Burr cells on a blood smear most commonly indicate kidney disease, liver disease, or simply a lab artifact. Also called echinocytes, these are red blood cells whose outer membrane has developed small, evenly spaced spikes, giving them the appearance of a tiny sea urchin. Finding them on your lab results can be clinically meaningful or completely harmless, depending on context.
What Burr Cells Look Like
Under a microscope, burr cells are red blood cells covered in short, uniform projections distributed evenly across the cell surface. Unlike normal red blood cells, which look like smooth discs with a pale center, burr cells have a spiky or scalloped edge. The pale center is still visible, which is one key feature that helps lab technicians identify them correctly.
The name “echinocyte” comes from the Greek word for sea urchin, which gives you a good mental image of their shape. They’re distinct from acanthocytes (spur cells), which have longer, irregularly spaced projections and a dense center with no pale area. This distinction matters because acanthocytes point to a different set of conditions, particularly severe liver disease and certain neurological disorders. When a lab report says “burr cells,” it specifically means the evenly spiky version.
The Most Common Cause: A Lab Artifact
Before assuming something is wrong, it’s worth knowing that burr cells frequently appear as a lab artifact rather than a sign of disease. Blood samples that sit too long before being processed, come into contact with glass slides, or experience a shift in pH can develop the characteristic spiky appearance. These are sometimes called crenated cells. If burr cells show up on your blood smear but your kidney and liver function tests are normal, the finding is very likely artifactual and not a concern.
Kidney Disease and Uremia
When burr cells are genuinely present in your bloodstream (not just a lab effect), the most well-established association is with kidney disease, particularly end-stage renal disease. As kidney function declines, waste products like urea build up in the blood, a condition called uremia. This toxic environment appears to alter the red blood cell membrane, causing it to deform into the burr cell shape.
Research from a tertiary care hospital studying chronic kidney disease patients found a direct relationship between burr cells and the severity of kidney impairment. Patients who had burr cells on their blood smear had an average serum urea level of 232, compared to just 97 in patients without burr cells. That’s a statistically significant difference, suggesting that burr cells don’t just signal kidney disease in general but tend to appear when kidney function has deteriorated substantially. The more waste products accumulating in the blood, the more likely red blood cells are to take on this abnormal shape.
Liver Disease
Liver disease is the other major condition linked to burr cells. A classic study described a pattern in patients with cirrhosis, most of them with alcoholic cirrhosis, where persistent burr cells appeared alongside signs of hemolytic anemia. These patients had ongoing destruction of red blood cells, elevated bilirubin (a marker of red cell breakdown), low platelet counts, and a sustained presence of burr cells on their blood smears. In this context, spotting burr cells on a smear from a patient with cirrhosis can flag an active hemolytic process that might otherwise go unrecognized.
Less Common Causes
Beyond kidney and liver disease, several other conditions can produce burr cells:
- Vitamin E deficiency: Vitamin E protects red blood cell membranes from damage. Without enough of it, the membrane becomes unstable and more prone to forming spicules.
- Pyruvate kinase deficiency: This inherited enzyme deficiency impairs the red blood cell’s ability to produce energy, which is directly tied to membrane shape. When energy stores run out, the cell loses its ability to maintain a smooth disc shape and transforms into a burr cell.
- Multisystem inflammatory syndrome in children (MIS-C): A rare inflammatory condition linked to viral infections that can produce burr cells among other blood abnormalities.
Why Red Blood Cells Change Shape
Red blood cells maintain their smooth, flexible disc shape by actively using energy (in the form of ATP) to keep their membranes properly organized. When ATP is depleted, the cell loses volume relative to its surface area and begins to form spikes, eventually becoming a sphere covered in projections. Calcium buildup inside the cell can trigger a similar transformation, though the process happens much faster than energy depletion alone.
In kidney disease, the accumulation of uremic toxins in the blood disrupts the membrane’s lipid balance. In liver disease, abnormal cholesterol and fat metabolism alters the composition of the membrane from the outside. Both pathways lead to the same visible result: a spiky, less flexible red blood cell that doesn’t move through small blood vessels as easily as a healthy one.
What Burr Cells Mean for You
If burr cells appear on your blood smear, the next step depends entirely on the rest of your lab work. Your doctor will look at kidney function markers (creatinine, blood urea nitrogen) and liver function tests to determine whether the finding reflects real disease or just a quirk of sample handling. A small number of burr cells on an otherwise normal panel is rarely cause for alarm.
When burr cells show up alongside abnormal kidney or liver values, they serve as a visual confirmation of what the numbers already suggest. In kidney disease specifically, their presence correlates with higher urea levels, meaning they tend to appear when the disease is more advanced rather than in its earliest stages. They’re a piece of the diagnostic puzzle rather than a diagnosis on their own.