Tuberculin skin testing ( TST PPD )

  

What is this?
Tuberculin skin test ( Mantoux test ) is the standard method to determine whether or not a person is infected with Mycobacterium tuberculosis. Tuberculin administration and reading outcome requires standardization of procedures, training , supervision and practice .

How to Manage ?
The TST is performed by injecting 0.1 ml of tuberculin purified protein derivative (PPD ) into the inner forearm . The injection should be made with a tuberculin syringe with the needle bevel facing upward. TST is an intradermal injection . When done correctly , the injection produces a small elevation of the skin ( a wheal ) from 6 to 10 mm diameter.

 How to read TST ?
Skin test should be read between 48 and 72 hours after dosing. A patient who comes within 72 hours will be rescheduled for another test . Reaction should be measured in millimeters of induration ( palpable , raised , endured or swelling) . The reader should not measure erythema ( redness ) . The diameter of induration should be measured across the forearm ( perpendicular to the long axis ) .

 

How to interpret the tuberculin skin test ?
Interpretation of skin test depends on two factors:
• measurement in millimeters of induration
• The risk of being infected person tested with BK and disease progression if infected

Classification of tuberculin skin reactions
A induration of 5 mm or more is considered to be positive :
• HIV
• A recent contact with a person with TB
• people with type fibrous pulmonary radiological changes as a result of previous TB
• transplant patients
• individuals who are immunosuppressed from other causes (eg, those receiving the equivalent of prednisone   15mg/zi for a month or more, those receiving treatment with TNF -alpha antagonists )
A hardness of 10 mm or more is considered as positive :
• Recent immigrants (5 years) from countries with high prevalence of TB
• IDUs
• residents and employees of high-risk institutions
• Laboratory staff mycobacteriologic
• people with clinical conditions that place them at high risk
• Children 5 years 
• Children and adolescents exposed to adults in high-risk categories A induration of 15 mm or more is considered positive to any person , including those with a known risk of TB. However, targeted skin testing programs should be directed only to high-risk groups .

What are false-positive reactions ?
Some people may react to the skin test even if you are infected with M. tuberculosis. The cause of false-positive reactions may include, but are not limited to the following :
• nontuberculoase mycobacterial infection
• previous BCG vaccination
• Improper method of administration of tuberculin
• Incorrect interpretation of reaction
• Incorrect vial antigen used in the test
What are false -negative reactions ?
Some people may not react to the skin test even if you are infected with M. tuberculosis. The causes of these false -negative reactions may include , but are not limited to, the following :
• Cutaneous anergy ( anergy is the inability to react to skin tests because of a weakened immune system )
• Recent TB infection ( 8-10 weeks after exposure)
• age very low (less than 6 months)
• recent vaccination with live virus (eg, measles and smallpox )
• particularly severe form of TB
• Some viral diseases (eg measles and chickenpox )
• Improper method of administration of skin test
• Incorrect interpretation of reaction

Who can do a skin test ?
Most people can get a test . IDR is only contraindicated in people who have had a severe reaction (eg, necrosis , blistering, anaphylactic shock , or ulcerations ) to a previous test . It is contraindicated in any other person , including children , pregnant women , people living with HIV or those vaccinated with BCG .

 How often can be repeated skin test ?
In general there is no risk associated with repeat tuberculin test . If a person does not return in 48-72 hours reading test, a new test can be done as quickly . There is no contraindication to repeating IDR ‘s , unless there was a previous severe reaction .

 What is an amplified reaction ( ” booster ” ) ?
In some people infected with M. tuberculosis ability to react to tuberculin may decrease over time. When you are making a TST few years after infection , these people may have a false-negative reaction . However, the skin test may stimulate the immune system , causing a positive reaction and amplified (effect ” booster ” ) in subsequent tests . Perform a second trial after an initial negative reaction is called two-step test .

Why testing is done two times ?
Two-stroke testing is required for initial skin testing of adults who will be retested periodically and employees of health care or institutionalized persons . This two-stage approach can reduce the likelihood of a reaction ” booster ” in subsequent tests to be misinterpreted as a recent infection .

 Skin tests can be administered to persons who receive vaccines ?
Vaccination with live viruses may interfere with skin reaction . In case of skin testing scheduled for testing is as follows:
• either the same day or after live virus vaccination 4-6 weeks after their administration
• After at least one month after vaccination against smallpox.

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