"A rash on the arms" tells a dermatologist almost nothing, and it is the reason so many Advice and Guidance requests come back with unhelpful answers. Describing skin precisely is not innate talent; it is a learnable framework, and it does two things at once: it gets you a genuinely useful answer from teledermatology or a specialist, and it forces the systematic looking that sharpens your own diagnosis. Here is the vocabulary dermatologists use, organised so you can apply it in a couple of minutes, with a worked example and a checklist for photo-based requests.
Key takeaways
- Precise description gets useful remote advice; vague description wastes an Advice and Guidance request.
- Work through distribution, then configuration, then morphology, then colour and secondary change.
- Name the primary lesion accurately: macule, papule, plaque, vesicle, pustule, nodule or wheal.
- Always test for blanching, because non-blanching purpura changes the whole differential.
- Good photos plus a structured description plus a specific question is what gets a fast, accurate answer.
Why this matters
Teledermatology and Advice and Guidance now handle a large share of skin referrals, which means a specialist is often diagnosing from your words and your photos rather than the patient in front of them. The quality of their answer is capped by the quality of your description. A precise description lets them recognise a pattern remotely and give you a confident answer, while a vague one forces them to hedge or ask you to refer anyway. Beyond the referral, the act of describing skin methodically is itself diagnostic, because it makes you look at features you would otherwise skip. Learning the vocabulary is therefore both a communication skill and a clinical one.
The framework
Work through the same sequence every time, and the description writes itself.
Distribution and site. Where is it, and what is the pattern? Localised or generalised; symmetrical or asymmetrical; flexural (in the creases) or extensor (over the surfaces); photodistributed (sun-exposed areas); acral (hands and feet); dermatomal (following a nerve root); or centripetal versus centrifugal. Distribution alone often narrows the differential dramatically.
Configuration and arrangement. How are the individual lesions arranged relative to each other? Discrete (separate), confluent (merging), grouped or clustered, annular (ring-shaped), targetoid (concentric rings), linear, reticulate (net-like), or serpiginous (snake-like). A grouped vesicular pattern suggests something very different from a scattered one.
Morphology: name the primary lesion. This is the core skill. A macule is a flat area of colour change under about a centimetre, a patch is the same but larger. A papule is a raised lesion under a centimetre, a plaque is a raised flat-topped lesion larger than that. A nodule is a deeper, larger raised lesion. A vesicle is a small fluid-filled blister, a bulla a large one, a pustule contains pus, and a wheal is a transient raised area of dermal oedema, as in urticaria. Naming the primary lesion correctly is the single most useful thing you can do.
Secondary changes. What has happened to the surface? Scale, crust, erosion (loss of epidermis), ulcer (loss into dermis or deeper), excoriation (scratch marks), lichenification (thickened skin with accentuated markings from chronic rubbing), fissure, or atrophy. Secondary changes tell you about chronicity and behaviour.
Colour, and the blanching test. Describe the colour: erythematous (red, blanching), violaceous (purple), hyperpigmented or hypopigmented, or purpuric. Then press the skin, or use a glass slide: erythema blanches, purpura does not. Non-blanching purpura is a red flag that shifts the differential toward vasculitis, meningococcaemia and bleeding disorders, so this simple test genuinely changes management.
Round it off with size, number, and border definition (well-demarcated or ill-defined), and you have a complete picture.
A worked example
Take "a rash on the arms" and apply the framework. It becomes: a symmetrical eruption on the extensor surfaces of both forearms, of discrete and confluent well-demarcated erythematous plaques with overlying silvery scale, non-tender, no secondary infection, present for six weeks. That description points a dermatologist toward psoriasis almost immediately, whereas the original told them nothing. The transformation took under two minutes and used only the framework above. That is the difference between a useful request and a wasted one.
What to include in a teledermatology or Advice and Guidance request
A good remote request has four parts. First, photographs: a well-lit overview showing distribution, a close-up showing morphology, and where relevant a dermoscopic image, all in focus, in natural light, with a scale reference. Second, the structured description above. Third, the relevant history: duration, evolution, symptoms such as itch or pain, systemic features, drug history, and what has been tried. Fourth, and most often forgotten, a specific question: "Is this consistent with psoriasis and can it be managed in primary care?" is answerable; "please advise" is not. The specific question is what gets you a specific answer.
Where iatroX fits
Learning this vocabulary and the differentials it points to is exactly the kind of structured clinical reasoning iatroX supports. You can work through skin presentations with a Socratic tutor that builds the pattern recognition, and practise the differentials with adaptive questions, with free sample questions to try at iatroX. For the resources that pair with this skill, see the best dermatology resources for UK GPs, and for the wider picture of getting help, how doctors find help online.
Frequently asked questions
How do you describe a rash properly? Work through distribution and site, configuration and arrangement, the primary lesion morphology, secondary changes, and colour, then add size, number and border definition. This sequence produces a complete, dermatologist-ready description.
What is the difference between a macule, papule and plaque? A macule is a flat area of colour change under about a centimetre, a papule is a raised lesion under a centimetre, and a plaque is a raised, flat-topped lesion larger than a centimetre. Naming the primary lesion accurately is the key skill.
Why is the blanching test important? Because it distinguishes erythema, which blanches, from purpura, which does not. Non-blanching purpura shifts the differential toward vasculitis, meningococcaemia and bleeding disorders, so it changes management. Always test with pressure or a glass slide.
What makes a good teledermatology request? Clear, well-lit, in-focus photographs showing both distribution and morphology, a structured description, the relevant history, and a specific answerable question rather than "please advise".
Can describing a rash well improve my own diagnosis? Yes. Working through the framework forces systematic observation of features you would otherwise skip, which is itself diagnostic. The communication skill and the clinical skill are the same skill.