TB Skin Test Reading: Measure Only Induration
Yes, when reading a TB skin test, you must measure only the indurated (hard, raised, palpable) area and completely ignore any surrounding erythema (redness). 1, 2
Why Induration Only Matters
The basis of tuberculin skin test interpretation is exclusively the presence and size of induration, not erythema. 1 This is because:
- Induration represents the true delayed-type hypersensitivity reaction caused by T cells and inflammatory cells accumulating at the injection site, which indicates prior exposure to M. tuberculosis. 1, 3
- Erythema alone has no diagnostic value and can be confounded by factors like atopy (allergic conditions), making it an unreliable marker. 4
- Recording erythema instead of induration is a common and serious error that leads to misdiagnosis. 2, 5
Proper Measurement Technique
Read the test between 48-72 hours after injection when induration reaches maximum size. 1, 2 Tests read after 72 hours underestimate the true induration size. 1, 2
To measure correctly:
- Use good lighting with the forearm slightly flexed at the elbow. 1
- Determine induration by both inspection and palpation - look from a side view against the light and feel for the hard, raised area. 1
- Measure only the transverse diameter (perpendicular to the long axis of the forearm) of the indurated area in millimeters. 1, 2
- The ball-point pen method can decrease interobserver variability - gently push a ballpoint pen toward the induration from multiple directions until resistance is felt. 1, 2
- Record "0 mm" if no induration is present, not "negative." 1, 2
Critical Pitfall to Avoid
The most common error is measuring or recording erythema instead of induration. 2, 6 Studies show that 93% of healthcare professionals underread tuberculin reactions, with many mistaking erythema for the relevant finding. 7 Even if erythema extends beyond 10mm, if there is no palpable induration, the test should be recorded as 0mm and may indicate the injection was placed too deeply. 5
Risk-Stratified Interpretation
Once you have measured the induration diameter, interpret using these cutoffs:
- ≥5 mm is positive for HIV-infected persons, recent TB contacts, those with fibrotic chest X-ray changes, organ transplant recipients, and immunosuppressed patients on ≥15mg prednisone daily for ≥1 month. 1, 2, 6
- ≥10 mm is positive for recent immigrants from high-prevalence countries, injection drug users, residents/employees of high-risk settings, and those with medical conditions increasing TB risk. 1, 2
- ≥15 mm is positive for persons with no TB risk factors. 1, 2
Special Considerations
Never dismiss a positive test in BCG-vaccinated individuals - tuberculin testing is not contraindicated in BCG recipients, and positive reactions should be interpreted as M. tuberculosis infection when the person is at increased risk. 1, 2, 6 BCG-induced reactivity typically wanes over time and is unlikely to persist beyond 10 years. 3
Patient self-reading is unacceptable - only trained healthcare personnel should read and interpret tuberculin skin tests. 2, 6