What is the purpose and procedure of a PPD (Purified Protein Derivative) screen for tuberculosis?

PPD Screening for Tuberculosis

The PPD (Purified Protein Derivative) skin test is the primary screening method for detecting tuberculosis infection by measuring the immune response to tuberculin injected intradermally, with results interpreted based on risk factors and induration size. 1

Procedure for PPD Administration

• The Mantoux technique is the preferred method for administering the PPD test, involving the intracutaneous injection of 0.1 mL of PPD (5 tuberculin units) just beneath the skin surface on the volar or dorsal forearm 1
• A proper injection should produce a pale elevation of the skin (wheal) measuring 6-10 mm in diameter 1
• The test must be read by trained healthcare personnel between 48 and 72 hours after injection; self-reading by patients is not acceptable 1
• The reading measures only the induration (hardened area) at the injection site in millimeters, not the erythema (redness) 1

Interpretation of Results

The interpretation of PPD test results varies based on risk factors:

• For high-risk individuals (HIV-infected persons, recent close contacts of TB patients, persons with fibrotic changes on chest X-ray), ≥5 mm induration is considered positive 1
• For moderate-risk individuals (injecting drug users who are HIV-negative, persons with certain medical conditions, residents of correctional facilities, healthcare workers with TB exposure), ≥10 mm induration is considered positive 1
• For low-risk individuals with no known risk factors, ≥15 mm induration is considered positive 1

Clinical Applications

• The primary purpose of PPD testing is to identify individuals with latent tuberculosis infection who may benefit from preventive therapy 1
• PPD testing is particularly important for screening high-risk populations such as healthcare workers, correctional facility residents, and immunocompromised patients 1
• For healthcare workers, baseline PPD testing should be performed during pre-employment physical examinations, with the two-step method recommended for those without documented negative PPD in the preceding 12 months 1
• PPD testing should be followed by chest radiography and clinical evaluation in individuals with positive results to exclude active tuberculosis 1

Follow-up Actions Based on Results

• Negative PPD test: No further treatment required if the test is valid 1
• Positive PPD test without evidence of active TB: Preventive therapy with isoniazid (INH) for 9-12 months is recommended, particularly for HIV-infected individuals 1
• Positive PPD test with suspicious symptoms or abnormal chest radiograph: Further evaluation for active TB is required, including sputum examination 1

Special Considerations

• The two-step testing method may be necessary for baseline testing to detect boosting phenomena that might be misinterpreted as a skin test conversion 1
• PPD testing has limitations in immunocompromised patients who may have anergy (inability to mount an immune response), potentially leading to false-negative results 1, 2
• Previous BCG vaccination can cause false-positive PPD results, but this should not prevent PPD testing in individuals who need screening 1
• PPD shortages have occurred in Europe, affecting TB screening practices and necessitating alternative approaches 3

Common Pitfalls and Caveats

• Improper administration technique can lead to false-negative results 1
• Reading the test outside the recommended 48-72 hour window reduces accuracy 1
• Measuring erythema instead of induration leads to incorrect interpretation 1
• Failure to consider risk factors when interpreting results can lead to inappropriate clinical decisions 1
• Relying on patient self-reading of results is unreliable and not recommended 1