TB Testing Interpretation (TST + IGRA)
TB Testing Interpretation (TST + IGRA) - a deterministic tool in Sophie Well's Immunization & Infectious Disease group.
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What this is
Interpret tuberculin skin test (TST) induration and interferon-gamma release assay (IGRA) results against CDC risk-stratified cutoffs. 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.
References
CDC TST cutoffs (5 / 10 / 15 mm) by risk category; IGRA interpretation.
Source: CDC TB testing interpretation
Worked example: TST: 12 mm vs cutoff 10 mm -> POSITIVE.
Sophie Well is a reference and educational tool. Not medical, legal, or financial advice. Does not replace clinician judgment, professional billing review, or legal counsel.