Susceptibility Testing

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Also known as: Sensitivity Testing; Drug Resistance Testing; Culture and Sensitivity; C & S; Antimicrobial Susceptibility
Formal name: Bacterial and Fungal Susceptibility Testing

At a Glance

Why Get Tested?

To determine the likelihood that a particular antibiotic or antifungal drug will be effective in stopping the growth of the bacteria or fungi causing your infection

When to Get Tested?

As follow up to a positive bacterial or fungal culture; when you have an infection and one or more types of bacteria or fungi have been grown and isolated in a culture from a sample obtained from the site of suspected infection; when your infection is not responding to treatment

Sample Required?

A sample of a pure culture of bacteria or fungi grown and isolated from an infected body site

Test Preparation Needed?

None

The Test Sample

What is being tested?

Susceptibility is a term used when microorganisms such as bacteria and fungi are unable to grow in the presence of one or more antimicrobial drugs. Susceptibility testing is performed primarily on bacteria but also on fungi that have been identified with a culture as causing an individual's infection. Testing is used to determine the potential effectiveness of specific antibiotics on the bacteria and/or to determine if the bacteria have developed resistance to certain antibiotics. The results of this test can be used to help select the drug(s) that will likely be most effective in treating an infection.

Although viruses are microorganisms, testing for their resistance to antiviral drugs is performed differently, so this article is limited to the discussion of bacterial and fungal susceptibility testing.

Bacteria and fungi have the potential to develop resistance to antibiotics and antifungal drugs at any time. This means that antibiotics once used to kill or inhibit their growth may no longer be effective. Susceptibility testing is a way to determine if this is the case when a culture of a sample collected from the site of a suspected infection is positive for the presence of one or more pathogens. (For more about cultures, see specific articles: Blood Culture, Urine Culture, Wound Culture, AFB Smear and Culture, Fungal Tests).

During the culture process, pathogens – if present – are isolated (separated out from any other microorganisms present) and each is identified using biochemical, enzymatic, or molecular tests. Once the pathogens have been identified, a determination can be made as to whether susceptibility testing is required. Susceptibility testing is not performed on every pathogen; there are some that respond to established standard treatments. An example of this is strep throat, an infection caused by Streptococcus pyogenes (also known as group A streptococcus) that can be treated with penicillin.

Susceptibility testing is performed on each type of bacteria or fungi that may be clinically significant in the specimen and whose susceptibility to treatment may not be known. Each pathogen is tested individually to determine the ability of antimicrobials to inhibit its growth. This is can be measured directly by bringing the pathogen and the antibiotic together in a growing environment, such as nutrient media in a test tube or agar plate, to observe the effect of the antibiotic on the growth of the bacteria.

How is the sample collected for testing?

A sample is obtained from a pure culture of bacteria or fungi suspected of causing an infection.

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

(© 1995-2013). Antimicrobial Susceptibility, Aerobic Bacteria, MIC. Mayo Clinic Mayo Medical Laboratories [On-line information]. Available online at http://www.mayomedicallaboratories.com/test-catalog/Overview/8073 through http://www.mayomedicallaboratories.com. Accessed August 2013.

Vorvick, L. (Updated 2013 January 22). Sensitivity analysis. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003741.htm. Accessed August 2013.

Hazen, K. (Revised 2013 February). Susceptibility Testing. Merck Manual for Healthcare Professionals [On-line information]. Available online through http://www.merckmanuals.com. Accessed August 2013.

Street, T. and Schmidt, S. (Updated 2012 October 18) Antimicrobial Susceptibility. Medscape Reference [On-line information]. Available online at http://emedicine.medscape.com/article/2103786-overview through http://emedicine.medscape.com. Accessed August 2013.

McPherson, R. and Pincus, M. (© 2011). Henry's Clinical Diagnosis and Management by Laboratory Methods 22nd Edition: Elsevier Saunders, Philadelphia, PA. Pp 11117-1128.

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.

(2004 April). The Problem of Antibiotic Resistance. National Institute of Allergy and Infectious Diseases [On-line information]. Available online at http://www.niaid.nih.gov/factsheets/antimicro.htm through http://www.niaid.nih.gov.

(2003 April 4). Antibiotic Resistance, A Growing Threat. US Food and Drug Administration [On-line information]. Available online at http://www.fda.gov/oc/opacom/hottopics/anti_resist.html through http://www.fda.gov.

(2004 February 19). Using Antibiotics Sensibly. Mayoclinic.com, Infectious Disease Center [On-line information]. Available online at http://www.mayoclinic.com/invoke.cfm?id=FL00075 through http://www.mayoclinic.com.

(2003 November). Multidrug-Resistant Tuberculosis Fact Sheet. American Lung Association [On-line information]. Available online at http://www.lungusa.org/site/pp.asp?c=dvLUK9O0E&b=35815 through http://www.lungusa.org.

Bren, L. (2003 September, Revised). The Battle of the Bugs: Fighting Antibiotic Resistance. U.S. Food and Drug Administration, FDA Consumer magazine [On-line information]. Available online at http://www.fda.gov/fdac/features/2002/402_bugs.html through http://www.fda.gov.

Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. McPherson RA and Pincus MR, eds. Philadelphia: 2007, Pp 1048-1057.

(March 14, 2009) MedlinePlus Medical Encyclopedia. Sensitivity Analysis. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003741.htm. Accessed May 2009.

Forbes, B. et. al. (© 2007). Bailey & Scott's Diagnostic Microbiology, Twelfth Edition: Mosby Elsevier Press, St. Louis, Missouri. Pp 187-214.

(2007 August 6). Get Smart: Know When Antibiotics Work, Frequently Asked Questions. CDC [On-line information]. Available online at http://www.cdc.gov/getsmart/index.html through http://www.cdc.gov. Accessed June 2009.

Sutphen, S (2007 August 30). Antibiotic Resistance in the Emergency Room: The First Line of Defense. Medscape CME [On-line information]. Available online at http://www.medscape.com/viewarticle/562056 through http://www.medscape.com. Accessed 8-24-08.

Barclay, L. (2008 July 3). Medscape Medical Household Antibacterial Product Use May Promote Microbial Resistance. Medscape Medical News [On-line information]. Available online at http://www.medscape.com/viewarticle/577055 through http://www.medscape.com. Accessed June 2009.

Nicasio, A. et. al. (2008 May 13). The Current State of Multidrug-Resistant Gram-Negative Bacilli in North America. Medscape from Pharmacotherapy [On-line information]. Available online at http://www.medscape.com/viewarticle/572674 through http://www.medscape.com. Accessed June 2009.

(2009 August 27). Treatment Multi-drug Resistant and Extensively Drug-Resistant Tuberculosis: Current Status and Future Prospects. Medscape Reuters Health Information [On-line information]. Available online at http://www.medscape.com/viewarticle/706826 through http://www.medscape.com. Accessed June 2009.