Patients presenting with “red rashes” are a challenge for any provider, even those who have practiced dermatology for years. These patients are often highly symptomatic and understandably anxious about their condition. Many have been seen in multiple primary-care settings, heard a number of conflicting diagnoses, and gone through several unsuccessful treatments. Though the cause of their complaint may not be immediately obvious, clinicians can bring an organized approach to the problem by focusing on the differential diagnosis.

Figure 1. Vesiculobullous allergic contact dermatitis caused by hair dye

Contact vs. irritant dermatitis

True contact dermatitis (Figure 1) is caused by direct contact with an allergen that elicits a cell-mediated or delayed hypersensitivity reaction (Gell and Coombs type IV). Poison ivy is the classic example, but hundreds of potential allergens exist, including other members of the Rhus genus as well as other plants (fig trees, ficus, dieffenbachia, carrot tops, parsley, cow parsnips, fern), hair dye, nail polish, nickel, latex, perfumes, aftershaves, and neomycin. These are true allergies manifested by an intensely pruritic papulovesicular rash, often with transudative (clear, nonpurulent, low specific gravity) drainage. Contact dermatitis is rarely painful, and there is no adenopathy associated with it.

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Look for sharply demarcated patterns of involvement (e.g., affecting both feet, the linearity seen with poison ivy). There is no model of “infection” that looks or acts like contact dermatitis, so covering the patient with an antibiotic salve is unnecessary. In addition to counseling the patient about the offending substance, treat with systemic steroids, aluminum acetate compresses (Burow’s solution), and perhaps topical steroids. Do not prescribe prednisone dosepaks—these are too weak (the dose tapers down from 24.5 mg prednisone). Duration of therapy is also too short (only six days). A better strategy is 40 mg prednisone tapered over two weeks every morning after meals (for a 125- to 150-lb adult, assuming there are no contraindications, such as severe osteoporosis, diabetes, congestive heart failure, gastric ulcers, or severe depression).

Figure 2. Irritant dermatitis following a gasoline spill on the arm

Irritant dermatitis (Figure 2) is a nonallergic event triggered by chemical irritation of the skin, such as one might see on hands after exposure to paint thinners, harsh cleansers, alcohol or liquid fuel. Treatment with class II or III topical steroids (such as betamethasone) in ointment form will usually suffice.


“Red man syndrome,” in which the patient presents with whole-body erythema, often with desquamation, can be due to any of the following: psoriasis, cutaneous T-cell lymphoma, Sézary syndrome, seborrhea, drug reaction (e.g., vancomycin), or acute eczema (Figure 3). It is not unusual for these patients to require hospital admission for workup and treatment.

Figure 3. Erythroderma following a course of oral steroids

They are frequently ill and sometimes chilling. Some have high-output heart failure. Ask if there is a history of similar problems. For best results, include a history, physical (check for nodes and abdominal organomegaly), biopsy, and lab (complete blood count, including a manual differential, and chemistry screen). Inquire about the patient’s stress level, increase in alcohol intake, smoking, or recent changes in medication, which can worsen seborrhea/psoriasis. Erythroderma patients need prompt dermatologic referral to establish a diagnosis.

Pityriasis rosea

Pityriasis rosea (PR) is likely caused by one of the human herpesviruses, but this has never been proven.

Figure 4. Characteristic oval lesions of pityriasis rosea

Histopathologic examination shows the kinds of changes seen with other viral eruptions. PR is quite common, generally affecting patients in their second and third decades. Clinicians frequently have to search for the characteristic oval lesions (Figure 4) with centripetal scaling and ask about a herald patch (a single, round-to-oval, scaly, pink patch with a raised border), which is only seen in about a third of cases.Typically, PR looks as if it itches more than it actually does.

Treatment consists of oral erythromycin (400 mg t.i.d. for two weeks) and sunlight/UVA light coupled with plenty of patient education. This condition can last up to as long as nine weeks, during which time the patient may become understandably anxious.

