Skin cytology is the microscopic examination of cells collected from the skin surface, from within skin lesions, or from masses just beneath the skin. It serves as a rapid, minimally invasive diagnostic tool in both human and veterinary medicine, often providing answers within minutes rather than the days required for a tissue biopsy. In veterinary dermatology especially, it is considered a first-line test for infections, inflammatory conditions, and tumors, while in human medicine a Cochrane review found that exfoliative cytology can detect basal cell carcinoma with roughly 97% sensitivity when clinical suspicion is already high. The technique’s real value lies in its speed and accessibility, but its accuracy depends heavily on how samples are collected, stained, and interpreted.
How Skin Samples Are Collected
There is no single “right” way to gather cells from the skin. The method depends on what the clinician suspects and where the lesion is located. The most common approaches fall into a handful of categories, each with different strengths.
- Impression smear: A glass slide is pressed directly against a moist or ulcerated lesion. This works well for oozing sores and exposed surfaces and tends to pick up inflammatory cells like neutrophils more effectively than some alternatives.
- Acetate tape strip: A piece of clear adhesive tape is pressed onto the skin, pulled away, and placed on a slide. This is especially useful for detecting surface-dwelling yeast organisms and for sampling areas that are hard to press a rigid slide against, such as skin folds or lip margins.
- Skin scraping: A scalpel blade is drawn across the skin to collect superficial or deeper cell layers. Superficial scrapings target fungi and surface mites, while deep scrapings that draw a small amount of blood are used to find burrowing mites like Demodex.
- Fine needle aspirate: A needle attached to a syringe is inserted into a mass or nodule, and cells are suctioned out. This is the go-to technique for evaluating lumps and bumps suspected to be tumors.
Each method captures a slightly different population of cells, which means the choice of technique can influence the diagnosis. A study comparing impression smears and acetate tape preparations found that the two methods agreed well on bacteria and yeast detection, but impression smears picked up significantly more neutrophils, while yeast detection was roughly equivalent across tape-strip staining methods.1PubMed. Impression Smear Agreement with Acetate Tape Preparation for Cytologic Sampling In some clinical contexts, the tape strip technique produces clearer differences between affected and unaffected sites, making it particularly useful when the question is whether a specific skin fold or lip margin is infected.2PubMed. Clinical features, cytology and bacterial culture results in dogs with and without cheilitis and comparison of three sampling techniques
A newer “slurry” technique, which involves mixing a swab sample with a small amount of liquid before spreading it on a slide, has been shown to detect more bacteria per sample than either impression smears or tape strips. In one study, the slurry approach found an average of about 13 additional bacteria per cytological sample compared to the standard methods. For yeast, though, tape strips still had the edge.3PubMed. A novel cytological technique for bacterial detection on canine skin
Bacterial Skin Infections
One of the most frequent reasons to perform skin cytology is to confirm a bacterial infection, especially superficial pyoderma in dogs. The hallmark finding is cocci (round bacteria) sitting on or inside neutrophils. When bacteria are found inside these white blood cells, it confirms that the immune system is actively fighting an infection, rather than the bacteria simply colonizing the skin surface. Researchers have tested how reproducible these readings are and found that when clinicians use a standardized counting method, the technique identifies superficial pyoderma with about 93% sensitivity. The tradeoff is that specificity for identifying truly normal skin is lower at around 51%, meaning some healthy dogs get flagged. If the criteria are tightened so that “normal” is defined strictly as the absence of both neutrophils and intracellular bacteria, specificity jumps to 98%, though sensitivity drops to about 64%.4PubMed. Reproducibility of a quantitative cutaneous cytological technique
This tradeoff is worth understanding because it reflects a practical decision every clinician faces: cast a wide net and catch nearly every infection (but over-treat some normal patients), or set the bar high and miss a chunk of real infections (but almost never treat unnecessarily). In practice, most veterinary dermatologists lean toward the sensitive end of the spectrum and treat when cytology looks suspicious, using culture and sensitivity testing to guide antibiotic selection when infections are deep or recurrent.
