What is the protocol for a patient with potential exposure to tuberculosis, including the use of a PPD (Purified Protein Derivative) test?

PPD (Purified Protein Derivative) Testing Protocol

Administration and Reading

The PPD test should be administered using the Mantoux method with 5 TU (tuberculin units) of purified protein derivative injected intracutaneously (0.1 ml) on the volar or dorsal surface of the forearm, producing a 6-10 mm wheal. 1

• The test must be read by trained personnel (not patient self-reading) between 48-72 hours after injection 1
• Only measure the transverse diameter of induration in millimeters; erythema alone does not count 1
• The delayed hypersensitivity reaction typically begins 5-6 hours after injection, peaks at 48-72 hours, and may persist up to 1 week 1

Interpretation Criteria Based on Risk Category

The definition of a positive PPD varies by patient risk factors, with lower thresholds for higher-risk populations:

≥5 mm Induration is Positive for:

• HIV-infected persons or those at increased risk for HIV infection 1, 2
• Recent close contacts of persons with active infectious tuberculosis 2, 3
• Persons with chest radiograph findings consistent with old healed tuberculosis 2, 3

≥10 mm Induration is Positive for:

• Recent converters (≥10 mm increase within 2 years for age <35; ≥15 mm increase for age ≥35) 2
• Intravenous drug users known to be HIV-seronegative 2
• Persons with medical conditions increasing TB risk: silicosis, diabetes mellitus, chronic corticosteroid therapy, immunosuppressive therapy, hematologic malignancies, end-stage renal disease, malnutrition, gastrointestinal surgery 2
• Foreign-born persons from high-prevalence countries, medically underserved populations, residents of long-term care facilities, children <4 years old 2

≥15 mm Induration is Positive for:

• All other persons without specific risk factors 2, 3

Management of Positive PPD Results

All patients with a positive PPD must undergo chest radiography and clinical evaluation to exclude active tuberculosis before initiating preventive therapy. 1, 4

• If chest x-ray shows evidence of old healed TB or active disease, obtain sputum for acid-fast bacilli smear and mycobacterial culture 4
• Active TB must be completely ruled out before starting isoniazid preventive therapy 2

Preventive Therapy Regimens:

Standard regimen: Isoniazid 300 mg daily for adults (10 mg/kg up to 300 mg daily for children) for 6-12 months depending on risk category. 2

• HIV-infected persons: 12 months of isoniazid 2
• Persons with fibrotic pulmonary lesions or silicosis: 12 months of isoniazid OR 4 months of isoniazid plus rifampin 2
• All other positive reactors: 6-9 months of isoniazid 2, 3
• Administer pyridoxine (vitamin B6) concomitantly to prevent peripheral neuropathy, especially in malnourished patients, alcoholics, and diabetics 2

For contacts of multidrug-resistant TB: Use pyrazinamide plus either ethambutol, ofloxacin, or ciprofloxacin instead of isoniazid. 3

Special Considerations for HIV-Infected Patients

HIV-infected persons with negative PPD (<5 mm) who are anergic may still require preventive therapy based on epidemiologic risk assessment. 1

• Anergy testing is no longer routinely recommended by CDC, as it has limited clinical utility 1, 5
• Consider preventive therapy for anergic HIV-infected persons who are close contacts of infectious TB cases or from groups with ≥10% TB infection prevalence 1
• HIV-infected persons should receive PPD testing as soon as possible after HIV diagnosis and within 7 days of any TB exposure 1

Common Pitfalls to Avoid

• Do not use multiple puncture tests—only the Mantoux method is acceptable 5
• Do not accept patient self-reading of results—trained personnel must measure induration 1
• Do not interpret BCG vaccination history as a reason to ignore positive PPD—treat positive results the same regardless of BCG status 1
• Do not use a negative PPD to exclude active tuberculosis—false negatives occur in immunocompromised patients, those with overwhelming disease, and anergic individuals 1, 5, 6
• Do not delay PPD testing in pregnant women who need it—the test is safe during pregnancy with no documented fetal harm 5
• Do not withhold preventive therapy in persons >35 years old if they have additional risk factors (HIV infection, recent conversion, close contact, abnormal chest x-ray, high-risk medical conditions) 2