Positive vs. Negative TB Skin Test: Key Differences Explained
A Positive TB skin test shows a raised, firm bump ≥5–15 mm after 48–72 h, signaling possible exposure. A Negative result is flat or <5 mm, suggesting no current reaction.
Patients panic when they see any redness and assume “positive.” Clinics also vary in how they measure, so people leave uncertain and search online before follow-up.
Key Differences
Positive: measurable induration meets size thresholds; prompts chest X-ray and possibly treatment. Negative: no significant swelling; usually no further tests unless immunocompromised.
Which One Should You Choose?
You don’t choose—your immune system does. If high-risk, repeat annually; if newly positive, start clinical work-up to rule out active TB.
Examples and Daily Life
A teacher gets 7 mm, recalls a student with TB, and undergoes X-ray—clear. A college athlete shows 2 mm, continues play, no isolation. Both share the same waiting-room photo online.
Can redness without a bump be positive?
No—only firm induration counts; simple redness is irritation, not infection.
Does a past vaccine always cause a positive result?
BCG can wane; size and risk factors decide interpretation, not vaccine history alone.