TST Interpretation: Positivity thresholds vary by risk group. Measure induration (swelling), NOT erythema.
Measured Induration
5mm
0mm5mm10mm15mm20mm30mm
Patient Risk Category
Interpretation Result
Positive TST
Indicates TB infection at the 5mm threshold for this risk group. Rule out active disease (CXR/Symptoms).
Guidelines & Evidence
Clinical Details
Section 1
Positivity Thresholds
≥ 5 mm is Positive for:
["HIV-positive persons.","Recent contacts of active TB cases.","Fibrotic changes on CXR consistent with old TB.","Immunosuppressed (e.g., organ transplant, >15mg/day prednisone for 1mo)."]
≥ 10 mm is Positive for:
["Recent arrivals (<5y) from high-prevalence countries.","Injection drug users.","Residents/employees of high-risk congregate settings.","Persons with clinical conditions (DM, CKD, Malignancy).","Children <4 years of age."]
≥ 15 mm is Positive for:
Persons with no known risk factors for TB.
Section 2
Procedure
Administration
0.1 mL of PPD (5 TU) injected intradermally to form a wheal.
Reading
Read after 48–72 hours. Measure the diameter of induration (palpable swelling), NOT erythema (redness).
