PPD Induration Size Interpretation for Healthcare Workers
For healthcare workers, a PPD induration of less than 10 mm is generally considered negative, unless the worker has specific risk factors that would lower this threshold to 5 mm. 1
Risk-Stratified Interpretation of PPD Results
The interpretation of tuberculin skin test (TST) results is based on a person's risk factors, with different cut-off values for different risk groups:
Induration ≥5 mm is considered positive in:
- HIV-infected healthcare workers
- Recent contacts of TB cases
- Healthcare workers with fibrotic changes on chest X-ray
- Immunosuppressed healthcare workers 2, 1
Induration ≥10 mm is considered positive in:
- Most healthcare workers, especially those working in facilities where TB patients receive care
- Recent immigrants (within 5 years) from high-prevalence countries
- Injection drug users
- Residents of high-risk congregate settings 2, 1
Induration ≥15 mm is considered positive in:
- Healthcare workers with no known risk factors, particularly those in minimal-risk facilities where there is essentially no risk for exposure to TB patients 2, 1
Interpretation Based on Facility Risk Level
The CDC guidelines specifically address healthcare facility risk levels:
- In facilities where TB patients receive care, the appropriate cut-point for healthcare workers with no other risk factors is ≥10 mm 2
- In minimal-risk or very low-risk facilities (where there is essentially no risk for exposure to TB patients), an induration ≥15 mm may be an appropriate cut-point for healthcare workers with no other risk factors 2
Special Considerations
PPD Test Conversion
- A PPD test conversion in a healthcare worker is generally defined as an increase of ≥10 mm in induration size within a 2-year period 2
- For healthcare workers in facilities where exposure to TB is very unlikely (minimal-risk facilities), an increase of ≥15 mm within a 2-year period may be more appropriate for defining a recent conversion 2
Two-Step Testing
- Two-step testing should be performed on newly employed healthcare workers who have an initial negative PPD test result and no documented negative PPD test result during the preceding 12 months 2
- This helps distinguish between a boosted reaction and a true conversion due to new infection 2
BCG Vaccination
- Prior BCG vaccination can cause false-positive TST results, but this effect typically diminishes with time 1
- In healthcare workers with a history of BCG vaccination, it is usually prudent to consider positive reactions as indicating infection with M. tuberculosis, especially if they are from countries with a high prevalence of tuberculosis 2
Management of Positive Results
If a healthcare worker has a positive PPD test result:
- A clinical evaluation and chest radiograph should be performed promptly to rule out active TB 2, 1
- If symptoms compatible with TB are present, the healthcare worker should be excluded from the workplace until either active TB is ruled out or the worker is on therapy and determined to be noninfectious 2
- Healthcare workers who do not have active TB should be evaluated for preventive therapy according to published guidelines 2
Common Pitfalls to Avoid
- Misinterpreting boosted reactions as new infections, which can lead to unnecessary investigations and preventive therapy 2, 1
- Failing to consider risk factors that might lower the threshold for a positive test 1
- Overlooking the possibility of anergy in immunocompromised individuals, which may result in false-negative results 2, 1
- Inconsistent reading techniques, which can lead to measurement variability (studies show significant variability in repeated measurements) 3
By following these risk-stratified guidelines for interpreting PPD test results in healthcare workers, facilities can effectively identify those with latent TB infection while minimizing unnecessary treatment.