1. TB Skin Testing

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

3. 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”

5. 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

6. 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”

7. 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)

11. Giving the Mantoux tuberculin skin test

15. 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)

16. How is the skin test read?
Test is read by a trained health worker
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)

23. Only the induration is being measured. This is CORRECT.

25. The erythema is being measured. This is INCORRECT.

27. 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

28. 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

29. 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

30. 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 weeks later

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

34. 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

35. 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

36. 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

37. Factors that can cause a false-negative reading
Recent TB infection
It takes 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)

38. 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)

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

41. 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

42. 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

43. 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 years
4% at ages years
7% of persons over 50 years
Thompson NJ, et al, Am Rev Resp Dis 1979;119(4):

44. Two-step Testing Baseline PPD test Repeat PPD 1 - 3 weeks later
PPD = 00 mm
(negative results)
Repeat PPD 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

45. 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

46. 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)

49. 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