At what induration size is a Purified Protein Derivative (PPD) test considered positive in an individual with close contact to a patient with active Tuberculosis (TB)?

PPD Interpretation in Close Contacts of Active TB Patients

For individuals with close contact to a patient with active tuberculosis, a PPD induration of ≥5 mm is considered positive. 1, 2

Risk-Stratified Interpretation of PPD Results

The interpretation of tuberculin skin test (TST) results is risk-stratified, with different cut-off values based on the individual's risk factors:

5 mm or greater (considered positive in):

• Recent close contacts of TB cases
• HIV-infected persons
• Persons with fibrotic changes on chest radiograph consistent with prior TB
• Organ transplant recipients and other immunosuppressed patients
• Persons receiving TNF-α inhibitors 1, 3

10 mm or greater (considered positive in):

• Recent immigrants from high-prevalence countries
• Injection drug users
• Residents and employees of high-risk congregate settings
• Mycobacteriology laboratory personnel
• Persons with clinical conditions that increase TB risk (diabetes, chronic renal failure, leukemias, etc.)
• Children younger than 4 years of age
• Infants, children, and adolescents exposed to adults in high-risk categories 1, 3

15 mm or greater (considered positive in):

• Persons with no known risk factors for TB 1, 3

Clinical Implications for Close Contacts

Close contacts of individuals with active TB represent a high-priority group for TB screening and preventive therapy due to:

• Higher likelihood of recent infection with M. tuberculosis
• Increased risk of progression to active TB disease
• Public health importance of breaking the chain of transmission 1

The lower threshold of 5 mm for close contacts ensures higher sensitivity for detecting latent TB infection in this high-risk population. This is particularly important since the risk of developing active TB is highest within the first two years following infection 1.

Management Considerations

When a close contact has a positive PPD result (≥5 mm):

• Rule out active TB disease with symptom assessment and chest radiography
• If active TB is excluded, initiate treatment for latent TB infection
• The recommended regimen for latent TB infection is isoniazid for 9 months, although alternative regimens may be considered 1, 3

Common Pitfalls

1. Misinterpreting BCG vaccination effects: Prior BCG vaccination can cause false-positive TST results, but this effect diminishes with time. In close contacts of active TB cases, a positive TST should be considered indicative of TB infection regardless of BCG history 1.

2. Failing to recognize anergy: Immunosuppressed individuals may have false-negative TST results due to anergy. Consider anergy testing in HIV-infected persons 1.

3. Delayed reading: TST should be read at 48-72 hours after administration for accurate results 1.

4. Improper measurement technique: Only the transverse diameter of induration (not erythema) should be measured 1, 3.

The 5 mm threshold for close contacts of active TB cases reflects the higher pre-test probability of infection and the critical importance of early identification and treatment to prevent progression to active disease and further transmission.