Tuberculin Skin Test

1. See Also
   1. Tuberculosis
   2. Tuberculosis Risk Factors
   3. Tuberculosis Screening in Children
   4. Mycobacterium Tuberculosis Antigen-Specific Interferon-Gamma Release Assay (IGRA)
2. Epidemiology
   1. Clinicians under-read PPDs
      1. Kendig (1998) Chest 113:1175-7
3. Mechanism
   1. Based on delayed Hypersensitivity Reaction
   2. Detectable 2-12 weeks after Tuberculosis infection
4. Efficacy: False Negatives
   1. False negative rate: 10-20% in immunocompetent patients
   2. Causes
      1. AIDS, Immunocompromised conditions or immunosuppressant use
      2. Alcoholism
      3. Bariatric Surgery
      4. Inaccurate reading of induration or incorrect placement of injection
      5. MMR Vaccine within the 2 months prior to PPD
      6. Malnutrition
      7. Chronic Kidney Disease
      8. Sarcoidosis
      9. Comorbid systemic infection
      10. Zinc Deficiency
5. Efficacy: False Positives
   1. Boosting reaction (baseline 2-step PPD will help avoid interpretation as conversion)
   2. Nontuberculous Mycobacterium (e.g. Mycobacterium Avium Complex in COPD patients)
   3. Prior BCG vaccine
6. Indication: Testing for Latent Tuberculosis
   1. See Tuberculosis Risk Factors
   2. Low risk patients
      1. Routine screening not recommended
      2. Screen only if symptoms suggest possible Tuberculosis
   3. High risk patients
      1. Medical risk factors for Tuberculosis
         1. Human Immunodeficiency Virus (HIV)
         2. Alcoholism
         3. Intravenous Drug Abuse
      2. Immigration from regions with high TB Prevalence
         1. Africa
         2. Asia
         3. Latin America
      3. Resident of long-term care facility
         1. Correctional institution
         2. Psychiatric institution
         3. Nursing Home
      4. Close contacts of high risk patients
      5. Health care workers
7. Contraindications
   1. BCG vaccine within 1 year of testing
      1. PPD should otherwise be placed as if BCG not given
8. Technique
   1. Inject 0.1 ml Purified Protein Derivative Intradermal
      1. Injection typically in volar Forearm
      2. Contains 5 tuberculin units
      3. Injection should raise initial wheal of 6-10 mm
   2. Read in 48 to 72 hours after injection
      1. Reaction is largest at 72 hours
      2. Reading after 72 hours carries risk of false negative and should be repeated
      3. Reading at 72 hours reduces false negative rate
         1. Singh (2002) Chest 122:1299-301
   3. Measure induration (palpable) across Forearm
      1. Perpendicular to long axis
      2. Pen Technique
         1. Draw with pen in from both lateral margins to edge
         2. Pen stops at induration
   4. Record result in millimeters (No induration: 0 mm)
   5. Additional factors
      1. Anergy testing is not recommended for HIV patients due to variability in results
      2. BCG vaccine will cause a false positive (reaction wanes over time)
         1. However BCG vaccination status should not be used to interpret the results
      3. Healthcare workers should have baseline distant exposure testing initially
         1. Two step performed with second test done at 1-3 weeks after negative result
         2. A negative on the second test suggests no prior exposure
9. Protocol
   1. Step 1: Tuberculin Skin Test
      1. Positive: Go to Step 2
      2. Negative: Go to Step 3
   2. Step 2: Obtain Chest XRay and examination
      1. Positive (cough, fever, abnormal Chest XRay): Treat as active Tuberculosis
      2. Negative: Treat as latent Tuberculosis
   3. Step 3: Does patient have exposure to active Tuberculosis?
      1. Yes: Go to Step 4
      2. No: No treatment or further testing needed at this time
   4. Step 4: Repeat Tuberculin Skin Test at 12 weeks after active Tuberculosis exposure
      1. Positive: Go to Step 2
      2. Negative: Treat as latent Tuberculosis if patient high risk (HIV, immunocompromised)
10. Interpretation: PPD under 5 mm
    1. Negative
    2. Observe Patient
11. Interpretation: PPD 5 mm or greater
    1. Positive if
       1. HIV Infection
       2. Recent Tuberculosis contact
       3. Immunosuppressed (e.g. HIV, Prednisone >15 mg/day for >1 month, Immunosuppressants)
       4. Apical fibronodular changes on Chest XRay (old Tb scarring) or other findings on Chest XRay
    2. Management
       1. Chest XRay and exam for disseminated disease
       2. Refer to Public Health or Infectious Disease
       3. See Tuberculosis Prophylaxis
12. Interpretation: PPD 10 mm or greater
    1. Positive if
       1. Health care workers
       2. New immigrant within last 5 years from developing nations
       3. Intravenous Drug Abuse
       4. Children under age 4 years old, or children/teens exposed to high risk adults
       5. Malnutrition
       6. Diabetes Mellitus
       7. Cancer
       8. Chronic Kidney Disease
       9. Body weight >10% below Ideal Weight
       10. Silicosis
       11. Tuberculosis endemic to region
       12. High risk living environments (prison, Nursing Homes, hospitals, homeless shelter)
    2. Management
       1. Chest XRay and exam for disseminated disease
       2. Refer to Public Health or Infectious Disease
       3. See Tuberculosis Prophylaxis
13. Interpretation: PPD 15 mm or greater
    1. Positive in all persons
    2. Management
       1. Chest XRay and exam for disseminated disease
       2. Refer to Public Health or Infectious Disease
       3. Isolate organism for drug susceptibility testing
       4. See multi-drug Tuberculosis treatment regimen
14. References
    1. Jasmer (2002) N Engl J Med 347:1860-6
    2. Hauck (2009) Am Fam Physician 79(10): 879-86