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AFB Testing

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Also known as: AFB Smear and Culture; TB Culture and Sensitivity; Mycobacteria Smear; Mycobacteria Culture; TB NAAT
Formal name: Acid-Fast Bacillus Smear and Culture and Sensitivity; Mycobacteria tuberculosis Nucleic Acid Amplification Test

At a Glance

Why Get Tested?

To help detect and identify infections caused by Mycobacterium tuberculosis, the cause of tuberculosis (TB), and other Mycobacterium species, which are known as acid-fast bacilli (AFB); to monitor the effectiveness of treatment

When to Get Tested?

When you have signs and symptoms of a lung infection, such as a chronic cough, weight loss, fever, chills, and weakness, that may be due to TB or a nontuberculous mycobacterial (NTM) infection; when you have a positive TB screening test and you are in a high-risk group for progressing to active disease; when you have a skin or other body site infection that may be due to mycobacteria; when you are undergoing treatment for TB

Sample Required?

For suspected cases of tuberculosis lung infections, usually three sputum samples are collected early in the morning on different days. If the affected person is unable to produce sputum, a bronchoscope may be used to collect fluid during a procedure called a bronchoscopy. In children, gastric washings/aspirates may be collected. Depending on symptoms, urine, an aspirate from the site of suspected infection, cerebrospinal fluid (CSF), other body fluids, or biopsied tissue samples may be submitted for AFB smear and culture.

Test Preparation Needed?

None

The Test Sample

What is being tested?

Most samples that are submitted for acid-fast bacilli (AFB) testing are collected because the health practitioner suspects that a person has tuberculosis (TB), a lung infection caused by Mycobacterium tuberculosis. Mycobacteria are called acid-fast bacilli because they are a group of rod-shaped bacteria (bacilli) that can be seen under the microscope following a staining procedure where the bacteria retain the color of the stain after an acid wash (acid-fast). AFB laboratory tests detect the bacteria in a person's sample and help identify an infection caused by AFB.

There are a several types of AFB that may be detected with this testing; however, the most common and medically important ones are members of the genus Mycobacterium. Mycobacterium tuberculosis is one of the most prevalent and infectious species of mycobacteria.

Since TB is transmitted through the air when an infected person sneezes, coughs, speaks, or sings, it is a public health risk. It can spread in confined populations, such as in the home and schools, correctional facilities, and nursing homes. Those who are very young, elderly, or have preexisting diseases and conditions, such as AIDS, that compromise their immune systems tend to be especially vulnerable. AFB testing can help track and minimize the spread of TB in these populations and help determine the effectiveness of treatment.

Another group of mycobacteria referred to as nontuberculous mycobacteria (NTM) can also cause infections. However, only a few of the more than 60 species of mycobacteria that have been identified cause infections in humans. Some examples include Mycobacterium avium-intracellulare complex (MAC), which can cause a lung infection in people with weakened immune systems, and Mycobacterium marinum, which can cause skin infections. (See the article on Nontuberculous Mycobacteria for more details on different types). In addition to TB, AFB testing can help identify infections caused by these nontuberculous mycobacteria.

AFB laboratory tests include:

  • AFB smear — a microscopic examination of a person's specimen that is stained to detect acid-fast bacteria. This test can provide presumptive results within a few hours. It is a valuable tool in helping to make decisions about treatment while culture results are pending.
  • Molecular tests for TB (nucleic acid amplification test, NAAT) — detect the genetic components of mycobacteria and are often done when the AFB smear is positive or TB is highly suspected. Like AFB smears, they can provide a presumptive diagnosis, which can aid in the decision of whether to begin treatment before culture results are available. Results of NAAT are typically available several hours after a sample is collected.
  • AFB cultures to grow the bacteria are set up at the same time as the AFB smears. Though more sensitive than AFB smears, results of cultures may take days to several weeks.
  • Susceptibility testing on the acid-fast bacteria grown in the cultures that are positive will determine the organism's susceptibility or resistance to drugs most commonly used to treat the infection.

See the "How is it used?" section for more details on these tests.

How is the sample collected for testing?

Since M. tuberculosis and M. avium most frequently infect the lungs (pulmonary disease), sputum is the most commonly tested sample. Sputum is phlegm, thick mucus that is coughed up from the lungs. Usually, three early morning samples are collected on consecutive days in individual sterile cups to increase the likelihood of detecting the bacteria.

