Histopathology is the diagnosis and study of disease by examining tissue under a microscope. When a doctor removes a tissue sample from your body, whether from a suspicious lump, an organ biopsy, or tissue taken during surgery, a specialist called a histopathologist examines that sample to determine what’s wrong. It is the primary method used to confirm or rule out cancer, and it plays a central role in diagnosing diseases of the skin, liver, kidney, and nearly every other organ.
What a Histopathologist Does
A histopathologist is a doctor who specializes in reading tissue samples. They examine thin slices of tissue under a microscope, looking for changes in cells that explain what’s causing illness. For cancer specifically, they identify whether a growth is malignant, determine what type of cancer it is, assign a grade reflecting how aggressive it looks, and in some cases assess whether the tumor is likely to respond to certain treatments. That information goes directly to the treating doctor and shapes every decision that follows.
Histopathologists don’t just work behind the scenes. Many attend multidisciplinary team meetings where their findings are discussed alongside imaging results and clinical observations. Some work directly with patients, performing procedures like fine needle aspiration in breast or head and neck clinics. They also carry key responsibilities in cancer screening programs for breast, bowel, and cervical cancer.
How a Tissue Sample Becomes a Slide
Getting from a piece of tissue to something a pathologist can read under a microscope takes four main steps: fixation, embedding, sectioning, and staining.
Fresh tissue breaks down quickly, so it first needs to be preserved. This step, called fixation, typically uses a formaldehyde solution (formalin) that stabilizes proteins and other molecules in the cells. The solution is carefully balanced to prevent the tissue from shrinking or swelling.
Next, the preserved tissue is embedded in a solid material so it can be sliced extremely thin. Water in the tissue is gradually replaced with solvents, then with melted paraffin wax. Once the wax cools and hardens, it forms a solid block with the tissue locked inside. A precision cutting tool called a microtome then shaves slices typically 5 to 10 micrometers thick, roughly one-tenth the width of a human hair. These wafer-thin sections are mounted onto glass slides.
At this point the tissue is nearly transparent, so it needs to be stained to make cellular structures visible. The standard stain used in virtually every histopathology lab is called H&E, short for hematoxylin and eosin. Hematoxylin dyes the genetic material inside cell nuclei a deep blue-purple. Eosin dyes the surrounding structures, including the cell body and connective tissue, an orange-pink-red. This color contrast lets pathologists quickly spot abnormalities in how cells are organized, shaped, and sized.
Beyond Standard Staining
Sometimes an H&E stain isn’t enough to make a definitive diagnosis. Immunohistochemistry, commonly called IHC, is an advanced technique that uses antibodies to detect specific proteins in a tissue sample. Think of it like a lock-and-key system: each antibody is designed to latch onto one particular protein. If that protein is present, the antibody binds to it and triggers a color change visible under the microscope. If the protein isn’t there, nothing happens.
IHC is most commonly used in cancer diagnosis, where it can pinpoint the exact type of tumor, identify where a cancer originated, or test whether a tumor carries proteins that specific therapies can target. It’s also used to diagnose conditions like Alzheimer’s disease, Parkinson’s disease, muscular dystrophy, and certain infections.
Frozen Sections During Surgery
Standard histopathology takes time because of the fixation and embedding process. But sometimes surgeons need an answer while a patient is still on the operating table. Frozen sections solve this problem. Instead of going through the full paraffin process, tissue is rapidly frozen, sliced, stained, and examined within minutes. A surgeon removing a tumor might use a frozen section to check whether the edges of the removed tissue are free of cancer cells, which determines whether more tissue needs to come out before closing the incision.
Frozen sections trade some quality for speed. The rapid freezing can create ice crystal gaps that distort the tissue’s architecture, making cells harder to interpret. They’re also unsuitable for hard tissues like bone, very fatty tissues, or samples from patients with certain infections. For these reasons, a standard paraffin-processed section almost always follows to confirm the frozen section result.
How Long Results Take
If you’re waiting for a histopathology report, the standard benchmark is two business days from the time the lab receives the specimen to the time the pathologist signs off on the final report. Many straightforward biopsies meet this window. However, several factors can extend it. Samples that need decalcification (dissolving calcium from bone tissue before it can be sliced), cases requiring IHC or molecular testing, or specimens that need a second opinion from another pathologist all take longer. Complex cancer resections with multiple areas to examine may take a week or more.
Histopathology vs. Cytology
Histopathology examines a block of tissue, preserving the full architecture of how cells are arranged relative to each other. Cytology, by contrast, examines individual cells or small clusters. A Pap smear is a classic cytology test: cells are scraped from the cervix and examined one by one. Urine samples, fluid drained from a cyst, and cells drawn through a fine needle also fall under cytology. Many histopathologists handle both disciplines, and the two often complement each other. Cytology is quicker and less invasive, while histopathology provides more structural detail and is generally needed for a definitive cancer diagnosis.
What Affects Sample Quality
The accuracy of a histopathology diagnosis depends partly on the quality of the tissue that reaches the lab. Several common problems can interfere. Crush artifact happens when tissue is compressed during removal, often by surgical forceps. It distorts the shape of cells, sometimes making them unrecognizable. This is especially problematic with tiny samples like endoscopic biopsies, where the crushed area may represent most of the specimen.
Cautery artifact occurs when electrocautery tools used to control bleeding during surgery heat the tissue at the edges of a sample. Under the microscope, cauterized cells look smudgy and dark, which can make it difficult to tell whether cancer extends to the surgical margin. Freezing artifact, as mentioned with frozen sections, creates ice crystal gaps that distort tissue structure. And if tissue isn’t placed in fixative quickly enough after removal, autolysis (self-digestion by the tissue’s own enzymes) degrades cells and obscures findings.
Reading Your Histopathology Report
If you receive a histopathology report, two terms that commonly appear in cancer diagnoses are grade and stage. Grade describes how abnormal the tumor cells look under the microscope. A low-grade tumor has cells that still resemble normal tissue and tend to grow more slowly. A high-grade tumor has cells that look very different from normal and typically behave more aggressively. The grade is determined by specific microscopic features and helps predict how the cancer may progress.
Stage describes how far the cancer has spread. It accounts for the size of the tumor, whether it has grown into nearby structures, and whether it has reached lymph nodes or distant organs. While grade comes from the pathologist’s microscopic examination, staging combines pathology findings with imaging and surgical observations. Together, grade and stage guide treatment decisions and give a clearer picture of prognosis.
The Role of Digital Pathology and AI
Histopathology has traditionally been entirely analog: a pathologist, a microscope, and a glass slide. That’s changing. Digital pathology converts glass slides into high-resolution digital images that can be viewed on a screen, shared instantly with colleagues worldwide, and analyzed by software. As of early 2025, at least 90 AI-powered diagnostic tools have received regulatory approval in Europe for analyzing digital pathology images. Some work with standard H&E slides, others with IHC-stained slides, and a smaller number with cytology specimens.
These tools are designed to assist pathologists rather than replace them. Some flag suspicious areas on a slide to speed up review. Others score IHC stains more consistently than the human eye can. Adoption is moving faster in Europe than in the United States, where very few AI products for histopathology have received FDA clearance so far. Independent validation studies remain limited for many of these tools, so the experienced pathologist at the microscope is still the cornerstone of every diagnosis.