a group of patients with pneumonia, and they found that probable pathogens occurred in numbers of 107 organisms per ml or greater. This work was confirmed and extended by Pirtle, Monroe, Smalley, Mohr, and Rhoades (1969).Received for publication 19 January 1972. A critical analysis of sputum culture results from our laboratory showed that there was frequently a good growth of a potential pathogen when clinical evidence for infection was equivocal and there was a tendency for the laboratory to make an excessive number of false positive reports using standard culture methods. These false positive reports are misleading and make it difficult to manage cases effectively, and they may lead to unwarranted and undesirable antibiotic usage.The combined technique of sputum homogenization and quantitative culture has definite advantages, but to be of use in the busy diagnostic laboratory it needs to be relatively simple and economical of time and materials. Mucoid sputum is a difficult material to process and with this in mind we have adopted a dilution technique which is practical, simple, and quick. The aim of the present study has been to utilize this technique for sputum dilution and quantitative culture in an endeavour to improve the correlation between the laboratory report and the patient's actual clinical condition. The presence of potential pathogens in sputum at a number greater than 107 organisms per ml was considered to be compatible with active infection, and this level of significance is the same as that adopted by Monroe et al (1969).A series of 200 consecutive specimens of sputum was cultured using the dilution technique. Results were compared with a standard straight plating method in which sputum was homogenized but not dilut...

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