In pathology, diffuse describes a change or finding that is spread throughout the tissue examined, rather than being limited to one small area. A diffuse change can be seen when a pathologist looks at the tissue by eye (called the gross examination) or under the microscope. The opposite of diffuse is focal, which describes a change limited to one or a few specific spots. Diffuse describes a pattern or distribution; it is not a diagnosis on its own, and it does not by itself mean that a finding is harmless or serious. This article explains what diffuse means, how it differs from focal, and where you are likely to see it on a pathology report.
Diffuse and focal are opposite terms that pathologists use to describe how widespread a finding is:
Pathologists note whether a change is diffuse or focal because the pattern can help tell conditions apart and can affect how a finding is interpreted. For example, inflammation spread evenly throughout a sample (diffuse) may point to a different cause than inflammation found in a single small area (focal).
For some diagnoses, the word diffuse is part of the name itself, describing how abnormal cells grow or spread. In these cases, diffuse usually means the cells spread throughout a tissue instead of forming a single, well-defined lump. Examples include:
In each of these examples, diffuse is describing the growth pattern of the disease and is only one part of the full diagnosis.
In immunohistochemistry, a test that uses special stains to detect specific proteins in the tissue, a result may be described as diffusely positive. This means the protein was found throughout the cells being examined, rather than in only a few scattered cells (which would be described as focal). Noting whether staining is diffuse or focal helps the pathologist interpret what the result means for the diagnosis.