Microbiology Test

Skin Test, Tuberculosis

Dr. Nithin Jayan
Medically Reviewed by Dr. Nithin Jayan, MBBS, DNB
Last Updated on Mar 29, 2014
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Test : Skin Test, Tuberculosis

Indications : A tuberculin skin test is ordered for those suspected of TB infection. They include the following -

People with symptoms of TB such as incessant cough, night sweats, and unexplained weight loss.
People with chest X-ray showing TB symptoms
People who have lived in proximity to a TB patient
Health care workers
Individuals who have undergone a recent organ transplant or those with HIV /AIDS

The site of test on the forearm is cleaned and allowed to dry after which a small shot of PPD is placed under the top layer of the skin. For the Mantoux test, 0.1 mL volume containing 5 TU (tuberculin units) PPD is administered into the top layers of skin on the forearm.

A slight bump can be noticed where the PPD has been placed. A small circle is marked out on this site. Do not apply bandage or scratch even if you feel itchy near the site. If itching persists place a wash cloth on the site and allow to dry.

Go to the doctor after 2-3 days to check the site. Consult a doctor if you have fever, swelling in the arm or nodes in the armpit.

The test must not be done for those with -

Known TB infection
A previous reaction to the TB antigens
Skin rashes, as it becomes difficult to detect the reaction to TB antigen

Certain infections, illnesses like cancer and vaccinations are likely to affect the test. Speak to your doctor regarding the dos and don’ts while doing the test.

Physiology : A skin test for tuberculosis (TB), also known as the tuberculin test, is done to detect immune response to the TB bacterium. The standard tuberculin test used universally is the Mantoux test.

The tuberculin test is usually done when clinical symptoms of TB are present and also to find out if an infection is active (contagious stage) or inactive (latent). The TB antigens used in the tuberculin skin test are known as purified protein derivative (PPD) and hence the test is also known as PPD test.

Risk groups for TB include:

High-risk groupHIV infected, those on long term on corticosteroids or on medications for rheumatoid arthritis or Crohn’s disease
Medium risk - Those who have come in contact with TB patients, young children or homeless people.
Low-risk group – others who have no exposure to risk factors

Normal Range : No reaction.

Interpretation :

Normal -Redness alone at the test site

Abnormal -A firm red bump at the site indicates infection at some time.

A measurement of the firm bump (not the red area) is taken 2 to 3 days after the test.

Mantoux tuberculin skin test
Normal (negative) No firm bump or a bump  that is smaller than 5 mm  may be present
Abnormal (positive) A firm bump that is  5 mm indicates TB infection in high-risk group individuals.
A firm bump that is 10 mm in size indicates TB infection in moderate-risk group individuals.
A firm bump that is 15 mm in size indicates a TB infection in low-risk group individuals.

A positive test does not indicate a current infection. It indicates if the infection is active or passive. Further tests may have to be done for further diagnosis.

Test Method : Tuberculin test

Related Tests : Acid fast stain, Mycobacterium test

References :

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