TB Skin Testing

Purpose of the TB skin test Tuberculin skin test is used to determine whether a person has TB infection

The Mantoux test Different types of tuberculin tests are available The Mantoux tuberculin skin test is the preferred type because it is the most accurate The tuberculin used in the Mantoux skin test is also known as purified protein derivative or “PPD”

Storage and handling of PPD Date and initial when vial is opened Discard 30 days after opening It is sensitive to light, keep out of light Draw up just prior to injection Store at 35 to 46 degrees F° in a refrigerator or cooler with ice packs

Clean bill of health to both PPD antigens Conclusion of a 2-part 4 year long CDC study “Both Tubersol and Aplisol have equally high specificity and sensitivity” “False-positives not a worry, study concludes” “Best way to avoid false-positives is not to test people unnecessarily”

How is the Mantoux skin test given? Health worker uses a needle and syringe Inject 0.1 ml of 5 tuberculin units of liquid tuberculin (PPD) between the layers of the skin (intradermally) Usually on the forearm (dorsal or volar surface)

Giving the Mantoux tuberculin skin test

TST: infection control reminders Needles should not be recapped, bent, broken, or removed from syringes Safety devices to prevent needlestick injuries are recommended Gloves are not necessary for administering intradermal injections (however, individual institution or agency policies may vary)

How is the skin test read? Test is read by a trained health worker 48 - 72 hours after the tuberculin injection Diameter of the indurated (swelling) area is measured across the forearm Erythema (redness) is not measured Test result is measured in millimeters (mm)

Only the induration is being measured. This is CORRECT.

The erythema is being measured. This is INCORRECT.

How is the skin test classified? Whether a reaction to the Mantoux tuberculin skin test is classified as positive depends on the size of the induration (swelling) and the person’s risk factors for TB

Definition of a positive tuberculin skin test > 5 or more millimeters (mm) Persons known or suspected to have HIV infection Contact to an infectious case of TB Person with an abnormal chest radiograph but no evidence of active TB

Definition of a positive tuberculin skin test (2) > 10 or more millimeters (mm) All other persons 15 or more mm In California, this cut-off is not recognized by public health departments CDC (April 2000) recommends the higher cut-off for groups who may be screened periodically, who have no other risk factors

Recording the PPD test reaction Record the size of the lump or induration in millimeters (mm) Don’t write “negative” or “neg” but record as 00 mm Do write positive results as a number, not positive,” such as 10 mm, 12 mm Refer all PPD test results 10 mm or more to your referral physician or local health department Remember: if not written down, “it didn’t happen” People who are contacts to active TB need a second PPD 10-12 weeks later

Factors that can cause a false-positive reading Infection with non-tuberculous mycobacteria (mycobacterium, other than M.tb or MOTT) Vaccination with BCG

Concerning BCG CDC guidelines state: Positive tuberculin reactions in BCG vaccinated persons usually indicate infection with M. tb San Francisco TB program: Disregard BCG unless given within the last year

Concerning BCG (2) BCG history should not preclude skin testing for TB infection Do not alter your interpretation of the tuberculin skin test reaction because of a history of BCG vaccination

Positive PPD in a person with past BCG vaccination Positive reaction more likely due to TB infection if: Reaction is large Person received BCG long ago Person comes from an area of the world where TB is common Person has been exposed to someone with TB disease Person has a family history of TB

Factors that can cause a false-negative reading Recent TB infection It takes 2 - 10 weeks after TB infection for the body’s immune system to be able to react to the tuberculin Anergy The inability to react to the TB skin test because of a weakened immune system In HIV, anergy increases with decreased immune response (if CD 4 < 200 = 72% anergic)

Factors that can cause a false-negative reading (2) Very young age (< 6 months old) Other viral, bacterial, fungal infections Live virus vaccination (e.g., MMR, varicella) Immunosuppressive drugs (corticosteroids) Overwhelming TB (i.e., miliary disease = up to 50% PPD negative)

False-negative PPD reactions Technique Improper storage Adsorption Improper administration (too deep, too little antigen) Failure to detect/interpret induration

Tuberculin reactor or tuberculin converter? An individual whose tuberculin skin test (PPD) reaction has been documented to change from negative to positive (or to increase by 10 mm or more) during the previous two years Reactor An individual with a positive tuberculin skin test reaction

What is a “boosted” reaction to a TB skin test? Some persons infected with TB in the past lose their ability to react quickly to tuberculin A first TB test may be negative Another test, one week or more later (up to one year) will show a positive reaction

What is a “boosted” reaction to a TB skin test? (2) A “boosted” TB skin test occurs when the first PPD “reminds” the immune system about TB infection from the past The booster effect increases with age: 1% of people ages 20 - 25 years 4% at ages 35 - 39 years 7% of persons over 50 years Thompson NJ, et al, Am Rev Resp Dis 1979;119(4):587-597

Two-step Testing Baseline PPD test Repeat PPD 1 - 3 weeks later PPD = 00 mm (negative results) Repeat PPD 1 - 3 weeks later POSITIVE PPD > 10 mm RESULTS: this is a “boosted” reaction due to TB infection a long time ago PPD < 10 mm RESULTS: person probably does NOT have TB infection

Common myths Myth: “My doctor says I’m allergic to TB skin tests and should not take them” Fact: repeated TB skin tests do not sensitize or make people “allergic” to tuberculin

Common myths (2) Myth: “I used to have a positive TB test, but now I’m negative” Fact: past positive PPD is considered TB infection – Later negative tests maybe due to: t inaccurate multi-puncture test t decreased immune response t should have 2-step test (PPD)

The Francis J. Curry National Tuberculosis Center gratefully acknowledges the permission to reproduce and distribute the graphic resources used in this PowerPoint presentation: Diane Bennett, M.D. Project Officer National Health and Nutrition Examination Survey (NHANES) Tuberculosis Skin Test Project Evelyn Lancaster, R.N., P.H.N. Tuberculosis Program Educator/Nurse Consultant San Diego County, CA Tuberculin Skin Testing videotape Centers for Disease Control and Prevention