Burr cells, scientifically known as echinocytes, are red blood cells (RBCs) that have developed an abnormal, spiky contour. Normally, RBCs are smooth, biconcave discs, but this spiculated appearance signals a disruption to the cell’s structure. Their presence on a microscopic slide requires investigation to determine if the cause is a serious underlying disease or a harmless error in laboratory processing. Understanding this shape change is a fundamental step in diagnosis, as burr cells can mark significant metabolic or microvascular issues.
Understanding the Burr Cell Structure
A burr cell is defined by its characteristic shape, featuring multiple short, blunt spicules distributed uniformly across the entire cell surface. This distinct morphology gives the cell a serrated or sea urchin-like appearance, leading to the alternative name, echinocyte. Crucially, the cell typically retains an area of central pallor, the paler region in the middle of a normal RBC.
The change from a biconcave disc to a burr cell stems from a disturbance in the red blood cell membrane’s equilibrium. The membrane is a lipid bilayer, and its shape is maintained by a balance between the inner and outer layers. When the outer layer expands relative to the inner layer, the membrane buckles outward to form the projections. This change can be triggered by internal issues, such as the depletion of adenosine triphosphate (ATP), the cell’s primary energy source necessary to maintain membrane integrity.
It is important to distinguish burr cells from acanthocytes, another type of spiky red cell commonly called spur cells. Unlike the burr cell’s numerous, uniform, and blunt projections, acanthocytes possess fewer spicules that are irregular in length and distribution. The formation mechanism also differs: the burr cell change is often reversible, while the acanthocyte’s alteration is typically permanent and associated with a profound defect in membrane lipid composition.
Reversible and Laboratory-Induced Formation
Many burr cells observed in a clinical setting are not a sign of disease but are artifacts caused by improper blood sample handling. This phenomenon is frequently termed the “crenation artifact” and occurs outside the body after the blood has been drawn. The formation of these artifactual burr cells is a reversible process; the cells could return to their normal biconcave shape if the external conditions were corrected.
One common laboratory cause is the use of an excessive amount of the anticoagulant EDTA, which alters the osmotic environment of the blood sample. Allowing the blood smear to dry too slowly during preparation can also induce the cells to crenate and take on the spiky appearance. Furthermore, the age of the blood sample plays a role, as red blood cells naturally deplete their ATP stores over time when stored, leading to membrane changes and the development of echinocyte morphology.
Changes in the blood’s pH or osmolarity during collection or storage also contribute to this shape transformation. For example, the “glass effect,” where basic substances leach from the glass slide, can induce echinocyte formation. Recognizing these technical errors is a significant part of laboratory medicine necessary to avoid misinterpreting the result as a sign of serious illness.
Systemic Conditions Causing Burr Cells
When burr cells are present in fresh, properly handled blood samples, they indicate a systemic health problem. The most frequent pathological association is with uremia, a condition resulting from severe kidney failure where metabolic waste products accumulate in the blood. Uremic toxins, such as indoxyl sulfate and p-cresol, disrupt the red blood cell membrane structure and function through oxidative stress.
This toxic environment causes the RBCs to undergo changes that lead to the formation of burr cells, often correlating with the severity of the patient’s renal dysfunction. The presence of burr cells in the context of renal disease can be a marker of increased risk and warrants aggressive management of the kidney condition.
Liver disease, particularly severe alcoholic liver disease, also commonly presents with burr cells due to alterations in the lipid composition of the red cell membrane.
Burr cells are also found in microangiopathic hemolytic anemias (MAHAs), such as Disseminated Intravascular Coagulation (DIC) and Thrombotic Thrombocytopenic Purpura (TTP). In these conditions, blood cells are mechanically damaged as they pass through small blood vessels partially obstructed by microscopic clots. While the primary finding in MAHA is schistocytes (fragmented RBCs), burr cells are also formed as the cells are stressed by the turbulent flow.
Other systemic conditions affecting red cell metabolism can also lead to burr cell formation. Severe hypophosphatemia, a low level of phosphate in the blood, can inhibit the production of ATP within the red blood cell. This energy deficit compromises the cell’s ability to maintain its normal biconcave shape, resulting in the spiky contour. Burr cells may also be noted in cases of significant gastrointestinal bleeding, often seen in conjunction with underlying uremia, and following extensive damage from severe burns.