Skin rash diagnosis
A skin rash is not a specific diagnosis. Instead, it refers to any type of inflammation and/or discoloration that distorts the normal appearance of the skin.
Common rashes include eczema, poison ivy, hives and athlete’s foot.
Infections that cause rashes can be fungal, bacterial, parasitic or viral.
Over-the-counter products can be helpful for many rashes.
Skin rashes that last longer than a few days and are unexplained should be evaluated by a doctor.
What are the causes, symptoms and signs of common types of non-infectious rashes?
Some common, non-infectious rashes are listed and described on the following pages. If you have a new rash and fever or other generalized disease is associated with it, it is best to consult your doctor.
Heat rash (miliaria)
Atopic dermatitis, often referred to as eczema, is a common childhood disease that causes red, itchy, weeping rashes on the insides of the elbows and the back of the knees as well as on the cheeks, neck, wrists and ankles. It is often found in patients who also have asthma and/or hay fever.
Seborrheic dermatitis is the most common rash in adults. It leads to red scaling and often to an itchy outbreak that typically affects the scalp, forehead, eyebrows, cheeks and outer ears. In infants it can also affect the scalp (milk crust) and the nappy area.
Contact dermatitis is a skin rash caused either by contact with a particular chemical to which the patient is clearly allergic or with a substance that directly irritates the skin. Some chemicals are both irritants and allergens. This rash tends to cause tears and weeping and affects the areas of skin that have come into direct contact with the attacking substance. Common examples of allergic contact dermatitis are poison ivy, poison sumac, poison oak (same chemical, different plant) and reactions to costume jewellery containing nickel.
This is a common type of irritant contact dermatitis, which occurs in most infants and some adults who wear diapers when feces and urine are in contact with the skin for too long.
This is a whiny, mucous dermatitis that occurs on the lower legs of people who have chronic swelling due to poor venous circulation.
This bumpy, scaly eruption that neither cries nor drips. Psoriasis tends to appear on the scalp, elbows and knees. The skin condition results in silvery skin flakes that flake and fall off.
These red, itchy bumps appear suddenly and then disappear in about eight hours. They tend to reappear frequently. If hives is caused by a medication, this medication should be avoided in the future.
It is a weeping, mucilaginous dermatitis that appears as coin-shaped plaques in winter and is associated with very dry skin.
Certain drugs (such as antibiotics) can cause a skin rash as an undesirable side effect. The common appearance is similar to rashes caused by certain common virus infections. On the other hand, drugs can cause a variety of other types of rashes.
Heat rash (miliaria)
This skin breakout is caused by the closure of sweat ducts in hot, humid weather. It can occur at any age, but is most common in infants who are kept too warm. The heat rash looks like a red acne ball or small blisters. It is more common on the neck and upper chest, groin, under the breasts and in the folds of the elbows. The person must be moved to a cooler environment for treatment.
Rashes that are not caused by infectious organisms can be treated with an over-the-counter 1% hydrocortisone cream for about a week before seeking medical help. Over-the-counter oral antihistamines such as diphenhydramine (Benadryl) or hydroxyzine (Vistaril, Atarax) may help control itching.
How do healthcare professionals diagnose common rashes?
The term rash has no precise meaning, but is often used to denote a variety of red skin rashes. A rash is any inflammatory disease of the skin. Dermatologists have developed various terms to describe rashes. The first is to identify a primary, most common feature. The configuration of the rash is then described with adjectives such as “circular”, “annular”, “linear” and “serpentine”. Other features of the rash that are recorded include density, color, size, consistency, softness, shape, and even temperature. Finally, the distribution of the rash on the body can be very useful in diagnosis, as many skin diseases have a preference for occurring in certain areas of the body. Although certain findings can be a very dramatic component of the skin disease, they are of limited value in making an accurate diagnosis. These include findings such as ulcers, scaling and scabbing. Using this framework, it is often possible to make a list, called a differential diagnosis, of the diseases in question. For an accurate diagnosis of a rash, a doctor or other health professional is often required. Based on the differential diagnosis, specific laboratory tests and procedures can be performed to determine the cause of a particular rash.