If a person is unable to produce sputum, a health practitioner may collect respiratory samples using a procedure called a bronchoscopy. Bronchoscopy allows the health practitioner to look at and collect samples from the bronchi and bronchioles. Once a local anesthetic has been sprayed onto the patient's upper airway, the practitioner can insert a tube into the bronchi and smaller bronchioles and aspirate fluid samples for testing. Sometimes, the health practitioner will introduce a small amount of saline through the tubing and into the bronchi and then aspirate it to collect a bronchial washing.

Since young children cannot produce a sputum sample, gastric washings/aspirates may be collected. This involves introducing saline into the stomach through a tube, followed by fluid aspiration.

If the health practitioner suspects TB is present outside of the lungs (extrapulmonary), a condition fairly common in AIDS patients, he or she may test the body fluids and tissues most likely affected. For instance, one or more urine samples may be collected if the practitioner suspects TB has infected the kidneys. A needle may used to collect fluid from joints or from other body cavities, such as the pericardium or abdomen. Occasionally, the practitioner may collect a sample of cerebrospinal fluid (CSF) or perform a minor surgical procedure to obtain a tissue biopsy.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

Common Questions

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Article Sources

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

Herchline, T. and Amorosa, J. (Updated 2014 June 9). Tuberculosis. Medscape Reference [On-line information]. Available online at http://emedicine.medscape.com/article/230802-overview through http://emedicine.medscape.com. Accessed August 2014.

(© 1995–2014). Acid-Fast Smear for Mycobacterium. Mayo Clinic Mayo Medical Laboratories [On-line information]. Available online at http://www.mayomedicallaboratories.com/test-catalog/Overview/8213 through http://www.mayomedicallaboratories.com. Accessed August 2014.

Koirala, J. (Updated 2014 March 14). Mycobacterium Avium-Intracellulare. Medscape Reference [On-line information]. Available online at http://emedicine.medscape.com/article/222664-overview through http://emedicine.medscape.com. Accessed May 2014.

Zieve, D. (Updated 2013 November 20). Acid-fast stain. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003766.htm through http://www.nlm.nih.gov. Accessed August 2014.

(Updated 2014 May). Mycobacterium tuberculosis – TB. ARUP Consult [On-line information]. Available online at http://www.arupconsult.com/Topics/TB.html through http://www.arupconsult.com. Accessed August 2014.

Dugdale, D. (Updated 2011 December 6). Sputum stain for mycobacteria. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003724.htm through http://www.nlm.nih.gov. Accessed August 2014.

Pagana, K. D. & Pagana, T. J. (© 2011). Mosby's Diagnostic and Laboratory Test Reference 10th Edition: Mosby, Inc., Saint Louis, MO. Pp 983-984.

Availability of an Assay for Detecting Mycobacterium tuberculosis, Including Rifampin-Resistant Strains, and Considerations for Its Use — United States, 2013. Morbidity and Mortality Weekly Report October 18, 2013 / 62(41);821-824. Available online at http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6241a1.htm?s_cid=mm6241a1_e through http://www.cdc.gov. Accessed Sept 2014.

Food and Drug Administrations News Release: FDA permits marketing of first U.S. test labeled for simultaneous detection of tuberculosis bacteria and resistance to the antibiotic rifampin. July 25, 2013. Available online http://www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm362602.htm through http://www.fda.gov. Accessed September 2014.

(February 11, 2014) Centers for Disease Control and Prevention. A New Tool to Diagnose Tuberculosis: The Xpert MTB/RIF Assay. Available online at http://www.cdc.gov/tb/publications/factsheets/testing/Xpert_MTB-RIF.htm through http://www.cdc.gov. Accessed September 2014.

Sources Used in Previous Reviews

Thomas, Clayton L., Editor (1997). Taber's Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].

Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby's Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.

Heimerdinger, M. (2000). Tuberculosis. ACP-ASIM [On-line session report]. Available online at http://www.acponline.org/vas2000/sessions/tb.htm through http://www.acponline.org.

Kirchner, J. (2000 February 1). Do Sputum Smears Positive for AFB Signify Treatment Failure? American Family Physician [On-line journal]. Available online at http://www.aafp.org/afp/20000201/tips/25.html through http://www.aafp.org.

Jerant, A. et.al. (2000 May 1). Identification and Management of Tuberculosis. American Family Physician [On-line journal]. Available online at http://www.aafp.org/afp/20000501/2667.html through http://www.aafp.org.

