Skin Rash: How to Work Out the Cause

How to describe a rash

Doctors identify rashes by their features. Noting these helps you and your doctor:

• Type of spot: flat (macule), raised (papule), larger raised plaque, fluid-filled blister (vesicle), pus spot (pustule), welt (wheal)
• Color: red, pink, purple, brown, white, silvery scale; on darker skin, redness may look brown, purple or gray
• Pattern: ring-shaped, in a line, in a band, clustered, symmetrical, in skin folds
• Symptoms: itching, burning, pain, or none
• Timing: sudden or gradual, constant or coming and going

Common causes of skin rash

Rash in the groin and on the scrotum

The groin and scrotum are warm, moist and subject to friction, so rashes are common.

• Jock itch (tinea cruris): a fungal infection causing an itchy red or brown rash on the inner thighs and groin with a raised, scaly edge. It often spares the scrotum itself. It is linked to sweating, tight clothing and athlete's foot. Treat with an over-the-counter antifungal cream (clotrimazole or terbinafine) twice daily for 2 to 4 weeks, continuing a week after it clears.
• Candida (yeast): a bright red, moist rash that does involve the scrotum, with small "satellite" spots at the edges. More common with diabetes, obesity and after antibiotics.
• Intertrigo: red, raw, moist skin in folds from rubbing and sweat; may become infected with yeast or bacteria.
• Contact dermatitis: from soaps, detergents, condoms, lubricants or wipes.
• Psoriasis and seborrheic dermatitis: smooth red patches in the folds without much scale.
• Scabies and pubic lice: intense itch, with nodules on the scrotum or penis in scabies.
• Lichen simplex chronicus: thickened, leathery skin from long-term scratching.
• Sexually transmitted infections: sores, blisters or warts rather than a widespread rash.

An itchy scrotum without a visible rash can result from dry skin, sweat, irritant products, early fungal infection or nerve irritation. Keep the area clean and dry, wear loose cotton underwear, avoid scented products, and see a doctor if it persists beyond 2 weeks. A swollen, painful scrotum with a rash needs prompt assessment; see scrotal swelling.

Painful skin with no rash

Burning, tingling or sensitive skin with no visible rash often comes from a nerve problem rather than the skin. A common cause is early shingles: pain or tingling in a band on one side of the back, chest or face 1 to 5 days before blisters appear. Other causes include nerve irritation from the spine, peripheral neuropathy, fibromyalgia and the aching skin that comes with flu and fever. A doctor should check painful skin on one side of the body, because antiviral treatment for shingles works best when started early.

When a rash is serious

The glass test helps identify non-blanching rashes: press the side of a clear glass firmly onto the rash. If the spots remain visible through the glass, they may be bleeding under the skin (petechiae or purpura), which with fever can mean meningococcal infection or sepsis. Call 911 or your local emergency number. On darker skin, check paler areas such as the palms, soles and inside the eyelids.

Other serious rashes include Stevens-Johnson syndrome (blistering and peeling with mouth sores after a medicine), DRESS (widespread rash with fever and swollen glands weeks after a new medicine), and cellulitis.

How a doctor diagnoses a rash

Most rashes are diagnosed by looking at them and asking questions: when it started, where, whether it itches, new products, medicines, travel, contacts, pets and other symptoms. Tests are used when needed:

• skin scraping for fungus or scabies mites
• swabs for bacteria or viruses
• patch testing for contact allergy
• blood tests for infection or autoimmune disease
• skin biopsy for unclear or persistent rashes

Treatment and home care

• Avoid triggers: stop new soaps, cosmetics or detergents; use fragrance-free products.
• Moisturize dry, eczema-prone skin with thick creams or ointments.
• Cool compresses and oatmeal baths ease itching.
• Hydrocortisone 1% cream helps mild contact dermatitis and eczema for short periods; do not use steroid creams on suspected fungal rashes without advice, as they can make fungus spread.
• Antifungal creams for jock itch, athlete's foot and ringworm.
• Non-drowsy antihistamines for hives and allergic itch.
• Loose cotton clothing and keeping folds dry.
• Do not scratch; keep nails short.

Frequently asked questions

What is causing my skin rash?

Most rashes are caused by eczema, contact with an irritant or allergen, hives, fungal infection, viral illness, heat or insect bites. Look at where the rash is, how it looks, whether it itches, and anything new you used or took. A rash lasting more than 2 weeks or with fever should be checked.

What causes an itchy rash on the scrotum?

Common causes include fungal infection (jock itch or candida), irritation from sweat, soaps or detergents, intertrigo from friction, psoriasis, eczema and scabies. Keep the area clean and dry and wear loose cotton underwear. Persistent or painful rashes need a doctor.

What is the best treatment for jock itch?

An over-the-counter antifungal cream such as terbinafine or clotrimazole, applied twice daily for 2 to 4 weeks and continued for a week after the rash clears, usually works. Keep the groin dry, change underwear daily, and treat any athlete's foot to prevent reinfection.

Why is my skin painful but there is no rash?

Painful or burning skin without a rash can be the early stage of shingles, which often appears as pain on one side of the body days before blisters. Nerve irritation, neuropathy and viral illness can also cause it. See a doctor promptly if it is in a band on one side.

When should I worry about a rash?

Get emergency help for a rash that does not fade when pressed, especially with fever, or with blistering, peeling skin, mouth sores, swelling of the face or breathing difficulty. See a doctor soon for a rash with fever, a painful spreading red area, or a rash lasting over 2 weeks.

What kind of doctor treats skin rashes?

A primary care doctor can diagnose and treat most rashes. For persistent, recurrent or unclear rashes, you may be referred to a dermatologist. A urologist is not usually needed for a scrotal rash unless there is swelling or pain in the testicles.