Acid-Fast Stain, Modified

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Subject: Acid-Fast Stain, Modified

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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% H2SO4 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.

  • Positive results with Nocardia: Delicate, branching filaments that retain the carbol-fuchsin stain.

Limitations

  • Nocardia may stain poorly in direct staining of patient specimens. Other species of aerobic actinomycetes, such as Rhodococcus equi and occasionally coryneform bacteria, may be modified acid-fast stain positive.

Suggested Readings

Al-Moamary  M, Black  W, Bessuille  E, et al. The significance of the persistent presence of acid-fast bacilli in sputum smears in pulmonary tuberculosis. Chest  1999;116:726–731.
Winn  WC Jr, Allen  SD, Janda  WM, et al. Koneman's Color Atlas and Textbook of Diagnostic Microbiology, 6th ed. Baltimore, MD: Lippincott Williams & Wilkins; 2006.
 
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