There are no hard-and-fast rules as to when you need to seek treatment for a rash. Oftentimes, you just need to follow your gut if you suspect something is unusual or wrong.
With that said, there are 10 signs that a skin rash may, in fact, be serious and requires immediate medical attention.
You Have a Fever
A fever is a common sign of an infection in which the body raises its temperature to kill a foreign invader. A rash that accompanied a fever suggests that the cause is infectious, but it may be due to a severe allergic reaction. In either instance, it is important to have it checked out immediately.
Examples include a bacterial infection called scarlet fever and a severe-whole body allergic reaction called anaphylaxis.
The Rash Covers Your Entire Body
A rash all over the body is always a cause for concern. Drug allergies commonly cause disseminated (widespread) rash, but it can also occur with Lyme disease (caused by a tick bite) or exanthems (reactive rashes mainly caused by viral infections like measles).
People who are immunocompromised (who lack immune defenses) can also have widespread rash when others might only have a localized rash. An example includes disseminated shingles in people with advanced HIV infection.
The Rash Is Painful
There is a difference between a rash being itchy and a rash being painful. Painful rashes warrant an immediate investigation as they may be the sign of a potentially serious but treatable infection like shingles or genital herpes.
In both cases, early treatment with antiviral drugs may reduce the duration and severity of the infection and, in the case of shingles, the risk of chronic nerve pain or vision damage.
You Have Difficulty Breathing
A sudden outbreak of rash or hives outbreak accompanied by shortness of breath and/or wheezing are classic symptoms of anaphylaxis. Anaphylaxis is a potentially life-threatening, whole-body allergy that requires immediate emergency care.
In addition to rash and breathing problems, symptoms of anaphylaxis may include abnormal heartbeats, diarrhea, nausea or vomiting, dizziness or fainting, and the swelling of the face, neck, or tongue.
If not treated immediately, anaphylaxis can lead to shock, coma, suffocation, heart or respiratory failure, or death. What Causes Anaphylaxis and Am I at Risk?
The Rash Is Blistering
A number of rashes are characterized by the formation of blisters, both small and large. These include chickenpox, shingles, and genital herpes. All of these need to be looked at by a healthcare provider without delay.
A blistering rash can also be a sign of a potentially life-threatening drug reaction. This includes Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN). Both cause skin swelling and pain in the early stages followed by widespread blistering and the peeling of the skin layers.
If left untreated, SJS and TEN can lead to shock and death due to the massive loss of fluid from open, exposed tissues.
The Rash Is Spreading Quickly
A fast-spreading rash is something you should never ignore. With some rashes, like shingles, you can sometimes watch the blistering lesions develop and spread before your eyes. The same can occur with a drug rash, which often starts on the chest and back first before spreading to the arms and finally the legs.
Arguably more concerning is a condition called cellulitis, a type of bacterial skin infection that initially starts as mildly inflamed skin but rapidly progresses, causing deep redness, swelling, warmth, and pain. As the infection spread, red streaks may begin to radiate outward accompanied by blistering or pus-filled bumps.
If not treated immediately with antibiotics, cellulitis can be fatal.
The Rash Appears Suddenly
Many rashes appear suddenly, such as a mosquito bite or heat rash, and are not a cause for alarm. However, when the sudden outbreak is severe and widespread, it is often a sign of a severe drug reaction. Generally speaking, the faster such an outbreak occurs, the more serious it is.
With SJS and TEN, the look of the rash may also be described as being “angry,” meaning that it is extremely red and painful looking. The same description may be applied to severe cases of cellulitis. In all of these situations, emergency medical care is needed.
The Rash Is Swelling or Bruising
Certain skin reactions, like hives, cause swelling due to the accumulation of fluids in the deeper layers of skin. In many cases, the swelling will resolve on its own with no harm to the tissues.
However, when the swelling is severe, you need to seek medical care immediately. This is especially true if you develop swelling of the tongue, face, or throat, all of which are signs of anaphylaxis. In cases like this, the swelling alone can block the airways and lead to suffocation and death.
A rash accompanied by bruising may be a sign of a serious condtion known as vasculitis (inflammation of blood vessels). Vasculitis limited to the skin may be the result of an infection (like a staph infection), a medication reaction (including to certain antibiotics), or an autoimmune disease (like lupus). In addition to bruising, there will often be raised bumps and reddish flat spots.
Depending on the cause, vasculitis can be mild, severe, or life-threatening.
The Rash Is Circular
Many rashes are round, but only a handful cause a circular or coin-shaped rash, referred to as nummular lesions. There is a type of eczema called discoid eczema that falls within this category ad may require medications to resolve. Ringworm also causes a circular rash that benefits from treatment with antifungal creams or lotions.
Arguably more concern are circular “bullseye” rashes in which a central lesion is surrounded by a circular ring. A bullseye rash is characteristic of Lyme disease, an infection caused by tick bites that, if left untreated, can lead to chronic fatigue, reactive arthritis, and mood or memory problems.
Erythema multiforme can also cause a bullseye rash, usually as a result of a drug reaction. Although erythema multiforme is generally self-limiting and not usually severe, it can sometimes be a prelude to SJS and TEN and should be looked at.
The Rash Is Infected
Rashes that are eruptive (meaning that they can break open or burst) leave the underlying tissues vulnerable to bacterial infections. These types of infections are called secondary infections because they occur on top of the first (primary) infection.
People with herpes, shingles, chickenpox, eczema, and psoriasis are espeically vulnerable to this unless they make effort to keep the broken skin clean.