Yeast and Fungal Infections
Malassezia yeast is a normal inhabitant of canine and feline skin, but it can overgrow and cause itchy, greasy dermatitis. Adhesive tape strip cytology examined at high magnification is the standard method for making this diagnosis. The organisms appear as small, peanut-shaped budding cells that stain dark purple with typical cytology stains.5PubMed Central. Repeatability and Reproducibility of Microscopic Examination of Adhesive Tape Strip Cytology Slides for the Diagnosis of Malassezia Overgrowth and/or Malassezia Dermatitis in Dogs The count of organisms per microscopic field guides the clinician. A few scattered yeast cells might be normal colonization, while clusters in every field suggest overgrowth that warrants antifungal therapy.
Deeper fungal infections are a different challenge. In human medicine, cytology has been evaluated for diagnosing conditions like chromomycosis, blastomycosis, sporotrichosis, candidiasis, and alternariosis. In a small case series, cytological methods correctly identified three out of five deep fungal infections, with the remaining two requiring biopsy or fungal culture for confirmation.6Journal of Cytology. Application of Cytology in the Diagnosis of Fungal Infections of the Skin The takeaway is that cytology can sometimes catch deep fungal disease quickly, but a negative cytology does not rule it out, and culture remains necessary for species identification and drug-susceptibility testing.
For superficial dermatophyte infections (ringworm), the standard screening test is a potassium hydroxide (KOH) preparation of skin scrapings, which dissolves skin cells and leaves fungal elements visible. This test’s sensitivity generally falls between 80% and 95% depending on sample quality and the examiner’s experience, and its specificity can exceed 90%.7PubMed Central. Advancements in Diagnosis of Neoplastic and Inflammatory Skin Diseases: Old and Emerging Approaches Despite being one of the oldest diagnostic tests in dermatology, KOH preparation remains a workhorse in clinics worldwide because of its speed and low cost.
Parasitic Skin Disease
Skin scrapings for mites are a form of cytological examination that every veterinary student learns early. The technique is especially important for diagnosing demodectic mange (caused by Demodex mites that live inside hair follicles) and sarcoptic mange (caused by Sarcoptes mites that burrow into the upper skin layers). For Demodex, a deep scraping that draws a tiny amount of capillary blood is standard, since these cigar-shaped mites live deep within follicles. For Sarcoptes, superficial scrapings from the edges of crusted lesions are more productive, though the mites are notoriously hard to find; you can have a full-blown sarcoptic mange infestation and still get a negative scraping.
One case report involving a cat with feline immunodeficiency virus highlighted the importance of performing both superficial and deep skin scrapings, because the cat turned out to be infected with two different species of Demodex simultaneously, each requiring a different scraping depth to detect.8PubMed. Deep and superficial skin scrapings from a feline immunodeficiency virus-positive cat Beyond mites, cytological examination of skin scrapings and impression smears has been found effective for diagnosing various forms of parasitic dermatitis, particularly when the inflammatory cell pattern on the slide points toward a parasitic cause even before the organism itself is spotted.9PubMed Central. Cytopathology of parasitic dermatitis in dogs
In human dermatology, skin scrapings for scabies follow a similar principle, but the sensitivity is more variable, ranging from about 40% to 90% depending on the anatomical site sampled and the examiner’s skill.10PubMed Central. Advancements in Diagnosis of Neoplastic and Inflammatory Skin Diseases: Old and Emerging Approaches Newer approaches like dermoscopy have started to supplement traditional scrapings, but the microscopic examination of scraped material remains the definitive confirmation method.
Autoimmune and Inflammatory Conditions
Cytology can also provide surprisingly useful clues when the problem is not infection but the immune system attacking the skin itself. Pemphigus foliaceus, the most common autoimmune skin disease in dogs and cats, produces a distinctive cytological picture. The hallmark finding is “acantholytic cells,” which are rounded keratinocytes that have lost their connections to neighboring cells and appear floating among clusters of neutrophils and eosinophils. These rafts of detached cells surrounded by inflammatory cells create a pattern that looks quite different from a typical bacterial infection.
A study comparing cytology samples from dogs with pemphigus foliaceus to samples from dogs with superficial pyoderma found that acantholytic cells were significantly more numerous in pemphigus. Rafts of these cells and eosinophils were also far more common in pemphigus than in pyoderma, while bacteria were rare in pemphigus samples, showing up in only about 5% of cases compared to being a defining feature in pyoderma.11PubMed Central. Comparison of selected cytomorphological features of canine pemphigus foliaceus and superficial pyoderma This distinction matters because pemphigus requires immunosuppressive treatment, which is the opposite of what you would give for a bacterial infection. Getting the cytology right can prevent a costly wrong turn in therapy.
