How do I know if I need to take my kid to the doctor for their rash? what rash needs medical attention?

What the evidence says

Call or see a doctor promptly if a rash comes with a high fever, if the child looks or acts very unwell, or if a rash covers most of the body on both sides. Also seek care if the area around the rash becomes increasingly painful, swollen, warm, or has red streaks spreading from it, or if pus is draining from it, since these can signal a skin infection that is getting worse. A few specific rash patterns deserve prompt medical attention: a sunburn-like rash with a high fever (possible toxic shock or scarlet fever), a rash that looks like a bruise or does not fade when you press a clear glass against it (a possible sign of a serious blood infection), an expanding red rash after a tick bite (possible Lyme disease), or any rash where the skin looks like it's peeling or blistering badly.

For babies under 3 months, any fever with a rash should be checked the same day, since infections can be more serious at this age. Certain children, such as those with diabetes, sickle cell disease, or on medicines that weaken the immune system, should also be seen sooner if a new rash appears with fever. Many common childhood rashes, like roseola, fifth disease (slapped cheek), heat rash, or mild diaper rash, are harmless and go away on their own with home care, so not every rash needs a doctor visit.

If you are ever unsure, calling a health line (like 8-1-1 in BC) or your child's doctor for advice is a reasonable first step.