TB Testing Interpretation (TST + IGRA)
Interpret tuberculin skin test (TST) induration and interferon-gamma release assay (IGRA) results against CDC risk-stratified cutoffs.
Open the tool → Runs in your browser. Data leaves only if you deliberately send a problem report from the interactive tool.
Example
- Induration
- 12 mm
- Risk-category cutoff
- Moderate (foreign-born, IVDU, healthcare workers, etc.)
Result: TST: 12 mm vs cutoff 10 mm -> POSITIVE.
The tool opens with these values already filled in. Replace them with your own.
What you enter
- Induration
- Risk-category cutoff (5 high / 10 moderate / 15 low)
What this is
The TST is read as the millimeters of induration (not erythema) at 48-72 hours and compared to the threshold for the patient's risk category: 5 mm for HIV, recent contact, fibrotic chest X-ray, organ-transplant or other immunosuppression; 10 mm for recent immigrants, IV drug users, residents or employees of high-risk congregate settings, mycobacteriology lab personnel, certain comorbidities, and children under 4 or exposed children and adolescents; 15 mm for everyone else. Sophie Well also reports IGRA positive / negative / indeterminate. Source: CDC TST cutoffs by risk category; IGRA interpretation guidance.
When to use it
Use this when screening for latent or active tuberculosis - new healthcare or correctional employment, post-exposure investigation, immigration medical exam, pre-biologic therapy, or evaluation of pulmonary symptoms in a higher-risk patient. The interpretation cutoff is determined by the patient's risk category, not the absolute millimeter reading, so a 9 mm induration is positive in a recent contact and negative in a healthy adult. IGRA is preferred over TST for BCG-vaccinated patients and people who are unlikely to return for the 48-72 hour TST read. A positive screen requires a chest X-ray and a symptom review to distinguish latent infection from active disease before any treatment decision.
How this is calculated
CDC TST cutoffs (5 / 10 / 15 mm) by risk category; IGRA interpretation.
Source: CDC TB testing interpretation
A reference and educational tool. Not medical, legal, or financial advice, and not a substitute for clinician judgment.