Quantitative detection of acid-fast bacilli in clinical specimens.
- Screening for the detection of active tuberculosisdisease
- Monitoring response to treatment
- Determination of infectiousness (discontinuation of airborne isolation)
Delayed laboratory confirmation of TB leads to delays in initiation of therapy, potentially inappropriate therapy, and missed opportunities to prevent transmission.
Response to treatment
All patients with smear- and culture-positive sputum should have repeat sputum examinations performed at the end of the second month of treatment. If culture remains positive, repeat after 4 months of treatment. In order to report treatment outcome as “cure”, there must be a negative culture at the completion of treatment. More frequent monitoring is recommended if the clinical and radiographic response is unfavorable. Treatment failure is defined as positive cultures after ≥4 months of treatment or two positive sputum cultures in different months during the last 3 months of treatment, even if the final culture is negative.
Discontinuation of airborne isolation in suspect TB cases
Three successive samples of sputum (spontaneous or induced) are negative on smear, unless TB is still strongly suspected, cultures are pending, and no other diagnosis has been made. Each 5-10 ml should be collected 8 – 24 hours apart (or longer of necessary). At least 1 should be collected early in the morning upon wakening.
Discontinuation of airborne isolation in confirmed TB cases
Smear-negative, culture-positive respiratory TB: After 2 weeks of appropriate therapy, as long as there is clinical evidence of improvement.
Smear-positive TB: Three consecutive sputum smears are negative. Each 5-10 ml should be collected 8 – 24 hours apart (or longer of necessary). At least 1 should be collected early in the morning upon wakening. In patients who are no longer able to spontaneously produce sputum specimen, sputum induction is useful and appropriate. More invasive testing, i.e. bronchoscopy, is not recommended for this purpose.
MDR-TB: must remain in airborne isolation for the duration of hospital stay or until three consecutive sputum cultures are negative after 6 weeks’ incubation.
XDR-TB: must remain in airborne isolation for the duration of hospital stay or until three consecutive sputum cultures are negative after 6 weeks’ incubation.
Interpretation AFB Microscopic
The threshold of detection of AFB in concentrated specimens using flourochrome stain is 5,000 – 10,000 bacteria/ml of sputum. The specificity of the AFB smear is high for mycobacteria, but it is important to remember that all nontuberculous mycobacteria (NTM) will be AFB positive. Other organisms, such as Nocardia and Actinomycetes, can be weakly acid-fast, but these are rare. Therefore, a positive AFB smear almost always indicates the presence of mycobacteria, but not necessarily M. tuberculosis.
Number of bacteria seen on microscopy and laboratory interpretation
|0 in 30 fields||Negative|
|1-2 per 30 fields||Report exact number|
|1-9 per 10 field||1+|
|1-9 per field||2+|
|10 – 90 per field||3+|
American Thoracic Society. Diagnostic Standards and Classification of Tuberculosis in Adults and Children. Am J Repir Crit Care Med. 2000; 161:1376-1395. Pfyffer, G, and F Palicova. 2011. Mycobacterium: general Characteristics, Laboratory Detection, and Staining Procedures, p 472 – 502, In Versalovic, J., Carroll, K. C., Funke, G., Jorgensen, J, H., Landry, M. L., and Warnock, D, W. Manual of Clinical Microbiology, 10th ed., vol. 2. ASM Press, American Society for Microbiology, Washington, DC.
Menzies D., and Khan, K. Diagnosis of tuberculosis infection and disease. In Long, R., and Ellis, E. Eds. Canadian Tuberculosis Standards 6th Ed. Public Health Agency of Canada, 2007.
Public Health Agency of Canada. Tuberculosis in Canada 2010. Public Health Agency of Canada, Ottawa, ON. 2012.
Maximum Laboratory Time
|Mon – Fri||24h||24h||1 month|
|Mon – Sun||24h||24h||1 month|
|Mon – Sun||24h||48h||1 month|
|Mon – Fri||8 weeks||10 weeks||1 month|
AFB Microscopic: Auramine fluorescent microscopy
TB PCR: Realtime TaqMan PCR with probe confirmation
Culture: 1) BACTTEC MGIT Liquid Media, 2) LJ Solid Media
Performing Laboratory Location
Labrador Public Health Laboratory
100 Forest Road
St. John’s, NL A1A 3Z9