Don't Miss That Melanoma: The ABCDE Rule (and Its Limits)
A patient once pointed to a mole on his back and said, "My wife's been telling me to get that checked for two years." It turned out to be nothing. But I've also had the opposite happen — a spot nobody thought twice about turned out to be exactly the thing worth catching early. Melanoma is one of the more treatable cancers when it's caught early, and one of the more dangerous ones when it isn't. Here's what to actually look for.
The ABCDE rule
This is the tool most doctors teach patients, because it's easy to remember and catches most melanomas:
- A — Asymmetry. One half of the mole doesn't match the other. A normal mole is roughly round or oval; draw a line down the middle and the two sides look alike.
- B — Border. The edges are ragged, notched, or blurry, rather than smooth and well-defined.
- C — Colour. More than one shade within the same spot — often shades of brown, black, tan, or (a real red flag) patches of red, white, or blue.
- D — Diameter. Larger than about 6mm — roughly the size of a pencil eraser — though melanomas can be caught smaller.
- E — Evolving. Any change in size, shape, colour, or texture over weeks to months. This is arguably the most important letter of the five. A mole that's been stable and unremarkable for years behaving itself is reassuring; a mole that's changing is the one I want to see.
The "ugly duckling" sign
Here's a shortcut I find just as useful as ABCDE, and easier to apply if you're not going to sit there measuring things with a ruler: most people's moles look like siblings of each other — similar colour, similar shape, a family resemblance. A mole that doesn't look like any of your others — the "ugly duckling" — deserves a second look even if it doesn't obviously break any of the ABCDE rules on its own. Your own pattern of moles is the baseline; the outlier is the one worth mentioning.
Check your own skin — it's worth the ten minutes
You don't need a formal ritual, but periodically looking over your own skin — scalp, back, soles, between the toes, all the places you don't normally see — is genuinely worthwhile. A partner, or a phone camera and a mirror, helps with the spots you can't see yourself. You're not trying to diagnose anything. You're just building familiarity with your own skin, so that something new or different actually registers as new or different.
Where ABCDE falls short
I want to be honest about this rule's limits, because I don't want anyone dismissing a spot just because it "doesn't fit." Some melanomas — particularly the nodular type, and amelanotic melanomas that have little or no pigment — don't look like the classic asymmetric, multicoloured spot at all. They can be a firm, pink or skin-coloured bump that grows steadily, sometimes bleeds, without ever checking most of the ABCDE boxes. So the rule is a helpful guide, not a filter you should trust completely. The real rule is simpler: anything new, anything changing, anything that won't heal, or anything that just worries you is worth a doctor's look — whether or not it fits a neat acronym.
Don't forget non-melanoma skin cancers
Melanoma gets most of the attention, but basal cell and squamous cell skin cancers are far more common — and they don't play by the ABCDE rules either. Watch for a spot that's new and won't heal, a sore that scabs over and reopens repeatedly, or anything that bleeds without an obvious cause (a bump against furniture, for instance). These are usually very treatable when caught, and rarely dangerous the way melanoma can be — but they still deserve a look, not a shrug.
A word on prevention
Most of this is common sense you've heard before, and it's still true: broad-spectrum sunscreen, seeking shade in the middle of the day, and a hat and sun-protective clothing when you're outdoors for a while. Tanning beds are a genuine, avoidable risk factor — there's no version of a "safe base tan." None of this guarantees anything, but it meaningfully shifts the odds in your favour over a lifetime.
When to see your doctor
Book a visit for any mole or spot that's new, changing, asymmetric, has an irregular border or multiple colours, is growing, doesn't look like your other moles, or is bleeding or not healing. You don't need to be certain something is wrong — "I'm not sure, but this is bugging me" is a completely reasonable reason to come in. Catching melanoma early is exactly why this kind of check is worth five minutes of a doctor's time, and yours.
General information, not a substitute for individual medical advice. If you're worried about a spot on your skin, don't wait — see your family doctor or a dermatologist.
— Dr. Carlos Yu, Ajax Harwood Clinic