Eczema is usually straightforward to diagnose with a simple doctor visit and without any lab tests. Eczema may be diagnosed and treated by a pediatrician, primary care physician, dermatologist, or allergist.
Eczema is diagnosed with a combination of a physical exam and a medical and family history. In some cases, the doctor may order blood tests, allergy tests, or a skin biopsy to confirm eczema or rule out other conditions. Some tests can provide evidence that supports a diagnosis of eczema or clues to what is triggering symptoms.
The doctor will take a thorough history, asking about symptoms over time, family medical history, and any exposures to chemicals. A clear picture may emerge from the medical history that will help a doctor assess risk factors that may strengthen the suspicion of eczema or rule out other conditions. If you or others in your family have asthma, food allergies, or hay fever, eczema may seem more likely.
The doctor will perform a physical exam to evaluate skin symptoms and look for any other signs of illness that may be related. The doctor may be confident in an eczema diagnosis or order more tests if the diagnosis is in question.
There are two types of skin testing that may be performed when eczema is suspected. Most commonly, a patch test can be used to confirm a diagnosis of contact dermatitis, a specific type of eczema. Patch testing can also help identify the substance that triggers contact dermatitis. In patch testing, one or more small patches are placed on your skin during an office visit. You will return to the doctor’s office 48 hours after the patch test is placed for the first reading and then 48 hours later for a delayed reading.
Rarely, the doctor may perform a skin biopsy — removing a tiny piece of skin for laboratory examination. Usually, a punch biopsy will be performed to confirm the diagnosis.
If the doctor suspects an allergic reaction is causing your eczema, a blood test may be administered. The specific IgE test — also called the RAST or ImmunoCAP test — measures the level of immunoglobulin E (IgE) in the blood that reacts to that allergen. Blood testing is a much less reliable test than skin testing, since it produces many false positives. Results often indicate that an allergy exists when it does not.
If the doctor suspects your rash may be caused by a food allergy, they may recommend you undergo allergy testing.
Other conditions can produce symptoms like those of eczema. The process of ruling out similar conditions is referred to as differential diagnosis. Conditions that can mimic symptoms of eczema include:
Your doctor may be able to quickly rule out many of these conditions based on your medical and family history or simple blood tests. Other disorders may require time and repeated tests before they can be confirmed or ruled out. The presence of other conditions in addition to eczema may complicate the diagnostic process and eventually result in multiple diagnoses.