How is a Montoux (Tuberculin Skin Test, TST) result interpreted?

How to Interpret the Mantoux (Tuberculin Skin Test)

The Mantoux test is interpreted by measuring only the transverse diameter of palpable induration (not erythema) at 48-72 hours, with positivity thresholds determined by the patient's risk category: ≥5 mm for high-risk individuals, ≥10 mm for moderate-risk, and ≥15 mm for low-risk persons. 1

Reading Technique and Timing

The test must be read between 48-72 hours after injection when induration is at maximum. 1 Reading should be performed in good light with the forearm slightly flexed at the elbow. 1

Measure only induration (palpable, raised, hardened area), never erythema (redness). 1, 2 The measurement should be:

• Determined by both inspection (viewing from the side against light) and palpation 1
• Measured transversely to the long axis of the forearm 1
• Recorded in millimeters (record "0 mm" if no induration present, not "negative") 1

Tests read after 72 hours tend to underestimate the true size of induration. 1

Risk-Stratified Interpretation Thresholds

≥5 mm is Positive for High-Risk Individuals:

• Close contacts of active TB cases 1
• HIV-infected persons 1
• Persons receiving TNF-blocking agents 1
• Individuals with clinical or radiographic evidence of current or prior TB 1
• Other immunosuppressed persons (organ transplant recipients, chronic corticosteroid therapy equivalent to ≥15 mg/day prednisone for ≥1 month) 1

≥10 mm is Positive for Moderate-Risk Individuals:

• Persons born in high TB incidence countries 1
• Healthcare workers and others with occupational TB exposure 1
• Residents and employees of high-risk congregate settings (correctional facilities, nursing homes, homeless shelters) 1
• Persons with medical conditions that increase TB progression risk: diabetes mellitus, silicosis, chronic renal failure, head/neck cancer, hematologic malignancies, gastrectomy, intestinal bypass, chronic malabsorption, low body weight 1
• Injection drug users 1

≥15 mm is Positive for Low-Risk Individuals:

• Persons with no known risk factors for TB 1

Critical Interpretation Considerations

BCG Vaccination

A positive TST in a BCG-vaccinated person should be interpreted as evidence of M. tuberculosis infection when they are contacts of infectious TB cases. 1 Prior BCG vaccination should not prevent appropriate interpretation—the larger the reaction, the greater the probability of true TB infection. 3 BCG's effect on TST is almost insignificant in adults above 30 years of age. 1

False-Negative Results (Reduced Sensitivity)

The TST has a false-negative rate of approximately 25% in persons with active TB. 1 False-negatives occur more frequently in: 1

• Infants and young children
• Early infection (<6-8 weeks after exposure)
• HIV infection (61% negative rate in HIV-infected TB patients) 4
• Immunosuppressive therapy (50% negative in those on immunosuppressants) 4
• Overwhelming or disseminated TB (64% negative) 4
• Recent live viral vaccination (measles, mumps, rubella, varicella, yellow fever)—wait 4-6 weeks after vaccination 1
• Elderly patients (27% negative in ages 60-74; 44% in those >74 years) 4
• Severe malnutrition or low protein states 1

False-Positive Results (Reduced Specificity)

False-positives occur with: 1

• Prior BCG vaccination (especially if given post-infancy or repeated vaccination)
• Infection with nontuberculous mycobacteria
• Improper test administration or reading

Special Testing Scenarios

Two-Step Testing (Booster Phenomenon)

Two-step testing should be performed for baseline screening in healthcare workers and others undergoing serial testing to detect boosted reactions that might otherwise be misinterpreted as new infections. 2

The procedure: 2

• If initial test is negative, repeat in 1-3 weeks
• A positive second test represents boosting (waned sensitivity restored), not new infection
• This establishes a true baseline for future comparison

Do NOT use two-step testing for TB contacts—a positive second test in contacts should be considered a true conversion. 1

Contact Investigation Context

For contacts of infectious TB cases, any induration ≥5 mm is considered positive. 1 If the initial test is negative and performed <8 weeks after exposure, repeat testing at 8-10 weeks after exposure ends is required. 2

Common Pitfalls to Avoid

• Measuring erythema instead of induration leads to incorrect interpretation 1, 2
• Reading outside the 48-72 hour window reduces accuracy 1, 2
• Allowing patient self-reading is unreliable and not recommended 2
• Ignoring risk stratification when applying cutoff values leads to inappropriate clinical decisions 1, 2
• Dismissing positive results in BCG-vaccinated persons as false-positives without considering epidemiologic risk 1
• Failing to recognize that a negative test does not rule out active TB, especially in immunocompromised patients 1

Next Steps After Interpretation

Positive TST without symptoms: Obtain chest radiograph to exclude active TB; if normal, treat for latent TB infection. 2

Positive TST with symptoms or abnormal chest radiograph: Evaluate for active TB with sputum examination and further diagnostic workup. 2

Negative TST in high-risk or immunocompromised patients: Consider repeat testing, two-step testing if appropriate, or interferon-gamma release assay (IGRA) as alternative. 2