Overview
Acid-fast staining demonstrates organisms that retain primary stain after acid-alcohol decolorization, used as early evidence in mycobacterial and selected related organism pathways.
Classification
| Category | Examples of use | Pathway note |
|---|---|---|
| Classic AFB smear | Mycobacteria screen | Follow with culture ± molecular ID |
| Fluorochrome screen | High-throughput respiratory smears | Confirm positives per SOP |
| Modified acid-fast | Partially AF organisms (context) | Not a mycobacterial species claim |
Morphologic Features
Classic mycobacteria appear as beaded acid-fast bacilli; branching partially acid-fast filaments suggest a different differential when Gram and colony features align, always site-contextualize.
Laboratory Characteristics
- Batch controls required
- Smear thickness and decolorization critical
- Concentration procedures per respiratory SOP
- Quantity grading is institutional
Reference Intervals
No universal numeric “normal” for AFB smears, interpret presence/quantity against site expectation and institutional reporting scales. Verify local confirmation and reading criteria.
Clinical and Laboratory Significance
Positive smears raise priority for mycobacterial workup and clinical risk assessment; they do not finalize species identification or guide MLS therapy recommendations.
Differential Considerations
Distinguish stain artifact, partially acid-fast organisms, and true mycobacteria using method, morphology, site, and follow-up culture/molecular results.
Comparison Tables
Smear interpretation cues
| Finding | Supports | Does not establish |
|---|---|---|
| Rare AFB, good quality | Acid-fast organisms present | M. tuberculosis complex species |
| Negative smear | Below smear detection in fields read | Absence of mycobacteria |
| Branching modified-AF filaments | Consider Nocardia-type pathway | Mycobacterial species ID |
Classification Frameworks
Not applicable.
Laboratory Notes
- Hedge quantity scales and fluorochrome-confirm algorithms as local
- Never report AFB smear as finished species ID
- Biosafety for smear prep, follow institutional assessment
References
Authoritative textbooks, guidelines, and reviews supporting this reference entry. Verify reference intervals, critical limits, and reflex criteria against institutional protocols and current guideline editions.
Textbooks
- Forbes BA, Sahm DF, Weissfeld AS. Bailey & Scott's Diagnostic Microbiology. 15th ed. Elsevier; 2020.
- Mahon CR, Lehman DC, Manuselis G. Textbook of Diagnostic Microbiology. 6th ed. Elsevier; 2018.
- Carroll KC, Hobden JA, Miller S, et al. Jawetz, Melnick, & Adelberg's Medical Microbiology. 28th ed. McGraw-Hill; 2019.
- Murray PR, Rosenthal KS, Pfaller MA. Medical Microbiology. 9th ed. Elsevier; 2021.
CLSI and Professional Guidelines
- Clinical and Laboratory Standards Institute. Performance Standards for Antimicrobial Susceptibility Testing. CLSI document M100 (verify current edition and institutional adoption).
- Centers for Disease Control and Prevention / NIH. Biosafety in Microbiological and Biomedical Laboratories (BMBL). Current edition (verify institutional adoption).
Frequently Asked Questions
Common questions about using this Acid Fast Stain foundational topics Medical Laboratory Library reference in Medical Laboratory Science education.
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