What is the minimum induration (mm) required for a positive Purified Protein Derivative (PPD) test result?

PPD Test Interpretation: Induration Thresholds for Positivity

The minimum induration required for a positive PPD test is risk-stratified: ≥5 mm for highest-risk patients (HIV-infected, recent TB contacts, or those with fibrotic chest radiograph changes), ≥10 mm for moderate-risk patients (immigrants from high-prevalence countries, healthcare workers, injection drug users, and those with high-risk medical conditions), and ≥15 mm for low-risk individuals with no identifiable risk factors. 1, 2

Risk-Stratified Interpretation Framework

High-Risk Category: ≥5 mm Induration is Positive

The following patients require only 5 mm of induration to be considered positive 1, 2:

• HIV-infected persons - This population has the highest priority due to increased risk of progression to active TB and potential for anergy 1
• Recent close contacts of infectious TB cases - Defined as household or unprotected occupational exposure similar in intensity to household contact 1
• Persons with chest radiographs showing fibrotic lesions consistent with old healed tuberculosis 1, 3
• Patients receiving immunosuppressive therapy or with conditions causing immunosuppression 1

Moderate-Risk Category: ≥10 mm Induration is Positive

The following patients require 10 mm of induration for positivity 1, 2:

• Recent immigrants (within 5 years) from high TB prevalence countries 1
• Injection drug users who are HIV-seronegative 1, 3
• Healthcare workers in facilities where TB patients receive care 1
• Residents and employees of high-risk congregate settings including correctional institutions, nursing homes, homeless shelters 1
• Mycobacteriology laboratory personnel 1
• Children younger than 4 years of age or adolescents exposed to high-risk adults 1, 4
• Persons with high-risk medical conditions including silicosis, diabetes mellitus, chronic renal failure, leukemia, lymphoma, head/neck/lung cancer, weight loss >10% ideal body weight, gastrectomy, jejunoileal bypass 1, 3
• Patients on chronic corticosteroid therapy (equivalent of >15 mg/day prednisone for >1 month) or other immunosuppressive therapy 1, 3

Low-Risk Category: ≥15 mm Induration is Positive

Persons with no identifiable risk factors for TB require 15 mm of induration to be considered positive 1, 2. This higher threshold is necessary because the positive predictive value of the test is lower in populations with low TB prevalence 1.

For healthcare workers in minimal-risk facilities (essentially no TB exposure), the 15 mm threshold may be more appropriate even for baseline screening 1.

Critical Technical Requirements

Proper Administration and Reading

• The test must use the Mantoux technique: 0.1 mL of PPD (5 tuberculin units) injected intracutaneously on the volar or dorsal forearm 1, 2
• Proper injection produces a pale wheal 6-10 mm in diameter 1, 2
• Only trained healthcare personnel should read results - patient self-reading is unreliable and unacceptable 1, 2, 5
• Reading must occur 48-72 hours after injection 1, 2
• Measure only induration (hardness), not erythema (redness) - record transverse diameter in millimeters 1, 2

Special Populations and Considerations

Children Under 4 Years

• Use the ≥10 mm threshold for positivity 1, 4, 3
• False-negative reactions occur more frequently in infants and young children, particularly within 6-8 weeks of infection 4
• Approximately 10% of children with culture-positive TB have negative PPD initially 4
• A negative PPD should not rule out tuberculosis in young children with clinical suspicion 4

HIV-Infected Patients

• The ≥5 mm threshold is mandatory regardless of anergy test results 1
• HIV-infected persons have substantially lower PPD positivity rates even with the 5 mm cutoff due to anergy 6, 7
• Anergy increases as CD4+ counts decline - occurs in 63% of HIV-infected hospitalized patients 6, 7
• Some evidence suggests a 2 mm threshold might reduce misclassification in HIV-positive patients, though this is not standard practice 6

BCG Vaccination History

• Prior BCG vaccination should not prevent PPD testing or alter interpretation thresholds 1, 2
• The probability that a positive PPD represents true TB infection increases with: larger reaction size, TB contact history, origin from high-prevalence country, and longer interval since BCG vaccination 1
• A reaction ≥10 mm in BCG-vaccinated adults from high-prevalence countries likely represents true TB infection 1

PPD Conversion Definition

• For persons <35 years: an increase of ≥10 mm within 2 years indicates conversion 1, 3
• For persons ≥35 years: an increase of ≥15 mm within 2 years indicates conversion 1
• For healthcare workers in minimal-risk facilities: a ≥15 mm increase may be more appropriate to avoid misclassifying boosted reactions 1

Common Pitfalls to Avoid

• Never accept patient self-reading - only 1 of 18 patients with positive tests (10-20 mm) correctly identified their result in one study 5
• Do not measure erythema - only induration counts 1, 2
• Reading outside 48-72 hour window reduces accuracy 2
• Improper injection technique (subcutaneous instead of intradermal) causes false-negative results 2
• Ignoring risk stratification leads to inappropriate clinical decisions - a 12 mm reaction is positive in a healthcare worker but negative in a person with no risk factors 1, 2
• Dismissing positive results in BCG-vaccinated persons - these patients still require evaluation and treatment 1, 2