II. Epidemiology
- Clinicians under-read PPDs
III. Mechanism
- Immune Allergic Contact Dermatitis after prior Mycobacterium tuberculosis exposure
- Based on Type 4 Hypersensitivity Reaction (Cell-Mediated, Delayed)
- Detectable 2-12 weeks after Tuberculosis infection
IV. Efficacy: General
- Accurate results are highly dependent on the reader's proficiency in measuring skin induration
- Accuracy statistics are far more variable than these values indicate
- Test Sensitivity: 68.9%
- Test Specificity: 59%
- Positive Likelihood Ratio: 1.68
- Negative Likelihood Ratio: 0.595
- Kendig (1998) Chest 113(5): 1175-7 [PubMed]
V. Efficacy: False Negatives
- False Negative Rate: 10-20% in immunocompetent patients
-
Immunocompromised state
- AIDS
- Immunosuppressant use
- Alcoholism
- Bariatric Surgery
- Malnutrition
- Chronic Kidney Disease
- Sarcoidosis
- Comorbid systemic infection
- Zinc Deficiency
- Lab errors and timing errors
- Testing too soon after exposure (within 6-8 weeks)
- Inaccurate reading of induration or incorrect placement of injection
- Improper handling of TST solution
- Live Vaccine (e.g. MMR Vaccine) within the 6 weeks prior to PPD
VI. Efficacy: False Positives
- Boosting reaction (baseline 2-step PPD will help avoid interpretation as conversion)
- Nontuberculous Mycobacterium (e.g. Mycobacterium Avium Complex in COPD patients)
- Prior BCG vaccine
- Inaccurate reading of induration (e.g. measuring erythema instead of induration)
VII. Indication: Testing for Latent Tuberculosis
-
Mycobacterium tuberculosis testing
- Low risk patients
- Routine screening not recommended
- Screen only if symptoms suggest possible Tuberculosis
- High risk patients
- Low risk patients
- Cases in which TST is preferred over IGRA
- Children younger than 5 years old
VIII. Contraindications
-
BCG vaccine within 1 year of testing
- PPD should otherwise be placed as if BCG not given
IX. Technique
- Inject 0.1 ml Purified Protein Derivative Intradermal
- Injection typically in volar Forearm
- Contains 5 tuberculin units
- Injection should raise initial wheal of 6-10 mm
- Read in 48 to 72 hours after injection
- Reaction is largest at 72 hours
- Reading after 72 hours carries risk of False Negative and should be repeated
- Reading at 72 hours reduces False Negative Rate
- Measure induration (palpable) across Forearm
- Perpendicular to long axis
- Pen Technique
- Draw with pen in from both lateral margins to edge
- Pen stops at induration
- Record result in millimeters (No induration: 0 mm)
- Additional factors
- Anergy testing is not recommended for HIV patients due to variability in results
- BCG vaccine will cause a False Positive (reaction wanes over time)
- However BCG Vaccination status should not be used to interpret the results
- Healthcare workers should have baseline distant exposure testing initially
- Two step performed with second test done at 1-3 weeks after negative result
- A negative on the second test suggests no prior exposure
X. Protocol
- Step 1: Tuberculin Skin Test
- Positive: Go to Step 2
- Negative: Go to Step 3
- Step 2: Obtain Chest XRay and examination
- Positive (cough, fever, abnormal Chest XRay): Treat as Active Tuberculosis
- Negative: Treat as Latent Tuberculosis
- Step 3: Does patient have exposure to Active Tuberculosis?
- Yes: Go to Step 4
- No: No treatment or further testing needed at this time
- Step 4: Repeat Tuberculin Skin Test at 12 weeks after Active Tuberculosis exposure
- Positive: Go to Step 2
- Negative: Treat as Latent Tuberculosis if patient high risk (HIV, Immunocompromised)
XI. Interpretation: PPD under 5 mm
- Negative
- Observe Patient
XII. Interpretation: PPD 5 mm or greater
- Positive if
- HIV Infection
- Recent close Tuberculosis contact
- Clinical findings suggestive of prior or current Tb
- Apical fibronodular changes on Chest XRay (old Tb scarring) or other findings on Chest XRay
- Immunosuppressed
- HIV Infection
- Prednisone >15 mg/day for >1 month (or equivalent other Corticosteroid)
- Immunosuppressants
- Organ transplant
- Management
- Chest XRay and exam for disseminated disease
- Refer to Public Health or Infectious Disease
- See Tuberculosis Prophylaxis
XIII. Interpretation: PPD 10 mm or greater
- Positive if
- Health care workers
- New Immigrant within last 5 years from developing nations
- Intravenous Drug Abuse
- Children under age 5 years old, or children/teens exposed to high risk adults
- Malnutrition
- Diabetes Mellitus
- Cancer
- Chronic Kidney Disease
- Body weight >10% below Ideal Weight
- Silicosis
- Tuberculosis endemic to region
- High risk living environments (prison, Nursing Homes, hospitals, homeless shelter)
- Management
- Chest XRay and exam for disseminated disease
- Refer to Public Health or Infectious Disease
- See Tuberculosis Prophylaxis
XIV. Interpretation: PPD 15 mm or greater
- Positive in all persons (regardless of risk factors)
- Management
- Chest XRay and exam for disseminated disease
- Refer to Public Health or Infectious Disease
- Isolate organism for drug susceptibility testing
- See multi-drug Tuberculosis treatment regimen