How is the Mantoux test interpreted?

Mantoux Test Interpretation

The Mantoux test is interpreted by measuring the transverse diameter of induration (not erythema) in millimeters at 48-72 hours post-injection, with positivity defined by three risk-stratified cutoff values: ≥5 mm for highest-risk individuals (HIV-positive, recent TB contacts, immunosuppressed), ≥10 mm for moderate-risk groups (immigrants from high-prevalence countries, injection drug users, high-risk congregate settings), and ≥15 mm for persons with no TB risk factors. 1, 2

Administration and Reading Technique

The test must be administered correctly to ensure valid interpretation:

• Inject 0.1 mL of 5-TU PPD intradermally into the volar or dorsal forearm surface using a 27-gauge needle, producing a discrete 6-10 mm wheal 1
• Read between 48-72 hours when induration reaches maximum; readings after 72 hours underestimate true induration size 1, 2
• Measure only induration (palpable, raised, hard swelling), not erythema, using inspection in good light with the forearm slightly flexed 1, 2
• Record the transverse diameter (perpendicular to the long axis of the forearm) in millimeters; document "0 mm" rather than "negative" for absent induration 1, 2
• The ball-point pen method of Sokal can reduce interobserver variability 1, 2

Risk-Stratified Interpretation Cutoffs

≥5 mm Induration (Highest Risk)

This cutoff applies to individuals at greatest risk for developing active TB disease if infected:

• HIV-positive persons (due to severe immunosuppression and high progression risk) 1, 2
• Recent close contacts of infectious TB cases 1, 2
• Persons with fibrotic changes on chest radiograph consistent with prior TB 1, 2
• Organ transplant recipients and other immunosuppressed patients (receiving ≥15 mg/day prednisone for >1 month or equivalent) 1, 2
• Children <4 years old or those exposed to adults in high-risk categories 1, 2

≥10 mm Induration (Moderate Risk)

This threshold identifies persons with increased probability of recent infection or clinical conditions increasing TB risk:

• Recent immigrants (<5 years) from high-prevalence countries 1, 2
• Injection drug users 1, 2
• Residents and employees of high-risk congregate settings (prisons, nursing homes, homeless shelters, hospitals) 1, 2
• Mycobacteriology laboratory personnel 1, 2
• Persons with high-risk clinical conditions: silicosis, diabetes mellitus, chronic renal failure, hematologic disorders (leukemias, lymphomas), head/neck/lung carcinoma, >10% weight loss, gastrectomy, jejunoileal bypass 1, 2

≥15 mm Induration (Low Risk)

This cutoff applies to persons with no known TB risk factors 1, 2

Special Interpretation Considerations

BCG Vaccination

A critical pitfall involves BCG-vaccinated individuals:

• Positive reactions should be interpreted as true TB infection, especially in persons from high-prevalence countries, because BCG-induced reactivity wanes over time (typically >10 years post-vaccination) 1, 3, 2
• The size of induration does not reliably distinguish between M. tuberculosis infection and BCG effect; mean BCG reactions are often <10 mm 1, 3
• Use the same risk-stratified cutoffs (≥5, ≥10, or ≥15 mm) for BCG-vaccinated persons based on their risk profile 3
• Interferon-gamma release assays (IGRAs) are preferred over TST in BCG-vaccinated individuals ≥5 years old due to superior specificity 1, 3

False-Negative Results

The test has significant limitations in sensitivity:

• 25% false-negative rate in immunocompetent persons with active TB; up to 60% in HIV/TB co-infected patients 1
• Causes of false-negatives include: immunosuppression (HIV, medications, malignancy), overwhelming acute illness, poor nutrition, recent live-virus vaccination (measles, mumps, rubella, varicella—wait 4-6 weeks post-vaccination), early TB infection (8-10 weeks needed for reactivity), elderly patients, active IBD 1
• A negative test does not exclude TB; proceed with chest radiography and clinical evaluation if suspicion remains 1

Two-Step Testing

For baseline testing of individuals who will undergo serial testing (healthcare workers, nursing home residents):

• Perform initial test; if negative, repeat 1-2 weeks later to detect boosted reactions from remote infection 1
• This identifies individuals whose waning hypersensitivity is "boosted" by the first test, preventing false interpretation of future tests as conversions 1

Skin Test Conversion

For serial testing programs:

• Conversion is defined as ≥10 mm increase in induration for persons <35 years old 1
• ≥15 mm increase for persons ≥35 years old 1
• Conversions indicate recent infection requiring evaluation and treatment 1

Clinical Action Following Interpretation

• All persons with positive tests (by risk-stratified criteria) require chest radiography to distinguish latent TB infection from active disease 1, 2
• Positive test indicates infection, not necessarily active disease; further evaluation determines appropriate treatment (preventive therapy for latent infection vs. multi-drug therapy for active disease) 2
• Multiple-puncture tests (Tine, Heaf) should never be used for diagnosis due to unreliable tuberculin dosing; if performed, repeat with Mantoux method regardless of result 1