What amount of induration indicates a positive purified protein derivative (PPD) skin test for tuberculosis (TB) in a patient with human immunodeficiency virus (HIV) infection?

From the Guidelines

For a patient with human immunodeficiency virus (HIV) infection, an induration of 5 mm or greater on a purified protein derivative (PPD) skin test indicates a positive result for tuberculosis infection. This threshold is based on the most recent and highest quality study, which emphasizes the importance of early detection and treatment of latent TB infection in immunocompromised individuals 1. The American Thoracic Society, Infectious Diseases Society of America, and Centers for Disease Control and Prevention clinical practice guidelines support this threshold, highlighting the increased risk of TB reactivation in HIV-positive individuals 1.

When interpreting the PPD test results, it is essential to measure only the firm, raised area (induration) rather than the entire reddened area (erythema) at 48-72 hours after placement. A positive result indicates TB infection but not necessarily active disease; further evaluation with chest X-ray and symptom assessment is required to distinguish between latent TB infection and active TB disease. Given the patient's HIV status, they should be closely monitored even with negative results, as false negatives can occur in immunocompromised individuals.

Key considerations in the diagnosis and management of TB in HIV-infected individuals include:

• The risk of false-negative TST results, particularly in those with advanced immunosuppression 1
• The importance of repeat testing in individuals with a negative TST who are at high risk of TB exposure or have a history of TB infection 1
• The need for prompt evaluation and treatment of latent TB infection to prevent progression to active disease, particularly in HIV-infected individuals with a positive TST result 1
• The potential for drug interactions between TB medications and antiretroviral therapy, which should be carefully considered when selecting a treatment regimen 1

From the FDA Drug Label

Isoniazid is recommended as preventive therapy for the following groups, regardless of age. (Note: the criterion for a positive reaction to a skin test (in millimeters of induration) for each group is given in parenthesis): 1 Persons with human immunodeficiency virus (HIV) infection (≥ 5 mm)

The amount of induration that indicates a positive Purified Protein Derivative (PPD) skin test for tuberculosis (TB) in a patient with human immunodeficiency virus (HIV) infection is ≥ 5 mm 2.

From the Research

Induration Amount for Positive PPD Skin Test in HIV-Infected Patients

• The amount of induration that indicates a positive purified protein derivative (PPD) skin test for tuberculosis (TB) in a patient with human immunodeficiency virus (HIV) infection is at least 5 mm 3.
• However, one study suggests that using the Centers for Disease Control (CDC)-recommended definition of an induration 5 mm or greater in diameter in HIV-1 seropositives may underestimate the "true" infection rate, and a definition of 2 mm or greater may be a better cutoff for reducing misclassification in HIV-1 seropositives 4.
• Another study found that the mean diameter induration in the HIV-1-seropositive group was 2.6 mm, compared to 5.4 mm in the seronegative group, and that skin test anergy appeared to explain the differential 4.

Factors Affecting PPD Positivity and Anergy

• PPD positivity and skin test anergy rates vary by HIV serostatus, CD4+ lymphocyte count, and other risk factors in intravenous drug users (IVDUs) 4.
• Being 35 years of age or older and being HIV-1 seronegative were significantly associated with PPD positivity, while history of previous police arrest was of borderline significance 4.
• HIV-1 seropositivity was significantly associated with anergy, and anergy testing is mandatory to properly assess a negative PPD test result 4.