Melanoma diagnosis typically follows a clear path: a visual skin check, a closer look with a magnifying device, a biopsy to remove the suspicious spot, and a pathology report that confirms whether it’s melanoma and how deep it goes. That depth measurement, called the Breslow thickness, is the single most important number in your diagnosis because it drives every decision that follows.
The Visual Skin Exam
The first step is a clinical exam where a doctor inspects the mole or lesion with the naked eye. Most clinicians use a set of visual criteria to decide whether a spot looks suspicious enough to warrant further testing. In North America and Australia, the most common framework is the ABCDE rule:
- Asymmetry: one half of the mole doesn’t match the other
- Border: edges are ragged, notched, or blurred
- Color: the color varies across the mole, with shades of brown, black, red, white, or blue
- Diameter: the spot is larger than about 6 mm (roughly the size of a pencil eraser)
- Evolution: the mole has changed in size, shape, or color over time
In the UK, doctors often use the Glasgow 7-point checklist instead. It assigns higher weight to three major features: a change in size, irregular pigmentation, and an irregular border. Four minor features round out the score: inflammation, itching or altered sensation, a diameter larger than 7 mm, and oozing or crusting. A combined score of 3 or more typically triggers a referral for biopsy. No visual checklist is a definitive diagnosis on its own. These tools exist to sort which spots need a closer look from the thousands that don’t.
Dermoscopy: A Closer Look at the Skin
If a spot raises suspicion during the visual exam, the next step is usually dermoscopy. A dermatoscope is a handheld device with a magnifying lens and a built-in light that lets a dermatologist see structures in the skin that are invisible to the naked eye. It’s painless and takes seconds.
What the dermatologist is looking for under magnification is quite specific. Normal moles tend to have a regular pigment network: a uniform grid-like pattern of lines with evenly spaced holes. Melanoma disrupts that pattern. An atypical pigment network shows irregular holes, thick lines, uneven spacing, and gray coloring distributed asymmetrically across the lesion. Other warning signs under dermoscopy include streaks radiating from the edges, irregular dots or globules, shiny white lines, pink structureless areas, and angular geometric patterns. Each of these structures helps the dermatologist estimate how likely the lesion is to be melanoma and whether a biopsy is necessary.
The Biopsy
A biopsy is the only way to definitively diagnose melanoma. Visual exams and dermoscopy can raise or lower suspicion, but the tissue itself has to be examined under a microscope by a pathologist.
The preferred method is an excisional biopsy, which removes the entire lesion along with a small margin of normal skin (at least 2 mm) and a layer of fat beneath it. This approach gives the pathologist the full picture: the breadth of the lesion, its symmetry, its borders, and critically, its full depth. Because melanoma prognosis depends heavily on how deep the tumor has grown, partial sampling methods like shave biopsies or curettage are not recommended. They risk cutting through the deepest part of the tumor, making it impossible to measure accurately.
An incisional biopsy, which removes only a portion of the lesion, is considered suboptimal for the same reason. It’s sometimes used when the spot is very large or located in a cosmetically sensitive area like the face, but it limits what the pathologist can assess. A punch biopsy, which takes a small cylindrical core of tissue, falls into a similar category. It can be useful for sampling, but it won’t capture the full architecture of the lesion the way excision does.
What the Pathology Report Tells You
Once the tissue is removed, it goes to a pathology lab. For a straightforward biopsy, results are typically available in two to three working days. More complex cases, or those requiring special staining techniques, can take a week or longer. If the pathologist anticipates a significant delay, they’ll often issue a preliminary report with a favored diagnosis while additional testing continues.
The pathology report contains several key findings, but the most important is the Breslow thickness. This is a direct measurement, in millimeters, of how deep the melanoma cells have penetrated into the skin. It’s measured from the surface of the skin down to the deepest cancer cell. If the surface is ulcerated (meaning the skin over the tumor has broken down), the measurement starts from the base of that ulcer instead. Thicker tumors are linked to lower survival rates, and the Breslow number directly determines the stage of the cancer.
The report also notes whether ulceration is present, which independently worsens the prognosis at any thickness. Other details include the rate at which cancer cells are dividing, whether cancer cells have invaded nearby blood or lymph vessels, and whether the surgical margins are clear, meaning no cancer cells were found at the edges of the removed tissue.
Sentinel Lymph Node Biopsy
For thinner melanomas (less than 0.8 mm with no ulceration), the cancer is very unlikely to have spread, and no further surgical investigation of nearby lymph nodes is needed. Once the Breslow thickness reaches 0.8 to 1.0 mm, or if ulceration is present even in a thinner tumor, a sentinel lymph node biopsy becomes a conversation worth having with your doctor.
For any melanoma thicker than 1.0 mm, a sentinel lymph node biopsy is recommended when no lymph node involvement is obvious on physical exam. This procedure identifies and removes the first lymph node (or nodes) that drain the area where the melanoma was found. If cancer cells have reached that node, the melanoma has begun to spread beyond the skin, and the staging and treatment plan change accordingly. If the sentinel node is clear, the chance that other nodes are involved is very low.
Imaging for Advanced Melanoma
Most people diagnosed with early-stage melanoma will not need any imaging scans. PET/CT scans and MRIs are low-yield for stage I and II melanomas, meaning they rarely find anything that changes management, and expert panels don’t recommend them at those stages.
The picture changes at stage III, when melanoma has reached the lymph nodes. At that point, imaging is recommended to establish a baseline and map the extent of the disease. PET/CT is the primary tool here because it’s better than standard CT or MRI at detecting melanoma that has spread to distant sites throughout the body. The one exception is the brain and liver, where MRI remains more sensitive. For stage IV melanoma, where cancer has spread to distant organs, imaging is strongly encouraged and plays an ongoing role in tracking how the disease responds to treatment.
Non-Invasive Testing
A newer approach uses adhesive tape strips pressed against a suspicious mole to collect skin cells from the surface. The cells are then analyzed for the activity of specific genes associated with melanoma. This technique, sometimes called tape-stripping, can potentially help distinguish melanoma from harmless pigmented spots without cutting into the skin. Early studies show promising accuracy when enough genetic material is collected, with around 80% sensitivity and specificity for detecting key mutations. However, this technology is not a replacement for biopsy. It’s positioned as a supplementary tool that may help reduce unnecessary biopsies on spots that look suspicious but turn out to be benign.