How is the Mantoux (tuberculin skin) test interpreted—what induration sizes are considered positive for various risk groups and what subsequent steps should be taken?

Mantoux Tuberculin Skin Test Interpretation

Test Administration and Reading Technique

The Mantoux test must be read between 48 and 72 hours after intradermal injection, measuring only the transverse diameter of palpable induration (not erythema) in millimeters, with results recorded as "0 mm" if no induration is present. 1, 2

• Administer 0.1 mL containing 5 TU of purified protein derivative intradermally on the volar or dorsal forearm, producing a discrete wheal 6–10 mm in diameter. 2, 3
• Read the test in good lighting with the forearm slightly flexed at the elbow, measuring the transverse diameter perpendicular to the long axis of the forearm. 1, 2
• Use the ball-point pen method to reduce inter-observer variability. 1, 2
• Tests read after 72 hours underestimate induration size and should be avoided. 1, 2
• Only trained healthcare personnel should perform readings—patient self-reading is unacceptable. 2, 3

Risk-Stratified Interpretation Cutoffs

Interpretation depends entirely on the patient's TB risk category, with three distinct cutoff thresholds: ≥5 mm for highest-risk patients, ≥10 mm for moderate-risk groups, and ≥15 mm for low-risk individuals. 2, 4

≥5 mm Induration is Positive in:

• HIV-infected individuals 2, 4, 3
• Recent close contacts of active TB cases 2, 4, 3
• Persons with fibrotic chest radiograph changes suggestive of prior TB 2, 4, 3
• Organ transplant recipients 2, 4
• Patients on ≥15 mg/day prednisone for ≥1 month or receiving TNF-blocking agents 2
• Children younger than 4 years or those exposed to high-risk adults 2, 4

≥10 mm Induration is Positive in:

• Recent immigrants (≤5 years) from high-TB-prevalence countries 2, 4, 3
• Injection drug users 2, 4, 3
• Residents or employees of high-risk congregate settings (prisons, nursing homes, hospitals, homeless shelters) 2, 4, 3
• Mycobacteriology laboratory staff 2, 4
• Persons with medical conditions increasing TB risk: silicosis, diabetes, chronic renal failure, hematologic malignancies, certain solid-organ cancers, >10% weight loss, gastrectomy, or jejuno-ileal bypass 2, 4

≥15 mm Induration is Positive in:

• Individuals with no identified TB risk factors 2, 4, 3
• Routine testing of low-risk populations is not recommended 2

Test Conversion

A rise in induration of ≥10 mm within a two-year interval after an initially negative TST defines skin-test conversion, indicating recent Mycobacterium tuberculosis infection. 2, 4

BCG Vaccination Considerations

Prior BCG vaccination does not preclude TST use; in BCG-vaccinated persons with TB risk factors, a positive reaction should be interpreted as M. tuberculosis infection. 2, 3

• BCG-induced tuberculin reactivity typically wanes over time and is unlikely to persist >10 years after vaccination. 4
• Specificity of the TST is reduced in individuals vaccinated after infancy or who received multiple BCG doses. 2
• The size of tuberculin reaction in BCG-vaccinated persons does not reliably distinguish between M. tuberculosis infection and prior BCG vaccination. 4
• The larger the reaction to PPD, the greater the probability of true TB infection regardless of BCG history. 5

False-Negative Results and Interfering Factors

The TST has an approximate 25% false-negative rate during initial evaluation of patients with active TB, with higher rates in immunosuppressed individuals. 1, 2

• False-negatives are more common in infants and young children, during early infection (<6–8 weeks after exposure), in persons with overwhelming illness or disseminated TB, and after recent viral or bacterial infections. 2
• Live-attenuated viral vaccines (measles, mumps, rubella, varicella, yellow fever) can suppress PPD response. 1, 2
• To avoid vaccine-related false-negatives, perform TST either on the same day as live-attenuated viral vaccine or 4–6 weeks after vaccination. 1, 2
• HIV infection causes anergy even in persons infected with M. tuberculosis, necessitating the lower ≥5 mm cutoff. 4
• Malnutrition, immunosuppressive medications, malignancy, and overwhelming acute illness increase false-negative risk. 1

Two-Step Testing Protocol

For baseline screening of healthcare workers and others requiring serial testing, use two-step testing to detect the booster phenomenon and prevent misinterpretation of boosted reactions as new infections. 3

• Place the first PPD test on Day 0 and read at 48–72 hours (Day 2–3). 3
• If the first test is negative, place the second PPD test 1–3 weeks after reading the first test (approximately Day 9–24) and read 48–72 hours later. 3
• Two-step testing is essential for newly employed healthcare workers without documented negative PPD within the preceding 12 months. 3
• The booster phenomenon is more common in older adults, individuals born in high-TB-prevalence countries, and those previously vaccinated with BCG. 3

Post-Exposure Testing Timing

In contact investigations, perform follow-up testing 8–10 weeks after the end of exposure; a negative TST performed less than 8 weeks after exposure is unreliable. 3

Subsequent Steps After Testing

Positive TST Result:

• Obtain chest radiograph to exclude active TB disease. 3
• If chest radiograph is normal and no symptoms of active TB, diagnose latent TB infection (LTBI). 4, 3
• Initiate preventive therapy with isoniazid for 9–12 months, particularly for HIV-infected individuals. 3
• If chest radiograph is abnormal or symptoms present, perform sputum examination and further evaluation for active TB. 3

Negative TST Result:

• No further treatment required if test is valid and patient is not in the window period post-exposure. 3
• Repeat testing at 8–10 weeks if exposure occurred within the preceding 8 weeks. 3

Common Pitfalls to Avoid

• Never measure or record erythema alone—only induration counts. 1, 2, 3
• Never accept patient self-reading—trained personnel must perform all readings. 2, 3
• Never use multiple puncture tests (Tine, Heaf)—they are insufficiently accurate. 1, 2
• Never interpret a positive test as "negative" in BCG-vaccinated individuals with TB risk factors. 2, 3
• Never delay reading beyond 72 hours—this underestimates induration size. 1, 2, 3
• Never ignore risk stratification—the same induration size has different meanings in different risk groups. 2, 4
• Never confuse baseline two-step testing with post-exposure testing—they serve distinct purposes with different timing requirements. 3