Tuberculin Skin Testing in Patients with HIV Infection: Limited Benefit of Reduced Cutoff Values

When determining eligibility for isoniazid preventive therapy of human immunodeficiency virus (HIV)-infected patients, the cutoff value of the tuberculin skin test (TST) is often reduced from an induration of 10 mm in diameter to one of 5 mm in diameter to compensate for loss of sensitivity. The effectiveness of this reduction depends on the underlying mechanism: a gradual decrease in skin test responsiveness with decreasing immunocompetence or an all-or-nothing switch to complete anergy. No published studies have assessed this directly in patients with tuberculosis.