Standard Mantoux Testing Uses 5 TU Only
The 1 TU Mantoux test is not sufficiently accurate and should not be used in clinical practice—only the 5 TU (tuberculin unit) PPD Mantoux test is recommended for tuberculosis screening. 1
Key Differences
Dosing and Administration
- 5 TU test: 0.1 ml containing 5 tuberculin units of PPD, administered intradermally
- 1 TU test: 0.1 ml containing 1 tuberculin unit of PPD, administered intradermally
Clinical Validity and Recommendations
The American Thoracic Society/CDC guidelines explicitly state that PPD strengths of 1 TU and 250 TU are not sufficiently accurate and should not be used 1. The standard Mantoux test uses only 5 TU PPD injected intradermally into the forearm, read at 48-72 hours.
Performance Characteristics
Research comparing the two strengths reveals critical differences:
Sensitivity: The 5 TU test detects significantly more tuberculosis cases than 1 TU. In pediatric TB patients, Mantoux positivity was 61.2% with 5 TU versus only 16.3% with 1 TU 2
Dose-response relationship: Skin induration does not increase linearly with tuberculin dose. Studies show that induration increases with higher doses but at a decreasing rate, with the response plateauing after 2 TU 3
Interobserver variability: Both strengths show significant variability between readers, with fair to moderate agreement (kappa 0.33-0.73), though this is a general limitation of tuberculin testing rather than dose-specific 2
Clinical Implications
Use only 5 TU PPD for all tuberculin skin testing. The 1 TU formulation misses the majority of tuberculosis cases and has no role in modern TB screening or diagnosis. The standard interpretation criteria (≥5 mm, ≥10 mm, or ≥15 mm cutoffs based on risk factors) are validated specifically for 5 TU testing 1.
Common Pitfall
Do not attempt to use 1 TU testing in immunocompromised patients thinking it will reduce false positives—this approach lacks validation and will miss true infections. The appropriate strategy is using 5 TU with risk-stratified interpretation cutoffs (≥5 mm for HIV-positive persons, recent contacts, and immunosuppressed patients) 1.