Impact of Laboratory Cross-Contamination on Molecular Epidemiology Studies of Tuberculosis

Abstract

Laboratory cross-contamination by Mycobacterium tuberculosis is known to be responsible for the misdiagnosis of tuberculosis, but its impact on other contexts has not been analyzed. We present the findings of a molecular epidemiology analysis in which the recent transmission events identified by a genotyping reference center were overestimated as a result of unnoticed laboratory cross-contamination in the original diagnostic laboratories.The phenomenon of misdiagnosing tuberculosis by laboratory cross-contamination when Mycobacterium tuberculosis is cultured has been widely reported (3, 4-8, 10, 11). The production of aerosolized particles after the processing of smearpositive specimens, cultures positive for M. tuberculosis, or positive control strains may be responsible for the inoculation of other specimens processed on the same day or of reagents used for the decontamination of specimens (5). False positivity is suspected (i) if M. tuberculosis is cultured from a sample processed together with a smear-positive specimen, (ii) if M. tuberculosis is cultured from only one of the cultures in the set (usually with a low yield of bacteria), and (iii) if the clinician is considering an alternative diagnosis, that is, a diagnosis other than tuberculosis (TB). Suspicion of false positivity is increased when two or more of these conditions are met. Finally, if molecular analysis is available, cross-contamination is confirmed when the strains cultured from both truly infected and contaminated specimens share the same genotypic pattern and no epidemiological links can be found between the cases. Several studies, some of which are based on molecular analysis, have estimated that the rate of laboratory cross-contamination for M. tuberculosis ranges from 0.1% to 3%, although massive contamination has caused up to 65% of false-positive cases (11).False-positive results for tuberculosis have been a matter of concern because of the clinical, therapeutic, and social impacts of the misdiagnosis of tuberculosis. The economic load associated with each misdiagnosed case of tuberculosis has been estimated to be $32,618 (9). However, another area on which false positivity has an impact but which has received little attention is the misidentification of recent transmission events by molecular epidemiology studies. Molecular epidemiology is based on the analysis of the genotypes of cultured M. tuberculosis isolates to identify cases infected by the same M. tuberculosis strain. These cases are defined as clustered and are considered to be caused by recent transmission events and to belong to the same transmission chain. If an analysis to determine the existence of potential false-positive cases is not performed before molecular analysis, as a quality control of microbiological procedures, there is a risk of misassigning clustered cases. This refined preanalysis is not usually performed because molecular epidemiology studies are generally run by laboratories which are different from those which culture M. tuberculosis from clinical spec...

Laboratory cross-contamination of Mycobacterium tuberculosis in Northwest of Iran

“…In some laboratories staff can distinguish possible false positive results by their experiences according to phenotypic methods, unusual antibiogram pattern and low number of colonies in Agar medium [12]. But molecular typing methods increased sensitivity of methods for detecting cross-contaminations and these methods were introduced for quality control of TB laboratories [4]. In this study we used IS6110-RFLP and VNTR-ETR methods for investigating possible cross-contamination.…”

“…The IS6110 fingerprinting patterns were compared by visual examination. A cluster of M. tuberculosis was defined as two or more isolates which exhibited the same number of copies of the IS6110 fragment with identical molecular sizes [4].…”

“…Hence, the possibility of false positive report is high. Considering the above mentioned reasons, we can consider possible cross-contamination [4]. Laboratories are using different methods for investigating cross-contamination of M. tuberculosis, but conventional methods are not able to detect all contaminations.…”

Molecular Characterization of Mycobacterium bovis and its Significance: Role for Control of Zoonotic Tuberculosis in Africa

“…In addition, it increased the epidemiological knowledge of M. bovis population and structure, evolutionary steps which is advantageous for the study of TB outbreaks and for understanding the dynamics of the disease in the area [57][58][59][60][61]. This will help in the control of TB strains distribution and treatment regimen by indicating the active transmission and reactivation of a Mycobacterium strain.…”

Qualitative Analysis To Ascertain Genotypic Identity of or Differences between Mycobacterium tuberculosis Isolates in Laboratories with Limited Resources

“…Although this strategy is of little use for universal molecular epidemiology programs, it might be able to solve many of the practical problems often faced by clinical mycobacteriology laboratories, namely, resolution of laboratory cross-contamination alerts (7)(8)(9), discrimination between reactivation and exogenous reinfection (10), analysis of suspected microepidemics (11,12), and tracking of the spread of specific high-risk strains (9,(13)(14)(15).…”