Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1314 page)

   Interpretation

   
Expected results:
Negative. Detection of mycobacteria requires 10,000 or more organisms per milliliter or gram of sample for consistent detection. Sensitivity may be improved by concentration of specimen, such as by centrifugation, and by examination of multiple specimens. Rapidly growing mycobacteria, such as
Mycobacterium fortuitum
, have relatively thin layers of cell wall mycolic acid and may decolorize with acid–alcohol decolorizing solutions. These organisms may be stained using a weaker acid in aqueous solution.

   
Positive results:
Positive specimens are very likely (>90%) to yield growth of mycobacteria in culture. In a minority of patients, usually with cavitary or extensive tuberculosis, sputum AFB stains may remain persistently positive for weeks after patients have converted to negative cultures. Nonviable organisms may be detected by AFB stains.
Nocardia
and related species are weakly acid fast and may give false-positive results if staining protocols are not followed closely.

   Limitations

   Standardized protocols, such as those published by the American Thoracic Society, should be followed carefully to ensure sensitive detection and accurate interpretation of smears.

   
Common pitfalls:
Care must be taken to avoid contaminating slides with acid-fast organisms. Common causes of slide contamination are use of tap water for solution preparation, carryover between slides with immersion oil, and use of common staining chambers.

ACID-FAST STAIN, MODIFIED

   Definition and Use

   This stain may be used for detection of
Nocardia
in patient specimens or culture isolates when nocardiosis is suspected on the basis of clinical presentation or because of typical morphology in culture isolates. The Gram stain is very sensitive for detection of
Nocardia
in patient specimens.

   The modified acid-fast stain is typically used to confirm nocardioform organisms detected by Gram stain. The modified acid-fast stain is useful for differentiating
Nocardia
(positive) from
Streptomyces
(negative), especially in culture isolates. The modified acid-fast stain is similar to the carbol-fuchsin– based acid-fast stains (Ziehl-Neelsen or Kinyoun stains) except that a less active decolorizer is used (1% H
₂
SO
₄
or 3% HCl in aqueous solution). Specimens should be collected and transported as appropriate for routine bacterial cultures for the specimen type.

   
Turnaround time:
24–72 hours

   Interpretation

   
Expected results:
Negative. Negative stains do not rule out nocardiosis. Rapidly growing mycobacteria, such as
M
.
fortuitum
, may be negative by routine acid-fast staining but positive by modified acid-fast staining.