Initial remarks: This blog by Lawrence F Muscarella, PhD, uses a question-and-answer format to reveal the difference between a true and pseudo-outbreak caused by one or more bacterium.
A hypothetical scenario: Several patients from the same critical care unit of a hospital undergo broncho-alveolar lavage (or, BAL) using the same bronchoscope. Each of these patients is being examined for possible pulmonary malignancy. None of these patients are colonized or infected with Mycobacterium tuberculosis. Respiratory specimens are collected from each patient for pathological analysis. Each patient’s respiratory specimen is also microbiologically evaluated for disease, in accordance with the hospital’s protocol, and is found to grow an identical strain of M. tuberculosis. The hospital concludes that it has identified a healthcare-associated outbreak of M. tuberculosis in its critical care unit, and an investigation into the outbreak’s source and cause is initiated. An aggressive course of antibiotic treatment for pulmonary tuberculosis is promptly ordered for each patient. Depending on a number of factors, including the strength of the patient’s immune system, pulmonary tuberculosis can cause permanent lung damage and, if untreated, it can be fatal.
Question: What is wrong with this hospital’s conclusion that it has identified an outbreak of pulmonary tuberculosis in its critical care unit? Why would initiation of an aggressive course of antibiotic treatment for each of these patients suspected of infection with M. tuberculosis be inappropriate?
Investigation and answer: While the conclusion that an outbreak of M. tuberculosis has been identified may be understandable, it is erroneous (i.e., a false-positive result). As mentioned previously, none of the patients are colonized or infected with M. tuberculosis. Use of aggressive anti-tuberculocidal medications to treat these “infected” patients and to stop the spread of this presumed (if phantom) “outbreak” would, therefore, be inappropriate and unjustifiably preemptive.(10,24-28)
Contamination of a patient’s respiratory specimen with a bacterium – for example, with M. tuberculosis – does not necessarily indicate pulmonary tuberculosis. Rather, this same result could indicate instead a pseudo-infection due to environmental contamination of the patient’s specimen via the instrumentation used to collect the specimen (e.g., a bronchoscope).
To be sure, each patient’s respiratory specimen is contaminated with M. tuberculosis. But, as this example demonstrates, contamination of a respiratory specimen does not necessarily indicate infection. Although the mode of transmission of M. tuberculosis is primarily from patient-to-patient,(3-5) the environment can contaminate respiratory specimens with M. tuberculosis(5,7,8,10-13,17,21,58)—a scenario known as a “pseudo-outbreak” of M. tuberculosis.
Examples of environmental sites and surfaces, which include medical instruments, that have been reported to contaminate respiratory specimens with M. tuberculosis and to cause pseudo-infections include bronchoscopes and the surfaces, equipment, and solutions used in the clinical microbiology laboratory to process and culture respiratory specimens.5,7,8,10-13,21
A blog by Dr. Muscarella that discusses a recall of several bronchoscope models in 2001 due to a loose, defective biopsy port can be read by clicking here.
During the hospital’s investigation to determine the source and cause of this presumed “outbreak” of M. tuberculosis, the working channel of the bronchoscope used to perform BAL on each of these patients and to collect a respiratory specimen from each was microbiologically sampled as recommended during an outbreak investigation.(29)
Sampling was achieved by flushing sterile water through the bronchoscope’s working channel via the biopsy port, and a sample of the water, known as the effluent, was aseptically collected at the bronchoscope’s distal tip for microbiological analysis. Cultures of the effluent grew the identical strain of M. tuberculosis identified in each patient’s respiratory specimen.
The investigation confirmed that this was a pseudo-outbreak caused by a damaged bronchoscope.
Closer examination of the bronchoscope revealed that its biopsy port was damaged and contaminated with this strain of M. tuberculosis. The damaged biopsy port protected and shielded the colonies of M. tuberculosis, preventing them from being destroyed during cleaning and sterilization of the bronchoscope after each use. Similar scenarios have been previously reported.(15,16,48) This investigation confirmed that this damaged bronchoscope was the source of this “outbreak.” During BAL, the bronchoscope contaminated each patient’s respiratory specimen with M. tuberculosis, yielding a false-positive, or “pseudo,” result. The aggressive course of antibiotic treatment ordered for each of these patients, therefore, was dubious and inappropriate.
Reference: Click here.
Blog by: Lawrence F Muscarella PhD posted 3-19-2013.