AFB Microscopic

Useful For

Quantitative detection of acid-fast bacilli in clinical specimens.

 

Indications

  • Screening for the detection of active tuberculosisdisease
  • Monitoring response to treatment
  • Determination of infectiousness (discontinuation of airborne isolation)

 

Clinical Information

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

Flourochrome (250X
magnification)

Laboratory Report

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+

References

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.

 

Performance

Status

Days

Analytic Time

Maximum Laboratory Time

Specimen Retention

AFB Microscopic

Routine

Mon – Fri 24h 24h 1 month

STAT

Mon – Sun 24h 24h 1 month
TB PCR

Routine

Mon – Sun 24h 48h 1 month
Culture

Routine

Mon – Fri 8 weeks 10 weeks 1 month

 

Method Description

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

Newfoundland &
Labrador Public Health Laboratory

100 Forest Road

St. John’s, NL A1A 3Z9

 
 

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