According to Penn Medicine, a burr cell is an abnormally shaped red blood cell – also known as an echinocyte – that can be caused by simple aging or by illnesses such as uremia. Echinocyte means "sea urchin" in Greek, so that is a clue as to the shape of the abnormal red blood cell, which is found through a blood smear test.
Uremia is caused by the kidneys not filtering blood properly, which itself can be caused by a variety of factors, including kidney failure, physical injury or high blood pressure. As waste products such as urea accumulate in the blood, the body is poisoned because little or no urine is being passed and abnormalities such as burr cells occur. In many cases, the symptoms are reversible with proper treatment.
Crenated cells, or red blood cells that mimic the appearance of burr cells, are frequent artifacts within blood smear tests. They can result from chemical contamination, high humidity or from the blood drying slowly. The key to differentiating crenated cells from true burr cells is the frequency with which crenated cells appear in the sample. Burr cells almost always constitute a small fraction of the sample, whereas crenated cells are much more numerous.