Falso diagnóstico de tuberculosis por cultivo / False diagnosis of tuberculosis by culture

Las herramientas de genotipificación intra-especie de Mycobacterium tuberculosis desarrolladas durante los años 90 no sólo dieron un impulso notable a la epidemiología de la tuberculosis, también pusieron de manifiesto un fenómeno hasta entonces soslayado en los laboratorios de tuberculosis: la contaminación cruzada de muestras. Este error consiste en la transferencia accidental de bacilos de una muestra con alta carga bacilar a la o las procesadas subsecuentemente. La consiguiente aparición de falsos cultivos positivos puede inducir al diagnóstico erróneo de tuberculosis y la instauración de tratamientos prolongados con drogas potencialmente tóxicas. Esa secuencia de errores conduce al mal manejo de los pacientes involucrados, la distracción de los recursos del sistema de salud y la distorsión de los resultados de análisis epidemiológicos. Se detectó contaminación cruzada en todos los laboratorios donde fue investigada sistemáticamente, con tasas de alrededor del 3% de los cultivos positivos. La confirmación requiere confrontar resultados bacteriológicos, clínicos, epidemiológicos y de genotipificación. Realizamos aquí una revisión de la información nacional e internacional sobre el tema y describimos las medidas recomendadas para minimizar el riesgo, vigilar la ocurrencia y evitar las consecuencias clínicas de este error de laboratorio que vulnera la certeza de un cultivo positivo.

A remarkable input to the epidemiology of tuberculosis was not the only benefit of the molecular tools developed in the early nineties for Mycobacterium tuberculosis intra-species differentiation. These genotyping methods served also to unveil specimen crosscontamination, which was until then overlooked in laboratories culturing mycobacteria. This error consists in the accidental carry-over of bacilli from a specimen with high bacterial load to that, or those, processed subsequently. The ensuing detection of falsely positive cultures can result in a wrong diagnosis of tuberculosis and the initiation of a long-lasting treatment with potentially toxic drugs. This series of errors implies the mismanagement of patients, the distraction of public health system resources, and the distortion of epidemiological data. M. tuberculosis laboratory cross-contamination was detected wherever investigated systematically, with a median rate of 3% of all positive cultures. The confirmation of this error requires a critical appraisal of bacteriological, clinical, epidemiological and genotyping results. We present here a review of national and international information on laboratory crosscontamination and describe measures recommended for minimizing the risk, surveying the occurrence, and avoiding clinical consequences of this laboratory error that raises a question on the reliability of a positive culture.

Incidence of False-Positive Cultures of Mycobacterium tuberculosis in A Microbiology Laboratory / 대한임상미생물학회지

BACKGROUND: Mycobacterial false-positive cultures have rarely been recognized in Korea, even though the rate of false-positive cultures of Mycobaterium tuberculosis has ranged from 0.4% to 4.0%. We estimated the false-positive rates by the review of medical records from whom mycobacterial cultures were requested, retrospeaively, after a bout of false-positive cultures was discovered in specimens treated in a single day. METHODS: Of the total 2,245 specimens, including 337 positive cultures of mycobacteria, during the period of January and June 1999, seventy-two specimens that showed colonies less than or equal to 5 colonies were reviewed, and classified as tuberculosis-likely group, tuberculosis-unlikely group and unclassifiable group by the clinical and radiological evidences, anti-tuberculosis therapy, and microbiological results. RESULTS: Tuberculosis-unlikely group was 21 specimens from 20 patients, and unclassifiable group was five specimens from four patients. So, the false-positive rates were estimated as 0.9- 1.1% of total cultures and 6.2-7.7% of positive cultures, according to excluding or including the unclassifiable group. CONCLUSION: Care should be taken for lowering false-positive mycobacterial cultures. Especially when a culture turned out to be positive with low colony isolates, more careful interpretations should be preceded before reporting the results by the review of medical records and communication with physician in charge.