Is a tuberculin skin test (TST) result below 5mm considered negative?

Tuberculin Skin Test Interpretation: Risk-Stratified Thresholds

No, a tuberculin skin test (TST) below 5mm is NOT universally negative—interpretation depends entirely on the patient's risk category, and for high-risk individuals, even <5mm may warrant clinical action.

Risk-Stratified Interpretation Thresholds

The TST must be interpreted using risk-stratified cutoffs, not a single universal threshold 1:

≥5mm is Positive for High-Risk Groups:

• Close contacts of active TB cases 1
• HIV-infected persons 1
• Immunosuppressed patients (organ transplant recipients, chronic corticosteroid users ≥15mg prednisone daily for ≥1 month) 1
• Patients on TNF-alpha blocking agents 1
• Persons with chest radiograph findings suggestive of prior TB (fibrotic changes, apical scarring) 1

≥10mm is Positive for Moderate-Risk Groups:

• Foreign-born persons from high TB prevalence countries 1, 2
• Healthcare workers with occupational TB exposure risk 1, 2
• Residents and employees of high-risk congregate settings (correctional facilities, nursing homes, homeless shelters) 1
• Injection drug users 1
• Persons with medical conditions increasing TB progression risk (diabetes, chronic renal failure, silicosis, malignancies, gastrectomy) 1
• Children <5 years exposed to high-risk adults 1

≥15mm is Positive for Low-Risk Groups:

• Persons with no known TB risk factors 1, 2
• Healthcare workers in minimal-risk facilities with essentially no TB exposure 1

Critical Clinical Caveat: False-Negative TST Results

A negative TST (<5mm) does NOT exclude TB disease or infection in high-risk populations 3. False-negative rates are substantial:

• 10% of immunocompetent children with culture-confirmed TB have negative TST initially 3
• Up to 50% of patients with miliary TB or TB meningitis have negative TST 3
• 61% of HIV-infected TB patients may have negative TST, with rates increasing as CD4 counts decline 4, 3
• 50% of patients on immunosuppressive therapy have negative TST despite active TB 4

When to Proceed Despite Negative TST:

For high-risk patients, clinical suspicion should drive diagnostic workup, not TST results 3. Proceed with full TB evaluation (chest radiograph, sputum AFB smear/culture, nucleic acid amplification testing) if the patient has 3:

• Persistent cough (>2-3 weeks)
• Abnormal chest radiograph
• Known exposure to active TB case
• HIV infection or other severe immunosuppression
• Membership in high-risk epidemiologic groups

Special Timing Considerations

Window Period for Recent Exposures:

• TST performed <8 weeks after TB exposure is unreliable and should be repeated at 8-10 weeks post-exposure 1, 2
• For children <5 years and immunocompromised contacts with initial TST <5mm and exposure <8 weeks prior, initiate "window prophylaxis" (presumptive LTBI treatment) after excluding active disease, then retest at 8-10 weeks 1

Booster Phenomenon:

• Delayed-type hypersensitivity can wane over time, and a subsequent TST can "boost" the response, mimicking conversion 1, 2
• Two-step testing (second test 1-3 weeks after initial negative) is recommended for baseline screening of healthcare workers without documented negative TST in preceding 12 months 2, 1

Common Pitfalls to Avoid

• Measuring erythema instead of induration leads to incorrect interpretation—only measure the palpable, raised, hardened area transverse to the long axis of the forearm 1, 2
• Reading outside the 48-72 hour window reduces accuracy 1, 2
• Ignoring risk stratification and applying a universal 10mm or 15mm cutoff misses high-risk patients who need treatment at ≥5mm 1
• Dismissing BCG vaccination history as explanation for positive TST—in adults vaccinated as children who are from high TB prevalence countries or are TB contacts, a ≥10mm reaction likely represents true M. tuberculosis infection, not BCG effect 1
• Relying on patient self-reading—TST must be read by trained healthcare personnel using proper palpation technique 1, 2

Documentation Requirements

Always record TST results in millimeters of induration, never simply as "positive" or "negative" 1, 2. Record 0mm if no induration is present 1.