Challenge of a false-positive acid-fast bacilli: a diagnostic conundrum

Infectious and autoimmune diseases are distinct entities that require opposite therapeutic approaches. However, differentiating between the two can be a challenge, especially when the histopathology misguides the clinician. We highlight the case of a 66-year-old female who presented with fever, shortness of breath and cough for three months duration. She had raised inflammatory markers, and imaging revealed bilateral cavitating nodules. Histopathology revealed necrotising granulomatous inflammation along with acid-fast bacilli on Ziehl-Neelsen staining. The patient was diagnosed with tuberculosis and started on anti-tubercular therapy. When no response was seen, a rheumatologist was consulted, and a suspected diagnosis of granulomatosis with polyangiitis was made on the basis of clinical and laboratory features. Dramatic response to steroids and a negative mycobacteria culture confirmed the diagnosis. The patient responded to a combination of steroids and cyclophosphamide. This case highlights the importance of recognising the possibility of a false-positive acid-fast bacilli report.

The realistic performance achievable with mycobacterial automated culture systems in high and low prevalence settings

Background

Diagnostic tests are generally used in situations with similar pre-test probability of disease to where they were developed. When these tests are applied in situations with very different pre-test probabilities of disease, it is informative to model the likely implications of known characteristics of test performance in the new situation. This is the case for automated Mycobacterium tuberculosis (MTB) liquid culture systems for tuberculosis case detection which were developed and are widely used in low burden settings but are only beginning to be applied on a large scale in high burden settings.

Methods

Here we model the performance of MTB liquid culture systems in high and low tuberculosis (TB) prevalence settings using detailed published data concentrating on the likely frequency of cross-contamination events.

Results

Our model predicts that as the TB prevalence in the suspect population increases there is an exponential increase in the risk of MTB cross-contamination events expected in otherwise negative samples, even with equivalent technical performance of the laboratories. Quality control and strict cross-contamination measures become increasingly critical as the burden of MTB infection among TB suspects increases. Even under optimal conditions the realistically achievable specificity of these systems in high burden settings will likely be significantly below that obtained in low TB burden laboratories.

Conclusions

Liquid culture systems can play a valuable role in TB case detection in laboratories in high burden settings, but laboratory workers, policy makers and clinicians should be aware of the increased risks, independent of laboratory proficiency, of cross-contamination events in high burden settings.

Investigation of Laboratory cross-contamination of Mycobacterium tuberculosis cultures

Many clinicians and laboratory personnel are unaware that a culture positive for Mycobacterium tuberculosis may represent contamination. Laboratory cross-contamination with the M. tuberculosis laboratory control strain (H37Ra) occurs infrequently and therefore demands heightened awareness and recognition. We report 3 occurrences of laboratory cross-contamination from the same laboratory. These occurrences illustrate the importance of interpreting laboratory results in conjunction with the patient's clinical presentation. Failure to recognize laboratory cross-contamination with M. tuberculosis leads to both erroneous administration of unnecessary medications and expenditure of resources required to conduct contact investigations.

Laboratory cross-contamination of Mycobacterium tuberculosis: an investigation and analysis of causes and consequences

BACKGROUND

The misdiagnosis of Mycobacterium tuberculosis infection has many ramifications. These include medical and psychological implications for patients and their families and financial and public health implications for health-care institutions. Microbiology laboratory procedures should minimize the possibility of laboratory cross-contamination of specimens and maximize the ability to recognize a cluster of false-positive cultures. Newer molecular typing methods provide rapid, accurate and effective means of identifying false-positive M. tuberculosis cultures.

AIMS

To investigate a cluster of patients with positive M. tuberculosis cultures that were processed in the mycobacteriology laboratory on the same day.

METHODS

Five patients' medical records and radiology results were reviewed to determine whether the cases were epidemiologically linked and whether there was clinical suspicion of tuberculosis. Restriction fragment length polymorphism (DNA fingerprinting) was performed using repetitive elements IS6110 and pTBN12. Laboratory processing procedures were analysed.

RESULTS

On the basis of DNA fingerprinting using IS6110, the isolates from all five patients were identical. Molecular typing using pTBN12 was performed on four of the five isolates. All four had identical patterns. There was no epidemiological link between the patients. At least three (and probably four) of the five patients were misdiagnosed with tuberculosis.