The differential includes secondary syphilis, so ask about sexual exposure and examine the palms and soles for the presence of papulosquamous brown lesions. Tinea corporis is the primary differential, but while PR will be KOH-negative, tinea corporis will be KOH-positive. Also, instead of the oval lesions with centripetal scaling of PR, tinea corporis has round lesions and peripheral scaling. Moreover, tinea corporis seldom presents with the numerous and widespread lesions typical of PR.

Figure 5. Atopic dermatitis in ahealthy infant

Atopic dermatitis

Atopic dermatitis (AD) is often mislabeled “eczema,” but it is more than simply a dry, itchy rash. AD features an allergic history plus an inherited form of eczema in characteristic distribution. This is an extremely common condition, and nearly every patient will have an atopic personal and family history. AD often appears in infancy and is a prominent entry in the differential for cradle cap, diaper rash, and chapped cheeks (Figure 5). Children with AD will exhibit Dennie-Morgan folds beneath the eye plus hyperlinear palms. Beyond infancy, AD is highly symmetrical, papulosquamous, and extremely pruritic. It favors the antecubital/popliteal fossae, neck, wrists, umbilical area, and eyelids. Similar to PR, the darker the patient’s skin, the more likely the rash is to be dark.

Recommended treatment includes topical steroids, greasy moisturizers, and, in select cases, oral antibiotics. Perhaps the most important aspect of treatment, however, is patient and family education regarding the nature of the problem, potential remedies, and likely chronicity. If this is not stressed, many parents will waste time looking for the cause rather than concentrating on prevention through daily bathing with proper soaps and shampoos, liberal use of the right moisturizers, and appropriate use of prescription medications, especially steroids.

Figure 6. Symmetric red papulopustules and crusts of scabies


Scabies is a mite (Sarcoptes scabiei) that lives and reproduces on human skin (Figure 6). It causes intense and relentless itching and must be acquired from another already infected person. The mites on pets are species-specific and cannot live or reproduce on human skin. While a patient does not necessarily have to be poorly washed to become infected, he does have to have had prolonged contact (in some cases sexual) with someone who has scabies. Nursing-home residents and other institutionalized individuals are particularly at risk, as are nurses and teachers.

The diagnosis of scabies requires microscopic confirmation. Examine volar wrists, thenar/hypothenar areas, and interdigital areas, and for vesicles. Using a No. 10 blade, scrape intact vesicles and/or burrows with vigor. Examine the scrapings in KOH at 10x power for eggs, adults, or droppings. On men, look for edematous papules on scrotal walls and on the penis for diagnostic corroboration.Scabies is treated with permethrin cream and requires minimal environmental cleanup. Be sure to identify the source of the infection and treat the entire family at the same time, lest your patient become reinfested.

Figure 7. Drug eruption from oral amoxicillin/clavulinic acid

Drug rash

Widespread and itchy, a drug rash often requires a biopsy and extensive history to diagnose (Figure 7). These rashes tend to be morbilliform (measleslike) and/or urticarial and may be bullous. Biopsy can help rule out items in the differential, such as lupus, contact dermatitis, psoriasis, eczema, and xerosis. Obviously, history is crucial in identifying the culprit. Natural history is variable—it can be weeks before the rash appears and weeks before it goes away after the drug is stopped.

Figure 8. Ringed, scaly, itchy tinea corporis eruption

Tinea corporis

Tinea corporis refers to dermatophytosis (superficial fungal infection) of the body exclusive of other tineas, such as tinea cruris, which localizes to the groin. The eruption can be widespread but will most often have only one or two lesions (Figure 8). These will almost always have annular/scaly margins that demonstrate fungal elements on KOH examination. Ask about steroids as an exacerbating factor, as well as tacrolimus/pimecrolimus ointment (in which case the fungal rash can be so extensive as to be difficult to diagnose). Treat with a combination of oral and topical terbinafine if severity warrants.

