of course, laboratories like Cook County's are exactly the kind of place where cross-contamination is most likely to occur: high volume, with a large number of ...
JOURNAL OF CLINICAL MICROBIOLOGY, Dec. 1996, p. 3257 0095-1137/96/$04.0010
Vol. 34, No. 12
Specimen Contamination in Mycobacteriology Laboratory Detected by PseudoOutbreak of Multidrug-Resistant Tuberculosis: Analysis by Routine Epidemiology and Confirmation by Molecular Technique A recent paper by Wurtz et al. (2) illustrates the usefulness of DNA fingerprinting in the detection of laboratory crosscontamination with Mycobacterium tuberculosis. However, the section comparing rates of cross-contamination in different studies illustrates the need to clearly define the events being compared. In the study by Wurtz et al., 12 of 441 positive sputum cultures were false positive, out of 4,075 sputum specimens processed. These figures were used to calculate a contamination rate of 0.33% (although 12/4,075 is 0.29%). The authors compare this to the rate obtained in a study by Fischl et al. (1) in which 16% of the patients who had positive cultures did not meet a clinical definition of tuberculosis (1). If one were to take this comparison at face value, one might conclude that the contamination rate in the study by Wurtz et al. was far lower, approximately 1/50 of that in the study by Fischl et al. (1). The numerators in these two contamination rates are the same (the number of patients who had presumed false-positive cultures), but the denominators are very different; Wurtz, et al. (2) use a rate whose denominator was the number of specimens cultured, whereas Fischl et al. used the number of patients who had a positive culture for M. tuberculosis as the denominator (1). Both of these rates are important, the first for the laboratory (number of false-positive cultures/total number of cultures) and the second for the clinician (number of patients having false-positive cultures/total number of patients whose cultures grew M. tuberculosis). Wurtz et al. (2) don’t include the number of patients from whom the 441 positive cultures came; therefore, one can’t calculate the true comparison with Fischl’s study. If one assumes an average of three positive cultures per patient (unpublished data from our laboratory), the percentage of patients having false-positive cultures would be approximately 8%, close to the figure reported by Fischl et al. (1). Wurtz et al. (2) highlight a crucial issue in clinical mycobacteriology, the occurrence of false-positive cultures for M. tuberculosis resulting from laboratory cross-contamination. This issue needs to be carefully studied, and it will be important for such studies to be comparable. We recommend that studies of cross-contamination use two rates: the percentage of falsepositive cultures (number of false-positive cultures/number of specimens which grew M. tuberculosis) and the percentage of patients having false-positive cultures (number of patients whose culture was false positive/total number of patients whose cultures grew M. tuberculosis).
tant tuberculosis: analysis by routine epidemiology and confirmation by molecular technique. J. Clin. Microbiol. 34:1017–1019.
William J. Burman, M.D. Michael L. Wilson, M.D. Randall R. Reves, M.D. Denver Public Health 605 Bannock St. Denver, Colorado 80204
Author’s Reply We appreciate the comments of Drs. Burman, Wilson, and Reves about our paper (1). They believe that (i) we made a mathematical error in calculating the contamination rate and (ii) we did not make the basis for a comparison of various contamination rates clear—we are confusing specimens and patients. To calculate our contamination rate, we took the total number of sputum specimens (4,075) and subtracted the number of true-positive specimens (429) to arrive at a denominator (3,646) for the rate: 4075 2 429 5 3,646. The contamination rate was then calculated as follows: 12/3,646 5 0.00329, or 0.33%. Presumably the number of true-positive cultures must be removed from the denominator, which represents the pool of specimens which can be assessed for contamination. It is not possible to determine if a true-positive specimen is also contaminated. This adjustment results in a higher, not lower, rate of contamination; our goal was to more accurately assess the problem. While the adjustment would not be large in many microbiology laboratories, in a laboratory like Cook County Hospital’s, where the annual number of true positives is quite high, the adjustment is significant. And of course, laboratories like Cook County’s are exactly the kind of place where cross-contamination is most likely to occur: high volume, with a large number of positive specimens. The basis for comparison of various contamination rates is given clearly in the first paragraph on p. 1019 of our article: “Studies have deliberately placed test sputum specimens among clinical specimens and found laboratory contamination rates ranging from 0.4 to 1.3% . . .. When cross-contamination was defined as a positive culture from a patient without signs or symptoms of TB who subsequently did well without therapy, contamination rates as high as 16% have been reported . . ..” Dr. Burman et al. assert that the numerators used are the same, but in fact the numerators are different: the number of culture-positive specimens in the former instance and the number of culture-positive patients in the latter instance. REFERENCE 1. Wurtz, R., P. Demarais, W. Trainor, et al. 1996. Specimen contamination in a mycobacteriology laboratory detected by pseudo-outbreak of multidrugresistant tuberculosis: analysis by routine epidemiology and confirmation by molecular technique. J. Clin. Microbiol. 34:1017–1019.
REFERENCES 1. Fischl, M. A., R. B. Uttamchandi, G. L. Daikos, et al. 1992. An outbreak of tuberculosis caused by multiple-drug-resistant tubercle bacilli among patients with HIV infection. Ann. Intern. Med. 117:177–182. 2. Wurtz, R., P. Demarais, W. Trainor, et al. 1996. Specimen contamination in mycobacteriology laboratory detected by pseudo-outbreak of multidrug-resis-
Rebecca Wurtz, M.D., M.P.H. Evanston Hospital Evanston, Illinois 60201
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