How Scabies Is Diagnosed

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Because the scabies infection causes such discomfort and can be spread so easily among close contacts, timely diagnosis is important. Of course, that starts with bringing any signs or symptoms of scabies (severe itching, rash, etc.) to your doctor's attention. Most often, she will use clinical judgment alone to make a scabies diagnosis, considering the appearance of your skin and your risk for exposure. She will also work to rule out other concerns that might instead be affecting you, such as a drug allergy, eczema, or dermatitis. In some cases, testing of the skin or evaluation of skin samples may be performed.

scabies diagnosis
Illustration by Joshua Seong. © Verywell, 2018. 

Self-Checks

The first step in the diagnosis of scabies is recognizing it at home. If you notice any of the signs or symptoms of scabies, even without having had any known exposure to the infection, see your doctor.

Look for:

  • Itching/Urge to Scratch: An intense itching sensation involves the chest, arms, hands, fingers, legs, breasts or genital area.
  • Rash: Small red bumps, nodules, or pimple-like irritations, which are most commonly located on the webbing between fingers, on the inner part of the wrist, under the arms, and on the elbows, knees, and along the belt line can occur. Scabies does not usually affect the face and scalp. 
  • Burrows: Short zigzag or S-shaped lines or burrows, which appear as small tunnels, may appear. The parasite creates these tunnels as it settles in the body. 
  • Skin Wounds: Scratches, sores, and open wounds caused by excessive scratching or by a secondary bacterial infection may be present. 
  • Crusting: Crusting of the skin generally means that you have advanced scabies, often referred to as Norwegian scabies, though it could indicate another skin issue. Interestingly, people who have Norwegian scabies might not have the more common symptoms (itching, rash) associated with uncomplicated scabies. 

    Labs and Tests

    Your doctor may rely on their judgment of your signs and symptoms, your medical history, and your history of exposure to diagnose scabies. If someone you're in close contact with has scabies and you go for an evaluation because of them, know that you may still be treated for the infection even if you're not actually diagnosed with it.

    If your doctor feels that diagnostic tests may be useful in forming a diagnosis, there are a few she can choose from.

    Microscopic Examination

    The only way to be certain that your rash and itching are caused by scabies is to identify the mite itself. (Diagnosing scabies with 100 percent certainty, however, is not required for treatment.) The rash that scabies induces is easy to see, but the actual scabies mite is very tiny and invisible to the naked eye.

    Your doctor might place a drop of mineral oil on a burrow, take a scraping, and examine the sample under a microscope to look for the mites or their eggs. It is not always possible to see mites in a scraping, and it depends on how many of them are present near the surface of your skin. Sometimes, scratching can destroy the burrow, making it more challenging to locate the mite on the skin. 

    If you have Norwegian scabies, characterized by crusting on the surface of the skin, a large number of mites are normally present in the crusted areas, however.

    Ink Test

    An ink test may identify the burrow created by the scabies mite. This involves placing special ink on an area of skin that appears to be a burrow, wiping the ink away, and then seeing if some of the ink remains down inside the burrow.

    Polymerase Chain Reaction (PCR) Test

    A scraping of skin is used for the PCR test, an advanced genetic test that can identify material from even a small part of the parasite's body. This test, which has been used for other conditions, is only recently being studied for scabies. So far, PCR for scabies shows promise in research studies, but it is not widely available at this time.

    Differential Diagnosis

    Scabies has an appearance that is similar to a number of other itchy rashes, most of which are not contagious. When scabies is misdiagnosed as another rash, and untreated, the mite has a greater chance of spreading and affecting more people, as it can complete its life cycle and find new hosts. 

    The most common causes of a skin rash that looks like scabies include:

    • Allergies to medications or food, which can cause the development of a sudden rash, typically characterized by redness with small bumps or swelling, and less commonly associated with a rash anywhere on the body, including the face.   
    • Contact dermatitis, a rash caused by a reaction to material touching the surface of the skin. Most of the time, contact dermatitis appears as a flat, red patch in the areas of the body that came into contact with the allergy-inducing material. 
    • Impetigo, a highly contagious bacterial skin infection. This can appear as clusters of open wounds, often with crusting. 
    • Eczema, a common rash that often appears without a known cause, is normally characterized by small, bumpy red spots, often with surrounding redness, which may be itchy. 
    • Psoriasis, an autoimmune condition that primarily affects the skin, is generally characterized by itchy, thick, light-colored flaky patches on the surface of the skin. 
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