Although people can often identify atypical moles on their own bodies (see the page on Typical and Atypical Moles), the diagnosis of melanoma must be made by a physician.
This process starts with a medical history and physical examination. The doctor will ask about when the abnormality on the skin first appeared and how it has changed since then, as well as about the patient's history of prior skin cancers and whether or not members of the patient's family have had skin cancer. The doctor will carefully examine the abnormality, looking at its size, shape, color, and surface and noticing whether it is scaly and whether it bleeds. During the initial examination, many dermatologists use a magnifying device called a dermatoscope to view the abnormality more closely and take photographs of it. The doctor will also check the rest of the patient's body for additional spots and moles. The doctor may also check lymph nodes in different parts of the body, such as under the arms, in the groin, and in the neck, to see whether they are enlarged. Enlargement may suggest that melanoma could have spread to the lymph nodes.
If the doctor's initial examination suggests that the mole may be a melanoma, a diagnostic test called a biopsy will be performed, in which the mole or a sample of it will be removed and sent to a laboratory for examination.
In some instances, other tests will also be performed. For example, if the physician suspects that the cancer may have spread to nearby lymph nodes, a procedure called sentinel lymph node mapping may be performed. In this procedure, the doctor injects radioactive material and/or a colored dye into the abnormal area and then determines which lymph node the injected substance travels to first. Wherever the injected material travels to is also where melanoma cells were most likely to travel. When the appropriate lymph node (called the sentinel lymph node) is identified, it can be removed and examined to determine whether melanoma has spread to it.
In instances where the doctor suspects that melanoma may have metastasized (spread to other parts of the body), various imaging techniques, such as a chest x-ray, a CT (computed tomography) scan, an MRI (magnetic resonance imaging) scan, or a PET (positron emission tomography) scan may be used to look for cancer in other parts of the body.