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Persistent tuberculosis or specimen contamination?
K Trakas1, V Stosor, S Dietrich
1Department of Pathology, Northwestern University Medical School, Chicago, IL 60611, USA.
Archives of Pathology & Laboratory Medicine
|June 3, 2000
Summary
Cross-contamination in sputum cultures can lead to false positives for tuberculosis. This case highlights how a susceptible Mycobacterium tuberculosis strain contaminated a resistant one, misdiagnosing a patient.
Area of Science:
- Microbiology
- Infectious Diseases
- Clinical Diagnostics
Background:
- Sputum culture is crucial for diagnosing tuberculosis (TB).
- Sequential processing of patient specimens can lead to cross-contamination.
- Mycobacterium tuberculosis (M. tuberculosis) is the causative agent of TB.
Observation:
- A patient with acquired immunodeficiency syndrome (AIDS) and suspected persistent TB had one positive sputum culture for rifampin-susceptible M. tuberculosis.
- The positive specimen was processed immediately after a sample from a patient with active pulmonary TB, which was positive for rifampin-resistant M. tuberculosis.
- Molecular strain typing confirmed both isolates were the same M. tuberculosis strain.
Findings:
- Agar proportion testing of the resistant isolate showed 1.5%–3.3% resistant organisms.
- The rifampin-susceptible M. tuberculosis (98% of the resistant isolate) contaminated the patient's sputum sample.
- The positive culture result was determined to be a false positive due to cross-contamination.
Implications:
- This case underscores the critical importance of stringent laboratory protocols to prevent cross-contamination in mycobacterial cultures.
- False-positive results can lead to unnecessary treatment and patient anxiety.
- Accurate diagnostic methods are essential for effective tuberculosis management, especially in immunocompromised individuals.