Often, this represents a 400-fold magnification when referenced in scientific papers.
Area per high-power field for some microscope types: The area provides a reference unit, for example in reference ranges for urine tests.
[3] Used for grading of soft tissue tumors: Grading, usually on a scale of I to III, is based on the degree of differentiation, the average number of mitoses per high-power field, cellularity, pleomorphism, and an estimate of the extent of necrosis (presumably a reflection of rate of growth).
Mitotic counts and necrosis are the most important predictors.
[4] The following grading is part of classification of breast cancer: