Recommended cutoff points for a positive TST reaction depend on the clinical setting: 5 mm: Patients at high risk of developing active TB if infected, such as those who have chest x-ray evidence of past TB, who are immunosuppressed because of HIV infection or drugs (eg, TNF-alpha inhibitors, corticosteroid use equivalent to prednisone 15 mg / day for > 1 month), or who are close contacts of patients with infectious TB 10 mm: Patients with some risk factors, such as injection drug users, recent immigrants from high-prevalence areas, residents of high-risk settings (eg, prisons, homeless shelters), patients with certain disorders (eg, silicosis, renal insufficiency, diabetes, head or neck cancer), and those who have had gastrectomy or jejunoileal bypass surgery 15 mm: Patients with no risk factors (who typically should not be tested) False-negative TST results can occur, most often in patients who are febrile, older, HIV-infected (especially if CD4 count is < 200 cells / mcL [0.2 x 10 9 / L]), immune-suppressed because of a disorder or use of certain drugs (eg, corticosteroids, certain biologic immune modulators, certain cancer drugs), or very ill.