Finding the cause of a skin problem usually takes three things: the story of the problem (the history), a careful look at the whole pet, and tests picked to fit what your vet sees [1][2]. Many skin diseases look alike on the surface. Even pustules, the typical sign of a bacterial skin infection, can be an early sign of an immune disease that has nothing to do with bacteria [2].
Expect more than one step. Your vet may need several tests, a treatment trial or a recheck before the cause is clear [1][2].
This page explains what each common test looks for, and links to our guides on the problems it can help find.
What your vet will ask
Your answers help your vet narrow down the list of possible causes [1]. Before the visit, think about:
- what is bothering your pet, and how long the problem has been there;
- how old your pet was when it started, and your pet’s breed;
- licking, rubbing, scratching or chewing of the skin;
- how the problem started, how it has changed, and where on the body you first saw it;
- whether it is worse at certain times of year or the same all year round;
- any treatments your pet has had, and how your pet responded;
- fleas or other parasites, recent travel, and any other animals your pet lives with or meets;
- changes at home, such as a new food, bedding or cleaning product;
- other signs, such as drinking or urinating more, a change in appetite or energy, or more panting.
Itch does not always look like scratching. Guidelines for dogs with skin allergies list licking, excessive grooming, rubbing, chewing, scooting and head shaking as signs of itch too [1]. Itch that comes with the seasons points more toward pollens, while itch all year round fits house dust mites or food better [1]. The age the problem began matters as well: when skin infections keep coming back in a dog that was young when they started, an allergy is one of the most likely causes behind them [2]. Also tell your vet about recent baths, shampoos or treatments, since they can change what the skin looks like on the day.
The physical exam
Your vet will look over the whole body, not just the sore spot, to see how far the problem reaches and where on the body it sits [2]. The pattern is a clue. In dogs with skin allergies, for example, the face, the inside of the ears, the underside of the body, the armpits, the groin, the area around the bottom and the paws are the places most often affected [1].
The tests and what each one looks for
Most of the basic tests are quick and can be done in the clinic: combing the coat, scraping the skin, plucking a few hairs and looking at skin samples under the microscope [1]. Your vet chooses which ones your pet needs.
Skin scrapings
Your vet gently scrapes the skin surface and looks at the material under a microscope to search for mites [1]. Demodex mites live deeper in the hair follicles, so the vet takes several deeper scrapings, and may add hair plucks or tape samples from squeezed skin [1]. Scabies mites are looked for with several shallow scrapings, and a blood test can help where it is available [1]. Mite problems include demodectic mange in dogs, sarcoptic mange (scabies) in dogs and mange in cats.
Hair combing
Combing or brushing the coat, often with a fine flea comb, collects fleas and flea dirt, and the debris can be checked for other parasites such as lice and walking dandruff mites [1]. The dog allergy guidelines say every itchy dog should be checked for fleas or flea dirt this way [1]. See flea allergy dermatitis and lice on dogs and cats.
Plucked hairs under the microscope (trichogram)
Your vet gently plucks a few hairs and looks at them under a microscope. Plucked hairs can show Demodex mites, lice eggs stuck to the hair shafts, and signs of ringworm fungus [1][2][3]. Ringworm is caused by fungi called dermatophytes; see ringworm. In one study of dogs and cats with ringworm, hair plucks found the fungus in about two out of three animals, so a negative result does not rule it out [3]. For coat problems that run in certain breeds, see color dilution alopecia.
Cytology (tape or slide samples)
Your vet presses a glass slide or clear tape on the skin, stains the sample and looks at it under a microscope for bacteria, yeast and inflammatory cells [1][2]. Guidelines on skin infections in dogs say this should be done for every suspected bacterial skin infection, and that it is cheap, fast and possible in any clinic [2]. It is also the most useful test for Malassezia yeast [1]. See Malassezia yeast skin infection in dogs, pyoderma in dogs and pyoderma in cats.
Fungal culture
For ringworm, no single test is perfect, so vets usually combine a microscope check of hairs and scale with a fungal culture [3]. To collect the sample, the coat over the affected skin is often brushed with a clean toothbrush [4]. A culture takes time to grow, so the result is not ready on the day of the visit.
Bacterial culture
A bacterial culture grows the germs from the skin and tests which medicines can still work against them [2]. It is not needed for every infection, but vets recommend it when an infection is deep, when earlier treatment has not worked, when rod-shaped bacteria are seen on cytology, or when resistant bacteria are a concern [1][2]. For deep infections, swabs from the skin surface often miss the bacteria that matter, so the sample may come from a small piece of tissue instead [2]. A culture result is always read together with the exam, because bacteria that are simply living on the skin are not the same as an infection [4].