CONCLUSION

Microbiology laboratories should ensure that appropriate methodologies are in place to avoid cross-contamination of specimens. Clinicians need to critically interpret any positive laboratory result, especially in an unlikely clinical setting.

Pseudo-outbreak of multidrug-resistant Mycobacterium tuberculosis associated with presumed laboratory processing contamination

An investigation prompted by six positive cultures for multidrug-resistant Mycobacterium tuberculosis during a 7-week period found that an unusual resistance pattern, temporal proximity of laboratory processing, and identical DNA fingerprints supported the theory of cross-contamination. Laboratory processing procedures included specimen batching and multi-use vials of buffer solution. Processing procedures were changed and no additional cases of suspected cross-contamination have been observed.

Pseudo-outbreak of Mycobacterium scrofulaceum linked to cross-contamination with a laboratory reference strain

Mycobacterium scrofulaceum was isolated from three clinical specimens over a period of 2 months. In the preceding 10 years, the institution had not reported any cases of this unusual mycobacterium. Investigation and confirmatory molecular testing indicated a laboratory control strain to be the source of contamination.

Rapid identification of laboratory contamination with Mycobacterium tuberculosis using variable number tandem repeat analysis

Compared with solid media, broth-based mycobacterial culture systems have increased sensitivity but also have higher false-positive rates due to cross-contamination. Systematic strain typing is rarely undertaken because the techniques are technically demanding and the data are difficult to organize. Variable number tandem repeat (VNTR) analysis by PCR is rapid and reproducible. The digital profile is easily manipulated in a database. We undertook a retrospective study of Mycobacterium tuberculosis isolates collected over an 18-month period following the introduction of the BACTEC MGIT 960 system. VNTR allele profiles were determined with early positive broth cultures and entered into a database with the specimen processing date and other specimen data. We found 36 distinct VNTR profiles in cultures from 144 patients. Three common VNTR profiles accounted for 45% of true-positive cases. By combining VNTR results with specimen data, we identified nine cross-contamination incidents, six of which were previously unsuspected. These nine incidents resulted in 34 false-positive cultures for 29 patients. False-positive cultures were identified for three patients who had previously been culture positive for tuberculosis and were receiving treatment. Identification of cross-contamination incidents requires careful documentation of specimen data and good communication between clinical and laboratory staff. Automated broth culture systems should be supplemented with molecular analysis to identify cross-contamination events. VNTR analysis is reproducible and provides timely results when applied to early positive broth cultures. This method should ensure that patients are not placed on unnecessary tuberculosis therapy or that cases are not falsely identified as treatment failures. In addition, areas where existing procedures may be improved can be identified.

Review of false-positive cultures for Mycobacterium tuberculosis and recommendations for avoiding unnecessary treatment

We reviewed reports of false-positive cultures for Mycobacterium tuberculosis and here propose guidelines for detecting and managing patients with possible false-positive cultures. Mechanisms of false-positive cultures included contamination of clinical devices, clerical errors, and laboratory cross-contamination. False-positive cultures were identified in 13 (93%) of the 14 studies that evaluated > or = 100 patients; the median false-positive rate was 3.1% (interquartile range, 2.2%-10.5%). Of the 236 patients with false-positive cultures reported in sufficient detail, 158 (67%) were treated, some of whom had toxicity from therapy, as well as unnecessary hospitalizations, tests, and contact investigations. Having a single positive culture was a sensitive but nonspecific criterion for detecting false-positive cultures. False-positive cultures for M. tuberculosis are not rare but are infrequently recognized by laboratory and clinical personnel. Laboratories and tuberculosis control programs should develop procedures to identify patients having only 1 positive culture. Such patients should be further evaluated for the possibility of a false-positive culture.

Nosocomial transmission of tuberculosis (TB) associated with care of an infant with peritoneal TB

Nosocomial transmission of tuberculosis (TB) after exposure to infected peritoneal fluid has not been described. We report the exposure of 111 healthcare workers to infected dialysate from an infant with TB peritonitis. Two (5%) of 39 primary-care nurses, but no doctors or environmental service workers, had apparent tuberculin skin test conversions, raising the concern that patients with peritoneal TB may be a source for nosocomial transmission of TB.