How is a tuberculosis (TB) skin test result interpreted in terms of induration diameter, considering patient risk factors and medical history, such as immunocompromised status or previous Bacille Calmette-Guerin (BCG) vaccination?

How to Read a TB Skin Test

Measure only the transverse diameter of induration (not erythema) between 48-72 hours after injection, record the result in millimeters, and interpret using risk-stratified cutoffs: ≥5 mm for highest-risk patients (HIV-infected, recent TB contacts, or those with fibrotic chest changes), ≥10 mm for moderate-risk groups, and ≥15 mm for low-risk individuals. 1

Timing of Test Reading

• Read the test between 48 and 72 hours after injection when induration reaches maximum size. 1, 2
• Tests read after 72 hours tend to underestimate the true size of induration and should be avoided. 1, 2
• The tuberculin reaction begins 5-6 hours post-injection as a delayed-type hypersensitivity response, with maximal induration occurring at 48-72 hours due to T-cell recruitment and local inflammatory cell accumulation. 2
• In elderly patients or first-time test recipients, the reaction may not peak until after 72 hours, though this delayed response does not alter test interpretation. 2

Proper Measurement Technique

• Measure only induration (palpable, raised, hardened area), not erythema (redness). 1, 2
• Measure the transverse diameter perpendicular to the long axis of the forearm using inspection and palpation. 1, 2
• Reading should be performed in good light with the forearm slightly flexed at the elbow. 1, 2
• Record measurements in millimeters; document "0 mm" rather than "negative" when no induration is present. 1, 2
• The ball-point pen method can be used to decrease interobserver variability. 1
• Trained personnel must perform the reading—patient or self-reading is not acceptable. 1

Risk-Stratified Interpretation Cutoffs

≥5 mm Induration is Positive for:

• HIV-infected persons 1
• Recent close contacts of persons with active TB (household or unprotected occupational exposure similar in intensity and duration to household contact) 1
• Persons with fibrotic changes on chest radiograph consistent with prior TB 1
• Organ transplant recipients and other immunosuppressed patients (receiving equivalent of >15 mg/day prednisone for ≥1 month) 1

≥10 mm Induration is Positive for:

• Injection drug users known to be HIV-seronegative 1
• Persons with medical conditions that increase risk for progression to active TB: silicosis, diabetes mellitus, chronic renal failure, hematologic disorders (leukemias, lymphomas), specific malignancies (head/neck or lung cancer), weight loss >10% ideal body weight, gastrectomy, jejunoileal bypass 1
• Recent immigrants (within last 5 years) from high-prevalence countries 1
• Residents and employees of high-risk congregate settings: prisons, jails, nursing homes, hospitals, healthcare facilities, residential facilities for AIDS patients, homeless shelters 1
• Mycobacteriology laboratory personnel 1
• Children younger than 4 years of age or infants, children, and adolescents exposed to adults at high risk 1

≥15 mm Induration is Positive for:

• Persons with no risk factors for TB 1
• This higher cutoff is used for low-risk populations to maintain specificity and avoid false-positives. 1

Skin Test Conversion

• An increase in induration size of ≥10 mm within a 2-year period is considered a skin-test conversion indicating recent infection. 1
• This applies to persons with negative tuberculin skin tests who undergo repeat testing (e.g., healthcare workers). 1

Special Considerations

BCG Vaccination History

• Tuberculin skin testing is NOT contraindicated in persons who received BCG vaccination. 1
• No method can reliably distinguish tuberculin reactions caused by BCG from those caused by M. tuberculosis infection. 1
• A positive reaction in BCG-vaccinated persons should be interpreted as M. tuberculosis infection when the person is at increased risk for recent infection or has medical conditions that increase risk for disease. 1
• The larger the reaction to PPD, the greater the probability of true TB infection regardless of BCG history. 3

False-Negative Results

• The PPD skin test has a reported false-negative rate of 25% during initial evaluation of persons with active tuberculosis. 1
• False-negatives can occur due to:
  • Immunosuppression (HIV infection, medications, malignancy) 1
  • Overwhelming acute illness or poor nutritional status 1
  • Viral infections (measles, mumps, chickenpox, HIV) 1
  • Live-attenuated virus vaccines given within 4-6 weeks (measles, mumps, rubella, oral polio, varicella, yellow fever, BCG, oral typhoid) 1
  • Metabolic derangements (chronic renal failure) 1
  • Age extremes (newborns, elderly with waned sensitivity) 1
  • Diseases affecting lymphoid organs (Hodgkin's disease, lymphoma, chronic leukemia, sarcoidosis) 1

Healthcare Worker Testing

• For healthcare workers without documented negative PPD in the preceding 12 months, use the two-step method for baseline testing to detect boosting phenomena that might be misinterpreted as skin-test conversion. 1
• Healthcare workers with indurations of 5-9 mm should be advised that such results may be considered positive if they are contacts of persons with infectious TB or have HIV infection or other severe immunosuppression. 1

Common Pitfalls to Avoid

• Do not measure or record erythema alone—only induration matters for interpretation. 1, 2
• Do not accept patient self-reading—only trained personnel should read results. 1
• Do not use multiple puncture tests (tine tests) for diagnostic purposes—they are unreliable and should be repeated with Mantoux method. 1
• Do not interpret a positive test as "negative" in BCG-vaccinated individuals—interpret based on risk factors. 1
• Do not delay reading beyond 72 hours—this underestimates true induration size. 1, 2