Fungal infection requires a source (e.g., a new cat, a child, the locker-room floor). Susceptibility may be evidenced by the concurrent presence of other fungal infections. The patient may be atopic and/or immunosuppressed. While diabetes does predispose to candidal infection, it does not predispose to fungal infection.

Figure 9. Patches of xerosis after visting a public bath


Characterized by extremely dry skin, xerosis (Figure 9) favors older, fair-skinned patients who bathe daily and often use scented or colored bar soaps. Most patients have a long history of dry skin that has worsened with age as sebaceous glands release sebum of poorer quality. Xerosis will first be noted as crackly patches (eczema craquelé) on the legs and arms and/or round, scaly, eczematoid patches (nummular eczema) on the anterior tibia and ankles. The condition is exacerbated in the winter months when the ambient humidity is low.If you suspect xerosis, ask about the duration and temperature of the patient’s showers. A typical patient with xerosis takes long, hot showers, especially during the winter months. Often, the patient has begun swimming for exercise.

Treatment centers around reversing the drying/irritating factors and adding copious amounts of moisturizer. A significant aspect of dealing with xerosis is reassuring these patients that they have not “caught something.” The differential includes thyroid disease, use of diuretics, antihistamines or other anticholinergic medications, and irritant dermatitis. Often, xerotic patients will make things worse through the application of numerous OTC remedies, especially triple-antibiotic ointment.

Figure 10. Discoid lupus plaques


Lupus is an autoimmune condition that can present in a number of ways depending on the type. In general, lupus plaques are annular, with scaly margins and clearing/atrophic centers, and usually confined to sun-exposed skin. If you see the telltale “butterfly” malar rash, the patient will likely be ill with fever, malaise, and joint pain.

Discoid lupus favors the face, neck, and scalp and often features pink scaly areas that demonstrate follicular accentuation with atrophic/scarring centers (Figure 10). Examine the cuticles with an ophthalmoscope. Look for tortuous, dilated capillary loops. While these are not pathognomonic for lupus, their presence will get you into the connective tissue disease ballpark.

Figure 11. Lupus plaques following sun exposure

While diabetes mellitus produces

Gottron’s papules, which cover the joints of the fingers, lupus will spare those areas and affect interphalangeal skin. Biopsy followed by a lupus panel is a must in order to nail down the diagnosis.Women, especially those with darker skin, are at particular risk for lupus. Also at risk are patients who spend a lot of time in the sun (Figure 11) and those on certain drugs, such as minocycline. Lupus is included in the differential for otherwise unexplained psychotic episodes, joint pain, hair loss, fever, and neutropenia.

Figure 12. T-cell lymphoma in a 72-year-old woman

Cancers presenting as red rashes

These include but are not limited to the following: cutaneous T-cell lymphoma (Figure 12), Paget’s disease (mammary and extramammary), metastatic cancers (breast, colon, etc.), hematologic cancers (leukemia, other lymphomas), and islet-cell cancer.

Any rash that cannot be explained, will not respond to steroids, or looks as though it ought to be symptomatic but is not, demands biopsy. Ask about previous history of cancer and the origin of any scars (e.g., from mastectomy or lung-cancer surgery). Get a review of systems, and check the history for smoking. Find out how many years it has been since the patient’s last Pap smear, pelvic exam, mammogram, or prostate exam. Finally, ask whether the patient is seeing any specialists (e.g., pulmonary, GI, gynecologic).

Rising to the challenge

As Osler said, “A diagnosis is seldom made if not entertained.” When presented with a diagnostic dilemma, a clinician must sort through all the possible explanations for the findings. To do this, one must become familiar with the various differentials and how they are likely to present. Red rashes are a particular challenge, but knowing the various diagnostic possibilities gives one a distinct advantage.

Mr. Monroe is a physician assistant specializing in dermatology at Warren Clinic in Tulsa, Okla., and founder of the Society of Dermatology Physician Assistants.