Eosinophilic granuloma complex in cats is another inflammatory condition where cytology proves valuable. The cellular composition of these lesions, dominated by eosinophils alongside macrophages and varying numbers of neutrophils, can be identified on cytological smears and helps confirm the diagnosis without the need for a surgical biopsy in many cases.12Scientific Messenger of LNU of Veterinary Medicine and Biotechnologies. Prevalence, diagnosis, patophysiological characteristics and cytomorphological evaluation of eosinophilic granuloma in cats
Skin Tumors and Masses
When a lump appears on or under the skin, cytology is often the first step in figuring out whether it is benign, malignant, or not a tumor at all. Fine needle aspiration can usually be performed in minutes without sedation, and the results frequently guide whether surgery, further staging, or a watch-and-wait approach is appropriate.
Round cell tumors are a category where cytology shines, because these tumors exfoliate readily and their cells have distinctive features. A classic study examining 64 canine cutaneous round cell tumors, including mast cell tumors, histiocytomas, lymphosarcomas, and transmissible venereal tumors, found that cytologic features were distinctive enough to allow a diagnostic opinion in most cases.13PubMed. Cytology of canine cutaneous round cell tumors. Mast cell tumor, histiocytoma, lymphosarcoma and transmissible venereal tumor Mast cell tumors, for instance, are identified by their characteristic dark-staining intracytoplasmic granules, while histiocytomas feature round to oval cells with kidney-shaped nuclei and pale cytoplasm.14Journal of Advances in Biology & Biotechnology. Cytomorphology of Canine Cutaneous Round Cell Tumours: Differentiating Histiocytoma from Mast Cell Tumour– A Case Report Getting this distinction right matters enormously: a histiocytoma in a young dog often resolves on its own, while a mast cell tumor may need surgery, radiation, or chemotherapy.
Epithelial tumors and mesenchymal tumors can be harder to classify on cytology alone. Epithelial tumors tend to exfoliate in clusters, while mesenchymal tumors often yield individual spindle-shaped cells that can look similar across different tumor types. Even so, cytology serves as a useful first-line tool for sorting masses into broad categories and determining whether a full biopsy is needed for a definitive grading and treatment plan.
Cysts and Keratinizing Lesions
Not every skin lump is a tumor. Follicular cysts and keratinizing growths are common, and they have their own cytological signatures. In a study of 46 canine samples from follicular tumors and cysts, follicular cysts and a type of benign growth called infundibular keratinizing acanthoma both produced sheets of keratinized squamous cells, some with visible nuclei and some without. A different type of growth, trichoepithelioma, had two distinct cytological presentations: about a third showed basaloid cells undergoing keratinization, while the remaining two-thirds looked more like the keratin-rich cysts.15PubMed Central. Cytologic features of cutaneous follicular tumors and cysts in dogs
From a practical standpoint, when a fine needle aspirate yields nothing but a mass of acellular keratin debris and a few squamous epithelial cells, the clinician can often reassure the owner that the lump is likely a benign cyst. That is a fast, inexpensive answer that spares the patient a surgical biopsy. However, because some keratinizing tumors can look deceptively similar to cysts on cytology, any lesion that grows rapidly or recurs after drainage still warrants a tissue biopsy for confirmation.
Viral Lesions and Unusual Mimics
In human medicine, two classic cytological techniques target viral skin infections. The Tzanck smear, performed by scraping the base of a fresh vesicle and staining the cells, detects herpes simplex and varicella-zoster virus by revealing giant multinucleated epithelial cells. The test’s sensitivity for herpes infections ranges from about 73% to 86%, with specificity around 85%.16PubMed Central. Advancements in Diagnosis of Neoplastic and Inflammatory Skin Diseases: Old and Emerging Approaches It is not perfect, but in settings where PCR testing is unavailable or too slow, a Tzanck smear can confirm a clinical suspicion within minutes.
