PPD Cutoff for Negative Test in Low-Risk Individuals
For asymptomatic individuals with no risk factors for tuberculosis, a PPD test showing less than 15 mm of induration is considered negative. 1, 2
Risk Stratification and Interpretation Thresholds
The interpretation of PPD testing follows a risk-based stratification system established by the CDC and American Thoracic Society, where the cutoff for positivity increases as risk decreases 1, 2:
Three-Tiered Interpretation System
≥5 mm induration is positive for:
- HIV-positive persons
- Recent contacts of active TB cases
- Persons with chest radiograph findings consistent with prior TB
- Organ transplant recipients and other immunosuppressed patients (equivalent of >15 mg/day prednisone for >1 month)
- Mycobacteriology laboratory personnel 1, 2
≥10 mm induration is positive for:
- Recent arrivals (<5 years) from high-prevalence countries
- Injection drug users
- Residents and employees of high-risk congregate settings (prisons, nursing homes, homeless shelters, healthcare facilities)
- Persons with clinical conditions increasing TB risk (silicosis, diabetes mellitus, chronic renal failure, leukemias, lymphomas, head/neck/lung cancer, >10% weight loss, gastrectomy, jejunoileal bypass)
- Children <4 years of age or adolescents exposed to high-risk adults 1, 2
≥15 mm induration is positive for:
Technical Requirements for Valid Testing
The test must meet specific technical standards to be considered valid 2:
- Administration: 0.1 mL of PPD (5 tuberculin units) injected intracutaneously using the Mantoux technique on the volar or dorsal forearm 2
- Proper injection technique: Should produce a pale wheal measuring 6-10 mm in diameter 2
- Reading timing: Must be read by trained healthcare personnel between 48-72 hours after injection 2
- Measurement: Only induration (hardened area) is measured in millimeters, not erythema (redness) 2
Important Clinical Caveats
When PPD Testing May Not Be Indicated
For persons at low risk for tuberculosis (no risk factors, asymptomatic), tuberculin testing is generally not indicated unless there is a specific clinical or epidemiologic reason 1. The 15 mm cutoff applies specifically when testing is performed in this population 1.
Healthcare Worker Exception
In healthcare facilities where there is essentially no risk for TB exposure (minimal- or very low-risk facilities), an induration ≥15 mm may be appropriate for healthcare workers with no other risk factors 1. However, in facilities where TB patients receive care, the appropriate cutoff drops to ≥10 mm 1.
Common Pitfalls to Avoid
- Improper reading timing: Reading outside the 48-72 hour window reduces accuracy 2
- Measuring erythema instead of induration: This leads to incorrect interpretation 2
- Patient self-reading: This is unreliable and not recommended 2
- Ignoring BCG vaccination history: Previous BCG can cause false-positive results, but the probability that a reaction results from M. tuberculosis infection increases with larger reaction size, contact history, origin from high-prevalence countries, and longer time since vaccination 1
Two-Step Testing Considerations
For individuals requiring baseline PPD testing (such as newly employed healthcare workers) without documented negative PPD in the preceding 12 months, two-step testing should be performed to detect the booster phenomenon 2. The second test should be administered 1-3 weeks after the first test if the initial test is negative 2, 3. Without two-step testing, a boosted reaction on subsequent testing might be misinterpreted as a new infection 2.
Management of Negative Results
If the PPD is negative (< 15 mm in low-risk individuals) and the test is valid, no further treatment is required 1. However, if there is uncertainty about test validity, options include repeating the PPD on the other arm within five weeks or considering alternative testing approaches 1.