Mantoux Test in Tuberculosis Diagnosis for At-Risk Patients
The Mantoux test (tuberculin skin test) is the preferred method for diagnosing latent TB infection in at-risk populations, using risk-stratified interpretation thresholds of ≥5 mm, ≥10 mm, or ≥15 mm induration based on the patient's specific risk profile. 1
Test Administration
The Mantoux technique requires intradermal injection of 0.1 mL of purified protein derivative (PPD) containing 5 tuberculin units, which should produce a discrete, pale wheal 6-10 mm in diameter at the injection site 1. The test must be read by trained personnel—not by patients themselves—between 48-72 hours after injection 1, 2. Only the transverse diameter of induration (hard, raised, palpable swelling) should be measured and recorded in millimeters; erythema or redness alone should never be measured 1, 2.
Risk-Stratified Interpretation Thresholds
The interpretation of induration size depends entirely on the patient's TB risk profile, not on an arbitrary universal cutoff 1:
≥5 mm is Positive for Highest-Risk Groups:
- HIV-infected persons (the strongest known risk factor for progression to active TB, with 7-10% annual reactivation rates) 1
- Recent close contacts of persons with active pulmonary TB 1, 2
- Persons with chest radiographs showing fibrotic changes consistent with old TB 1
- Organ transplant recipients and other immunosuppressed patients (including those on ≥15 mg prednisone daily for >1 month) 1, 2
≥10 mm is Positive for Moderate-Risk Groups:
- Recent immigrants (<5 years) from high TB prevalence countries 1, 2
- Injection drug users 1
- Residents and employees of high-risk congregate settings (prisons, homeless shelters, nursing homes) 1, 2
- Healthcare workers and mycobacteriology laboratory personnel 1, 2
- Persons with medical conditions increasing TB risk: diabetes mellitus, chronic renal failure requiring dialysis, silicosis, hematologic malignancies, head/neck/lung cancer, gastrectomy, or >10% weight loss 1
≥15 mm is Positive for Low-Risk Groups:
Systematic Testing Recommendations by Risk Group
Strong recommendations for systematic testing and treatment (WHO 2015 guidelines) 1:
- People living with HIV 1
- Adult and child contacts of pulmonary TB cases 1
- Persons initiating anti-TNF-alpha treatment 1
- Patients receiving dialysis 1
- Patients preparing for organ or hematological transplantation 1
- Persons with silicosis 1
Conditional recommendations for systematic testing 1:
- Prisoners 1
- Healthcare workers 1
- Immigrants from high TB burden countries 1
- Homeless persons 1
- Illicit drug users 1
Critical Pitfalls and Special Considerations
BCG Vaccination Does Not Invalidate Results
A common error is dismissing positive Mantoux tests in BCG-vaccinated individuals 1, 2. Any positive tuberculin reaction in BCG-vaccinated persons should be interpreted as indicating M. tuberculosis infection, especially in those from high-prevalence countries 1, 2. BCG-induced reactivity typically wanes beyond 10 years post-vaccination, and mean reaction sizes are often <10 mm 1, 2. The size of induration does not reliably distinguish BCG reaction from true TB infection 2.
Defining Skin Test Conversion
For persons with initial negative tests who undergo serial testing (e.g., healthcare workers), an increase of ≥10 mm induration within 2 years indicates skin test conversion and recent TB infection, regardless of baseline 1, 2. This accounts for measurement variability while identifying clinically meaningful changes 1.
Immunosuppression and False Negatives
Patients on corticosteroids (>1 month), thiopurines, or methotrexate (>3 months) may have false-negative results due to impaired delayed-type hypersensitivity 1. HIV-infected persons may be anergic despite true TB infection, which is why the lower 5 mm threshold is used to maximize sensitivity in this population 1, 2. Anergy testing with control antigens is not recommended, as it has poor validity and unpredictable variation over time 1.
Booster Testing
For patients on immunomodulators with an initial negative test, a booster test 1-2 weeks later may identify an additional 8-14% of latent TB cases 1. This is particularly relevant before initiating biologics like anti-TNF agents 1.
Next Steps After Positive Test
All persons with positive Mantoux tests require chest radiography to exclude active TB disease before considering treatment for latent TB infection 1. A positive test indicates TB infection but does not differentiate between latent infection and active disease 2. For HIV-infected persons with positive tests (≥5 mm), treatment of latent TB infection is indicated regardless of age, using isoniazid 300 mg daily for 9-12 months 1.