Rashes caused by fungal infections
Fungal infections are quite common. Yeasts are botanically related to fungi and can cause skin rashes. These tend to affect skin folds (like the skin under the breasts or groin). They look fiery red and have pustules (blisters) on the edges.
Fungal and yeast infections have little to do with hygiene – even clean people get them. Fungal skin rashes are not often transmitted by dogs or other animals. They seem to be easiest to get in gyms, showers, swimming pools or changing rooms or from other family members. Many effective antifungal creams can be bought in pharmacies without a prescription, including 1% clotrimazole (Lotrimin, Mycelex) and 1% terbinafine (Lamisil). In the case of extensive infections or if toenails are involved, a prescription drug such as oral Terbinafine may be useful.
If a fungus has been repeatedly treated without success, it is worth considering the possibility that it was never really a fungus to begin with, but rather a form of eczema. Eczema is treated quite differently. A fungal infection can be independently confirmed by a variety of simple tests.
Rashes caused by bacterial infections
The most common bacterial infections of the skin are folliculitis and impetigo. Staphylococcus or streptococcus germs can cause folliculitis and/or impetigo, two diseases that are much more common in children than in adults. Bacterial outbreaks are often pustular (the bumps are covered with pus) or can be plaque-like and quite painful (as with cellulitis). In rare cases, strep throat can cause scarlet fever, a rash that affects large areas of skin. Rashes caused by certain classes of bacteria, rickettsia or spirochetes, Rocky Mountain spotted fever or secondary syphilis can often be clinically suspected.
Browse our medical image collection on allergic skin diseases such as psoriasis and dermatitis and more, which are caused by allergies
Rash due to parasites
One of the most common rashes after a parasite infection is scabies. Scabies is produced by a small mite (related to a spider). This mite is usually contracted by prolonged contact with another infected person. The mite lives in the superficial layers of human skin. It produces
Rashes that typically occur as part of certain viral infections are called exanthema. Many rashes of viruses are more often symmetrical and affect the skin surface all over the body, including roseola and measles. Sometimes certain viral rashes are localised on the cheeks, such as parvovirus infections (fifth disease). Other viral infections, including herpes or shingles, are usually localised to a part of the body. Patients with such rashes may or may not have other symptoms such as coughing, sneezing, localized burning or upset stomach (nausea). Viral rashes usually last from a few days to two weeks and subside by themselves.
Non-prescription (over-the-counter) remedies include the following:
Anti-itching creams with 1% hydrocortisone cream may be effective
Oral antihistamines such as diphenhydramine and hydroxyzine can be helpful in controlling itching.
Fungal infections are best treated with topical antimycotics containing clotrimazole (Lotrimin), miconazole (Micatin) or terbinafine (Lamisil).
If these measures do not help, or if the rash persists or spreads, consultation with a general practitioner or dermatologist is recommended.
There are many, many other types of rashes that we have not covered in this article. So if you have questions about the cause or medical treatment of a rash, it is especially important to consult your doctor. This article is, as the title suggests, only an introduction to common rashes.
A word about smallpox vaccination in patients with rashes
People with atopic dermatitis or eczema should not be vaccinated against smallpox, regardless of whether the disease is active or not. Patients with atopic dermatitis are more susceptible to the virus spreading on their skin, which can lead to a serious, even life-threatening condition known as eczema vaccinatum. Other rashes have a much lower risk of medical complications. Talk to your doctor about the smallpox vaccine.
What is the prognosis for a rash?
The prognosis depends on the cause of the rash. Accurate identification is therefore very important in predicting how the rash will heal.
Is it possible to prevent rashes?
If the cause of a particular rash is known, it can be prevented. For example, a measles vaccination would be of great benefit in preventing the rash of measles, but also more serious consequences of measles infections.