Sadovsky, R. (2001 May 15). Making the Diagnosis of Tuberculosis in Children. American Family Physician [On-line journal]. Available online at http://www.aafp.org/afp/20010515/tips/9.html through http://www.aafp.org.

Centers for Disease Control and Prevention (2002 August 13, Reviewed). Frequently Asked Questions, Questions and Answers about TB. National Center for HIV, STD and TB Prevention Division of Tuberculosis Elimination [On-line information]. Available online at http://www.cdc.gov/nchstp/tb/faqs/qa.htm through http://www.cdc.gov.

Tuberculosis. The Merck Manual of Medical Information--Home Edition [On-line information]. Available online at http://www.merck.com/pubs/mmanual_home/sec17/181.htm through http://www.merck.com.

Tan, L.J. (2000 June 28, Updated). ATS/CDC Statement Revises Treatment and Testing for Latent TB Infection. American Medical Association [On-line information]. Available online at http://www.ama-assn.org/ama/pub/article/1865-2125.html through http://www.ama-assn.org.

Mazurek, G. & Villarino, M. (2002 December 18 Updated). Guidelines for Using the QuantiFERON®-TB Test for Diagnosing Latent Mycobacterium tuberculosis Infection. Centers for Disease Control and Prevention Morbidity and Mortality Weekly Report [On-line report]. Available online at http://www.cdc.gov/mmwr/preview/mmwrhtml/di51cha1.htm through http://www.cdc.gov.

Centers for Disease Control and Prevention (2002 March 23, Reviewed). TB and HIV Coinfection. National Center for HIV, STD and TB Prevention, Division of Tuberculosis Elimination [On-line information]. Available online at http://www.cdc.gov/nchstp/tb/pubs/TB_HIVcoinfection/default.htm through http://www.cdc.gov.

Tuberculosis: Ancient Enemy Present Threat, "I must die": Tuberculosis in History. NAIAD [On-line information]. Available online at http://www.niaid.nih.gov/newsroom/focuson/tb02/story.htm through http://www.niaid.nih.gov.

Tuberculosis (TB). American Lung Association [On-line information]. Available online at http://www.lungusa.org/diseases/lungtb.html through http://www.lungusa.org.

(2002 January). American Lung Association® Fact Sheet: Tuberculosis. ALA [On-line information]. Available online at http://www.lungusa.org/diseases/tbfac.html through http://www.lungusa.org.

(2002 January). American Lung Association® Fact Sheet: Tuberculosis Skin Test. ALA [On-line information]. Available online at http://www.lungusa.org/diseases/tbskinfac.html through http://www.lungusa.org.

(2002 January). American Lung Association® Fact Sheet: Pediatric Tuberculosis. ALA [On-line information]. Available online at http://www.lungusa.org/diseases/pedtbfac.html through http://www.lungusa.org.

NIAID (1999 December 1, Updated). The Global Burden of Tuberculosis. National Institute of Allergy and Infectious Diseases, National Institutes of Health [On-line publication]. Available online at http://www.niaid.nih.gov/publications/blueprint/page2.htm through http://www.niaid.nih.gov.

Tuberculin skin test. Hendrick Health System AccessMed [On-line information]. Available online at http://www.hendrickhealth.org/healthy/00069330.html through http://www.hendrickhealth.org.

Tuberculosis. Hendrick Health System AccessMed [On-line information]. Available online at http://www.hendrickhealth.org/healthy/00069340.html through http://www.hendrickhealth.org.

Mycobacteriology, Introduction to the Genus Mycobacteria. Microbiology Public Health for Northern Alberta, Bugs on the Web [On-line information]. Available online at http://www2.provlab.ab.ca/bugs/mycob/mbintro.htm through http://www2.provlab.ab.ca.

Mycobacteriology, Acid-Fast Staining Procedures for Mycobacteria. Microbiology Public Health for Northern Alberta, Bugs on the Web [On-line information]. Available online at http://www2.provlab.ab.ca/bugs/mycob/afb_stain/afb_tutorial.htm

Hall, G. (2001). Practical Microbiology, Update on Broth Media for the Isolation of Mycobacterium Species. Medscape from Infect Med 18(1):28-30, 2001 [On-line journal]. Available FTP: http://www.medscape.com/viewarticle/410131 through http://www2.provlab.ab.ca.

