From the Guidelines
The recommended approach for a urine test to detect mycobacteria involves collecting three to six consecutive early morning urine samples and performing both acid-fast bacilli (AFB) smear microscopy and culture, with consideration of nucleic acid amplification tests (NAATs) like PCR when available. This approach is based on the high sensitivity and specificity of mycobacterial cultures in diagnosing extrapulmonary TB, including urinary TB, as noted in the official American Thoracic Society/Infectious Diseases Society of America/Centers for Disease Control and Prevention clinical practice guidelines 1.
When collecting urine samples, it is essential to follow proper protocols to minimize contamination and maximize the detection of mycobacteria. This includes:
- Collecting samples in sterile containers after proper cleansing of the genital area
- Ensuring each sample is at least 40-50 mL in volume
- Processing samples within 24 hours of collection
- Centrifuging samples at high speed (3000g for 20 minutes) to concentrate mycobacteria
- Decontaminating urine samples with N-acetyl-L-cysteine-sodium hydroxide (NALC-NaOH) before processing
The use of both liquid and solid cultures is recommended, as it likely improves the sensitivity of mycobacterial cultures, with liquid cultures providing a more rapid answer and solid cultures serving as a safeguard against contamination 1. However, the quality of evidence regarding the preferred type of cultures is limited, and the recommendation is conditional.
Given the importance of obtaining mycobacterial isolates for drug susceptibility testing (DST) and the potential consequences of delayed diagnosis or incorrect treatment, the benefits of mycobacterial culture outweigh the costs and burdens, making it a crucial component of the diagnostic approach for urinary TB 1.
From the Research
Urine Test for Mycobacteria
- The recommended approach for a urine test to detect mycobacteria is still being researched and developed 2.
- Current methods for detecting mycobacteria in urine include testing for mycobacterial lipoarabinomannan (LAM) and mycobacterial DNA using PCR 2.
- Urine LAM has limited diagnostic utility in unselected tuberculosis suspects, but its test characteristics improve in HIV-infected patients, particularly those with advanced immunosuppression (CD4 cell count <200 cells/microl) 2.
- Methodologies for urine PCR for detection of mycobacterial DNA vary across studies, and there is a need for standardization of assays with respect to specimen collection, assay design, and processing methodology 2.
Diagnostic Approaches
- Traditional diagnostic methods for mycobacteria, such as acid-fast bacilli (AFB) smear, histopathology, and culture, remain the primary methods for detection 3.
- New molecular diagnostic tests, such as broad-range polymerase chain reaction (PCR) and sequencing, have the potential to improve diagnostics, but their value should be investigated to determine the best use of these tools 3.
- A predictive model for nontuberculous mycobacterial infections in AFB smear-positive patients has been established and validated, which may be useful for early differential diagnosis 4.
Clinical Implications
- The incidence and clinical implication of sputum with positive AFB smear but negative mycobacterial culture have been studied, and clinicians should consider the possibility of false positive AFB smear 5.
- The GeneXpert MTB/RIF assay has limited value in the diagnosis of tuberculous peritonitis in patients with end-stage renal failure 6.
- Urine tests for mycobacteria may be useful in specific clinical settings, such as in HIV-infected patients with advanced immunosuppression, but further developmental studies are required to enhance the performance of the assays 2.