Molluscum contagiosum, a poxvirus infection that usually appears as small dome-shaped papules, has a distinctive cytological appearance as well. The infected cells contain large intracytoplasmic inclusion bodies that push the nucleus to one side, creating what pathologists describe as a signet ring appearance. This pattern is recognizable enough to be diagnostic, and in atypical cases where the clinical presentation is confusing, fine needle aspiration cytology can establish the correct diagnosis and prevent unnecessary surgery for what might otherwise be mistaken for a skin tumor.17PubMed Central. The cytology of molluscum contagiosum mimicking skin adnexal tumor
How Accurate Is Skin Cytology for Cancer Screening
The accuracy question comes up most urgently in the context of skin cancer. A Cochrane systematic review pooled data from seven studies that used standard cytomorphological criteria to detect basal cell carcinoma in patients who already had a high clinical suspicion of the disease. The pooled sensitivity was about 97.5%, and specificity was roughly 90%.18Cochrane Database of Systematic Reviews. Diagnostic accuracy of exfoliative cytology for the diagnosis of cutaneous basal cell carcinoma and other skin cancers in adults Those numbers sound excellent, but a critical detail is that these patients were already selected by a dermatologist as likely having basal cell carcinoma. The test performs well as confirmation in a pre-screened population. Its performance in a general screening role, where the person walking in might have anything from a wart to melanoma, has not been studied as rigorously.
This distinction between “test accuracy in a specialized setting” and “test accuracy in everyday practice” matters for every cytological application. A veterinary dermatologist who sees pyoderma and Malassezia dermatitis every day will have sharper cytology skills for those conditions than a general practitioner who sees them once a month. Similarly, a pathologist experienced with cutaneous cytology will catch subtle patterns that a less experienced reader might miss. Technique and experience are not minor variables; they are central to how useful cytology turns out to be in any given case.
Artifacts, Contaminants, and Pitfalls
One of the underappreciated challenges in skin cytology is that slides can pick up extraneous material that mimics real findings. Pollen grains can look like fungal spores. Starch granules from gloves can resemble yeast. Cotton fibers from gauze can be confused with fungal hyphae. Contaminants and mimickers can make interpretation difficult enough that a second pathologist’s opinion or additional testing becomes necessary.19PubMed Central. Contaminants and Mimickers in Cytopathology
Preparation technique also introduces variability. Under-staining can make organisms invisible, while over-staining can obscure cell detail. Air-drying artifacts distort cell morphology, making normal cells look abnormal. Thick smears trap cells on top of each other, hiding individual features. Thin smears may not contain enough cells for a diagnosis. All of these factors contribute to why skin cytology is described as “operator dependent,” a polite way of saying that the person doing it has to be good at it for the results to mean anything.
For the pet owner or patient wondering whether to trust a cytology result, the practical advice is straightforward. A positive finding, such as bacteria inside neutrophils, mast cell granules in a lump aspirate, or abundant yeast on a tape strip, is usually reliable because those things are hard to hallucinate on a slide. A negative finding deserves more caution. The absence of organisms or abnormal cells on a single cytology sample does not guarantee that the skin is healthy, especially for conditions like deep fungal infections, scabies, or well-differentiated tumors that do not exfoliate easily.
Artificial Intelligence in Skin Cytology
The operator-dependence problem has made skin cytology an attractive target for artificial intelligence. If an algorithm can look at a slide image and classify what it sees with consistent accuracy, it could reduce the variability that comes with different examiners at different skill levels. One research group developed an AI classifier based on a Vision Transformer architecture to detect Malassezia yeast on tape strip samples. The best-performing model achieved a validation accuracy of 94%, with sensitivity of 94% and specificity of 93.5%.20PubMed. Next generation mycological diagnosis: Artificial intelligence-based classifier of the presence of Malassezia yeasts in tape strip samples
These numbers compare favorably with experienced human readers, and the appeal is obvious: an AI system does not get tired at the end of a long day or second-guess itself on borderline fields. However, AI in cytology is still in its early stages. Most published models have been validated on image datasets from a handful of institutions, and performance can drop when the algorithm encounters slides prepared with different stains, different scanners, or from populations with different prevalence rates of disease. The technology is moving fast, but for now it is best thought of as a promising assistant rather than a replacement for an experienced eye behind the microscope.