Skin biopsy
A biopsy removes a small piece of skin so it can be examined under a microscope in a lab. Vets consider one when lesions remain despite treatment or when other causes need to be checked [2], and it is a main test for immune-mediated diseases and for growths [2]. Autoimmune blistering diseases, for instance, are usually diagnosed by putting together the clinical signs, the biopsy and, where available, special immune tests on the tissue [5]. See pemphigus foliaceus in dogs and cats. For lumps, see lumps and bumps on dogs and lumps and bumps on cats.
Blood and urine tests
Blood and urine tests are not part of every skin workup. Your vet may suggest them when the cause is still unclear after the first steps, for example to check for hormone problems that can affect the skin [2]. In older dogs, some underlying diseases only show up as they progress, so blood and urine tests, imaging or a biopsy may be repeated over time [2]. See skin signs of internal cancer in cats.
Allergy tests (skin and blood)
Allergy tests are often misunderstood. In dogs, there is no test that can diagnose a skin allergy on its own: the diagnosis is made by ruling out other itchy skin diseases first [1]. Intradermal skin tests and blood tests for allergen-specific IgE are used after that, mainly to find which allergens to avoid or to include in allergy shots [1]. Blood tests often turn up positive results in dogs that are not allergic at all, and neither test is reliable for diagnosing a food allergy [1]. Results always have to be read alongside your pet’s history and signs [1]. See seasonal allergies in dogs and, for cats, feline atopic skin syndrome.
Why it can take time
Some results come back from an outside lab, and cultures need time to grow. Your vet may also use your pet’s response to treatment as part of the diagnosis. A yeast skin problem, for example, is confirmed when the clinical signs and cytology fit and the skin improves with antifungal treatment [1]. Skin infections are rechecked during treatment, and if lesions remain, cytology and sometimes culture are repeated [2]. So several tests, a treatment trial or repeat visits may be needed before your vet can name the cause.
When to see your vet
See your vet if your dog or cat has:
- itching, licking, rubbing, scratching or chewing of the skin [1];
- a skin problem along with drinking or urinating more, a change in appetite or energy, or more panting;
- a skin problem that has not improved with the treatment your vet gave, so your vet can look again [2].
FAQ
Will one test give the answer?
Not always. Many skin diseases look alike, and the cause is found by combining the history, the exam, tests and the response to treatment [1][2]. For ringworm, for example, vets usually use at least two tests [3].
Does a positive allergy test mean my pet is allergic to that thing?
Not on its own. Blood allergy tests can be positive in dogs that are not allergic, and allergy tests are only used after a skin allergy has been diagnosed by other means [1].
Should I bathe my pet before the visit?
Ask your vet first, and tell them when your pet was last bathed or treated.
The bottom line
Vets diagnose skin problems with a history, a full exam and tests chosen for your pet. Scrapings and hair samples look for mites and fungi, combing finds fleas and other parasites, cytology and cultures look for bacteria, yeast and fungi, and a biopsy shows the skin under the microscope. Many skin diseases look alike, so expect more than one test or visit.
This article is for general information only and does not replace advice from your veterinarian. If your pet has a skin problem, see your vet.
Sources
- Hensel P, Santoro D, Favrot C, Hill P, Griffin C. Canine atopic dermatitis: detailed guidelines for diagnosis and allergen identification. BMC Veterinary Research. 2015;11:196. Open access, CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC4531508/
- Loeffler A, Cain CL, Ferrer L, et al. Antimicrobial use guidelines for canine pyoderma by the International Society for Companion Animal Infectious Diseases (ISCAID). Veterinary Dermatology. 2025;36:234-282. Open access, CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC12058580/
- Bouza-Rapti P, Karafylia A, Tamvakis A, Farmaki R. Comparison of Adhesive Tape Impression Cytology, Hair Plucks, and Fungal Culture for the Diagnosis of Dermatophytosis in Dogs and Cats. Veterinary Sciences. 2023;10(3):183. Open access, CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC10055676/
- Cellier L, Guimarães H, Cardoso A, Campos C, Barradas P, Miranda C. Infectious Dermatitis in Companion Animals: A One Health Perspective. Animals. 2026;16:2284. Open access, CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC13463489/
- Bizikova P, Olivry T, Linder K, Rybnicek J. Spontaneous autoimmune subepidermal blistering diseases in animals: a comprehensive review. BMC Veterinary Research. 2023;19:55. Open access, CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC9969658/