How is a Tuberculosis (TB) skin test measured and interpreted in individuals with a history of exposure to TB, from a region with high TB prevalence, or with a weakened immune system, considering their demographic and medical history?

Tuberculin Skin Test Measurement and Interpretation

Administration Technique

The tuberculin skin test must be administered using the Mantoux technique with 0.1 mL of PPD (5 tuberculin units) injected intracutaneously on the volar or dorsal forearm, producing a discrete pale wheal of 6-10 mm diameter. 1, 2

• The injection should be placed just beneath the skin surface, creating visible elevation 3, 2
• If significant leakage occurs or the injection is given subcutaneously, repeat the test immediately at another site 4
• Proper technique is critical—improper administration leads to false-negative results 2

Reading the Test

Trained healthcare personnel must read the test between 48-72 hours after injection by measuring only the transverse diameter of induration in millimeters, completely ignoring any erythema. 1, 2

• Patient or self-reading is never acceptable 3, 2
• Tests read after 72 hours underestimate induration size and should be avoided 1
• Use good lighting with the forearm slightly flexed, employing both inspection and palpation 1
• The ball-point pen method can reduce interobserver variability 1, 5
• Record the measurement in millimeters, not as simply "positive" or "negative" 3

Risk-Stratified Interpretation

Interpretation depends entirely on the patient's risk category, not a single universal cutoff—this is the most critical concept in tuberculin testing. 1, 2

≥5 mm Induration is Positive for:

• HIV-infected persons 1, 2, 6
• Recent close contacts of persons with active TB 1, 2, 6
• Persons with fibrotic changes on chest radiograph consistent with prior TB 1, 2, 6
• Patients on immunosuppressive therapy or with conditions causing immunosuppression 6

≥10 mm Induration is Positive for:

• Injection drug users (HIV-negative) 1, 2, 6
• Persons with medical conditions increasing TB risk: diabetes mellitus, silicosis, chronic renal failure, hematologic malignancies, prolonged corticosteroid therapy, substantial weight loss, gastrectomy, intestinal bypass, chronic malabsorption 1, 2, 6
• Residents and employees of high-risk congregate settings (correctional facilities, nursing homes, healthcare facilities) 1, 2, 6
• Foreign-born persons from high TB prevalence countries 2, 6
• Medically underserved low-income populations 6
• Children under 4 years of age 6
• Recent converters under age 35 (≥10 mm increase within 2 years) 1, 6

≥15 mm Induration is Positive for:

• Persons with no known TB risk factors 1, 2, 6
• Recent converters age 35 or older (requires ≥15 mm increase) 6

Special Populations

Immunocompromised Patients

In immunocompromised hosts, tuberculin skin testing has reduced sensitivity, but all available clinical data should be used together—history, physical examination, chest radiography, and the tuberculin test—to assess TB infection risk. 3

• Sensitivity is particularly limited in patients on corticosteroids, calcineurin inhibitors, methotrexate, anti-TNF-α therapy, those with HIV infection (especially with low CD4 counts), chronic renal insufficiency, or rheumatoid arthritis 3
• A false-negative rate of 25% occurs even in persons with active TB 1
• Consider interferon-gamma release assays (IGRAs) as they may have superior sensitivity in immunocompromised patients 3, 2

BCG-Vaccinated Individuals

Prior BCG vaccination is NOT a contraindication to tuberculin testing, and positive reactions should be interpreted as M. tuberculosis infection when the person has increased risk factors. 1, 2, 4

• The larger the reaction, the greater the probability of true TB infection rather than BCG effect 4
• Use the same risk-stratified cutoffs as for non-BCG-vaccinated persons 1, 2

High TB Prevalence Regions

Persons from regions with high TB prevalence should be tested using the ≥10 mm cutoff if they are foreign-born and never received BCG vaccine, or if they belong to high-risk groups. 2, 6

• Maintain high index of suspicion in these populations 3
• Consider two-step testing for baseline establishment if no documented negative test in preceding 12 months 2

Two-Step Testing for Baseline Screening

For newly employed healthcare workers or others requiring serial testing who lack a documented negative PPD within 12 months, perform two-step testing to detect the booster phenomenon and prevent misclassification of boosted reactions as new infections. 1, 2

• If the first test is negative (<5 mm), administer a second test 1-3 weeks later 2
• The second test result becomes the baseline 2
• This prevents future boosted reactions from being misinterpreted as recent TB conversion 1, 2
• The booster phenomenon is more common in older adults, BCG-vaccinated persons, and those from high TB prevalence countries 2

Skin Test Conversion

A tuberculin skin test conversion is defined as an increase of ≥10 mm in induration within a 2-year period for persons under age 35, or ≥15 mm increase for those age 35 or older, indicating recent infection requiring preventive therapy. 1, 6

• All infants and children under 4 years with >10 mm induration are considered recent converters 6

Contact Investigation Timing

For contacts of infectious TB cases, a negative test obtained less than 8 weeks after exposure is unreliable; repeat testing at 8-10 weeks after exposure ends is mandatory. 2

• Tuberculin-negative (<5 mm) children and adolescents who are close contacts should receive preventive therapy until repeat testing at 12 weeks post-contact 6
• If the repeat test is positive (≥5 mm), continue therapy 6

Critical Pitfalls to Avoid

• Never measure or record erythema alone—only induration counts 3, 1, 2
• Never accept patient self-reading 3, 1, 2
• Never use a single universal cutoff—always apply risk-stratified interpretation 1, 2
• Never interpret a positive test as "negative" simply because the patient received BCG vaccination 1, 2
• Never read the test beyond 72 hours as this underestimates induration 1, 2
• Never skip two-step testing for baseline screening in healthcare workers without recent documented negative tests 1, 2
• Never rely on tuberculin testing alone in immunocompromised patients—integrate all clinical data 3

Follow-Up Actions

• Positive test without active TB symptoms or abnormal chest X-ray: Initiate preventive therapy with isoniazid for 9-12 months (12 months minimum for HIV-infected persons) 2, 6
• Positive test with suspicious symptoms or radiographic findings: Perform immediate evaluation for active TB including sputum examination 2
• Negative test in low-risk person: No further action required 2