Tuberculin Skin Test Positivity: Induration is the Only Criterion
The tuberculin skin test (TST) is read by measuring only the palpable induration (hardness), not erythema (redness), vesiculation, or any other skin changes at the injection site 1, 2.
What to Measure and What to Ignore
Measure Only Induration
- Palpate the injection site at 48-72 hours and measure the transverse diameter of palpable induration in millimeters 1, 2
- Use your fingers to feel for the raised, hardened area—this is what counts
- Record the measurement in millimeters, not as "positive" or "negative"
Explicitly Ignore These Features
- Erythema (redness) alone does not indicate a positive test 3
- Vesiculation and ulceration may occur in very strong reactions but are not required for positivity 1
- The absence of induration with erythema >10 mm diameter may actually indicate the injection was made too deeply (subcutaneously rather than intradermally), and retesting is indicated 3
Critical Pitfall to Avoid
A common error is measuring or considering erythema when reading the TST. The guidelines are explicit: only induration matters 2, 3. Some clinicians may be confused by prominent redness at the site, but if there is no palpable hardness, the test is negative regardless of how red it appears.
Special Consideration: Pseudoanergy
Research has identified rare cases of "pseudoanergy" where cellular infiltration occurs without palpable induration 4. However, this phenomenon does not change clinical practice—if you cannot palpate induration, the test is read as negative based on current guidelines 1, 2.
The Size Thresholds (For Context)
While you asked specifically about characteristics other than size, remember that positivity is defined entirely by induration size using risk-stratified cutoffs: ≥5 mm, ≥10 mm, or ≥15 mm depending on the patient's risk factors 1, 2. No other physical characteristic at the injection site contributes to this determination.