Bacteriologic smear (e.g. AFB stain) Laboratory Test

A bacteriologic smear is a lab test where a small amount of a sample (such as sputum from the lungs) is spread on a glass slide, stained with special dyes, and examined under a microscope.

Bacteriologic smear (e.g. AFB stain) procedure illustration

Overview

A bacteriologic smear is a lab test where a small amount of a sample (such as sputum from the lungs) is spread on a glass slide, stained with special dyes, and examined under a microscope. An AFB stain looks for acid-fast bacteria like Mycobacterium tuberculosis, the germ that causes tuberculosis (TB). This test helps quickly detect whether acid-fast bacteria are present in a specimen and can suggest if someone may be infectious. It is often used with molecular tests and culture, which can identify the exact bacteria and test for drug resistance.

Overview Preparation Clinical Info Recovery FAQ References

Also known as: AFB smear, Acid-fast stain, Ziehl-Neelsen stain, Kinyoun stain, Mycobacterial smear microscopy

Recovery
Same day
Return to Work
0–1 days

Preparation & Next Steps

Everything you need to know before and after your procedure

Before Care

  • Follow the collection instructions from your clinic or lab, including timing and container handling
  • If giving sputum, collect first thing in the morning if asked; rinse your mouth with water and avoid food, toothpaste, or mouthwash beforehand
  • Practice deep coughing to bring up sputum from the lungs rather than saliva
  • If multiple specimens are ordered, note the schedule; often 2 or 3 samples are collected 8–24 hours apart with at least one early morning sample
  • Label the container as instructed, keep it closed, and avoid touching the inside of the lid
  • Keep the sample cool and deliver it to the lab promptly or as directed
  • Confirm lab hours, location, and any required ID, paperwork, or shipping instructions
  • If induced sputum or bronchoscopy is planned, follow any fasting and transportation instructions from the facility
  • Tell staff about allergies and current medicines at check-in
  • Collect at home in a well-ventilated area if directed, and follow provided safety steps

After Care

  • If you collected sputum at home, seal the container, wash your hands, and clean any surfaces used
  • If you had induced sputum or bronchoscopy, follow the facility’s instructions about eating, activity, and when to resume normal routines
  • Complete any remaining scheduled specimens on time if more than one is ordered
  • Watch your patient portal, phone, or email for preliminary smear results; culture and some molecular results can take longer
  • Keep a simple log of symptoms to share at follow-up
  • Store and carry your lab receipt or requisition in case your insurer requests it
  • Contact the clinic if you develop increasing chest pain, trouble breathing, fever, or more than small streaks of blood after an invasive collection
  • Ask how and when results will be shared and whether a follow-up visit is planned

Clinical Information

Important medical details about this procedure

Indications

  • Symptoms or imaging that suggest tuberculosis (TB), such as a long-lasting cough, fever, night sweats, or weight loss
  • Evaluation of contagiousness in suspected or confirmed TB
  • Testing body fluids or tissues when extrapulmonary TB is suspected
  • Follow-up during TB treatment to monitor smear status
  • Assessment for nontuberculous mycobacterial infections

Alternatives

  • Molecular nucleic acid amplification tests (NAAT) on the specimen
  • Mycobacterial culture with species identification and drug susceptibility testing
  • Interferon-gamma release assay (IGRA) or tuberculin skin test to detect TB infection
  • Chest imaging to support diagnosis and guide sampling
  • Gram stain and standard bacterial culture when non-mycobacterial infection is suspected

Risks

  • Minimal physical risk from basic sputum collection (coughing or throat irritation)
  • Possible sore throat, bleeding, or sedation effects if bronchoscopy is used to collect a sample
  • False negatives if few bacteria are present
  • False positives from non-tuberculous mycobacteria
  • Delays or repeat testing if the specimen is contaminated or inadequate

Contraindications

  • No specific contraindications to the stain itself
  • Contraindications and precautions relate to how the sample is collected (for example, sedation or airway procedures may not be suitable for some people)

Recovery Timeline

What to expect during your recovery

There is no recovery time after routine sputum collection. If sedation or bronchoscopy is used to obtain a sample, many people take it easy the same day and return to normal activities by the next day.

Typical Range

Same day

Return to Work

0–1 days

Recovery Milestones

Day 0

Resume normal daily activities after simple sputum collection

Day 0–1

If sedation or bronchoscopy was used, rest the day of the procedure

Day 1–14

Review results and confirm the follow-up plan with your care team

Frequently Asked Questions

Common questions and expert answers about this procedure

What is an AFB smear?

It is a microscope exam of a stained sample to look for acid-fast bacteria like Mycobacterium tuberculosis, which causes TB.

How is the sample collected?

Most often by coughing up sputum from deep in the lungs. Depending on the situation, induced sputum, bronchoscopy, or tissue or fluid samples may be used.

Do I need more than one sample?

Often 2 or 3 specimens are collected over 1–2 days, with at least one early morning sample, to increase the chance of detecting bacteria.

How fast are the results?

Smear results are often available the same day or within 1 day. Culture can take several weeks, and some molecular tests may return results within hours to days.

Does a negative smear rule out TB?

No. A smear can be negative when few bacteria are present. Culture and molecular tests help confirm or exclude TB.

What does a positive smear mean?

It shows acid-fast bacteria are present. Further testing (molecular tests and culture) helps identify the species and check for drug resistance.

Do I need to fast or stop medicines?

Fasting is usually not needed for sputum collection. Follow any instructions from the lab or facility, especially if sedation or bronchoscopy is planned.

How does a smear differ from culture and NAAT?

A smear gives a quick look for bacteria. Culture grows the organism for identification and drug testing. NAAT detects bacterial genetic material and is faster than culture.