Keratocytes are fragments of red blood cells. They come in various shapes (such as half-moons or helmets) rather than the pale-centered biconcave disks that are characteristic of normal, intact red blood cells.
A check for abnormal red blood cells is performed systematically when irregularities are found in an automated complete blood count or when a blood smear (examination of the blood under the microscope) is specifically prescribed by the doctor. While all possible varieties of abnormal red blood cells are checked for (unless a specific request is made), the test results report mentions only the abnormal varieties found in the specimen.
Keratocytes usually indicate a disease of the blood vessel walls that causes the membrane of some red blood cells to rupture (in conditions such as disseminated intravascular coagulation, hemolytic uremic syndrome and other hemolytic syndromes).