Tuberculin Purified Protein Derivative (Mantoux) Tubersol. Aventis Pasteur Inc. (Swiftwater, PA) [Package Insert].

(1996 October 28, Rev L). AccuProbe, Mycobacterium Tuberculosis Complex Culture Identification Test. Gen-Probe [Package Insert]. Available online at http://www.gen-probe.com/mtd.html through http://www.gen-probe.com.

(2006 November 3). Notice to Readers: Revised Definition of Extensively Drug-Resistant Tuberculosis. CDC MMWR 55(43);1176 [On-line information]. Available online at http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5543a4.htm through http://www.cdc.gov. Accessed December 10, 2006.

(2006 April). Tuberculosis: General Information. National Center for HIV, STD, and TB Prevention, Division of Tuberculosis Elimination Fact Sheet [On-line information]. Available online at http://www.cdc.gov/nchstp/tb/pubs/tbfactsheets/250010.htm through http://www.cdc.gov. Accessed December 10, 2006.

(2006 March, Revised). Tuberculosis. World Health Organization [On-line information]. Available online at http://www.who.int/mediacentre/factsheets/fs104/en/index.html through http://www.who.int. Accessed December 10, 2006.

(2006 March). Tuberculosis. Accessed December 10, 2006. NIAID Fact Sheet [On-line information]. Available FTP: http://www.niaid.nih.gov/factsheets/tb.htm

(2006 April). Diagnosis of Tuberculosis Disease. National Center for HIV, STD, and TB Prevention, Division of Tuberculosis Elimination Fact Sheet [On-line information]. Available online at http://www.cdc.gov/nchstp/tb/pubs/tbfactsheets/250102.htm through http://www.cdc.gov. Accessed December 10, 2006.

Pagana, Kathleen D. & Pagana, Timothy J. (© 2007). Mosby's Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 955-957.

Moore, DAJ et al. Microscopic-Observation Drug-Susceptibility Assay for the Diagnosis of TB. New England Journal of Medicine 2006;355:1539-1550.

Forbes BA, Sahm DF, Weissfeld AS, Bailey & Scott's Diagnostic Microbiology 12th Edition: Mosby Elsevier, St. Louis, MO; 2007, Pp 478-508.

Centers for Disease Control and Prevention (July 1, 2010 Reviewed). Diagnosis of Tuberculous Disease. Available online at http://www.cdc.gov/tb/publications/factsheets/testing/diagnosis.htm through http://www.cdc.gov. Accessed January 2011.

Centers for Disease Control and Prevention. Updated Guidelines for the use of Nucleic Acid Amplification Tests in the Diagnosis of Tuberculosis. MMWR January 16, 2009 / 58(01); 7-10.  Available online at http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5801a3.htm?s_cid=mm5801a3_e through http://www.cdc.gov. Accessed January 2011.

Herchline, T (December 17, 2010). Tuberculosis. eMedicine article. Available online at http://emedicine.medscape.com/article/230802-overview through http://emedicine.medscape.com. Accessed January 2011.

(Jan. 29, 2009) Mayo Clinic. Tuberculosis. Available online at http://www.mayoclinic.com/health/tuberculosis/DS00372 through http://www.mayoclinic.com. Accessed January 2011.

(November 12, 2010) National Institute of Allergy and Infectious Diseases. Tuberculosis (TB). Available online at http://www.niaid.nih.gov/topics/tuberculosis/Pages/default.aspx through http://www.niaid.nih.gov. Accessed January 2011.

(November 2010) World Health Organization. Tuberculosis Fact Sheet. Available online at http://www.who.int/mediacentre/factsheets/fs104/en/index.html through http://www.who.int. Accessed January 2011.

Boehme C, et. al. Rapid Molecular Detection of Tuberculosis and Rifampin Resistance. N Engl J Med 2010; 363:1005-1015. Available online at http://www.nejm.org/doi/full/10.1056/NEJMoa0907847 through http://www.nejm.org. Accessed January 2011.

Minion J, Leung E, Menzies D, Pai M. Microscopic-observation drug susceptibility and thin layer agar assays for the detection of drug resistant tuberculosis: a systematic review and meta-analysis. Lancet Inf Dis, 2010 Oct;10(10):688-98. Epub 2010 Aug 31. Available online at http://www.ncbi.nlm.nih.gov/pubmed/20813587 through http://www.ncbi.nlm.nih.gov. Accessed January 2011.

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