DR14.3 | Dermographism Demonstration — Summary & Reflection

KEY TAKEAWAYS

Dermographism is the commonest inducible (physical) urticaria, and its diagnosis is made at the bedside by reproducing the patient's wheal through a controlled skin stroke. The test is indicated when a patient reports transient itchy linear wheals at sites of stroking, scratching, rubbing, or pressure. It works by eliciting the triple response of Lewis — red line (capillary dilation) → red flare (axon reflex) → wheal (histamine-mediated oedema) — using a firm blunt object (a dermographometer or, at the bedside, a wooden swab end), a timer, and good lighting, with consent. The technique is to apply a single firm, even linear stroke and observe the line at 3–5 minutes for a raised wheal, then ask whether it itches. Interpretation pivots on that itch: a wheal with itch is symptomatic dermographism (treat), a wheal without itch is asymptomatic simple dermographism (a ~5% normal variant — reassure), no wheal is negative, and recent antihistamine use is the classic cause of a false-negative. A positive symptomatic result is managed exactly as inducible urticaria — explanation, trigger avoidance, and a second-generation non-sedating H1-antihistamine first-line, up-titrated if needed — while an asymptomatic result needs reassurance only.

REFLECT

Think about how this single bedside manoeuvre changes your approach to a patient with a vague history of 'itchy lines on the skin.' Before learning it, you might have ordered allergy tests or referred the patient; now you can confirm or refute the diagnosis yourself in five minutes with a swab and a watch. Reflect on the discipline the test demands: applying a consistent, controlled stroke, waiting the full interval rather than reading the result too early, and — above all — asking the patient whether the wheal itches before deciding what it means. Consider, too, how you will avoid the two traps this module emphasised: over-labelling the healthy person whose skin simply writes, and being misled by a false-negative in someone who took an antihistamine that morning. Rehearsing the manoeuvre and its interpretation now, ideally on a willing colleague under supervision, is what will let you perform it smoothly and confidently when a real patient presents.