Anisocytosis means your red blood cells vary more in size than they should. It’s not a disease itself but a finding on a complete blood count (CBC) that signals something is affecting how your body produces or maintains red blood cells. You’ll typically see it reported as an elevated RDW (red cell distribution width), a number that appears on standard blood work.
How RDW Measures Anisocytosis
Your blood cells aren’t all identical in size, but in a healthy person they fall within a narrow range. The RDW captures how much variation exists. A normal RDW-CV (the most commonly reported version) falls between about 11.6% and 14.6% in adults. When that number climbs above the upper limit, your lab report may flag it, and your doctor may use the term anisocytosis to describe what’s happening.
There are actually two ways labs express RDW. The RDW-CV is a percentage that compares size variation to the average cell size. The RDW-SD is measured in femtoliters (a unit of volume) and captures the raw spread of cell sizes, with a normal range of roughly 39 to 46 fL. Most labs report one or both. Either way, a higher number means more size variation among your red blood cells.
When a pathologist examines a blood smear under a microscope, they can also grade anisocytosis on a 1+ to 4+ scale. The grading compares the largest red blood cell to the smallest: 1+ means the biggest cell is less than twice the size of the smallest, while 4+ means the largest is more than four times bigger. Most cases flagged on routine blood work fall on the milder end.
What Causes It
An elevated RDW tells you something is disrupting the normal production or lifespan of red blood cells, but it doesn’t tell you what. The list of possible causes is long, which is why doctors interpret RDW alongside other values on the CBC, especially the MCV (mean corpuscular volume), which measures average cell size.
High RDW with small cells (low MCV): The most common scenario. Iron deficiency anemia is the leading cause. When your body runs low on iron, it starts producing smaller-than-normal red blood cells while older, normal-sized ones are still circulating. That mix of sizes drives the RDW up. Inherited conditions like beta-thalassemia and sickle cell disease can also produce this pattern, though thalassemia sometimes shows a normal RDW, which helps distinguish it from iron deficiency.
High RDW with large cells (high MCV): This pattern points toward vitamin B12 or folate deficiency, liver disease, or heavy alcohol use. In these situations, the bone marrow releases oversized red blood cells. Mixed deficiencies, where you’re low on both iron and B12 for example, can produce especially high RDW values because you have both abnormally small and abnormally large cells in circulation at the same time.
High RDW with normal-sized cells: This can show up with early nutrient deficiencies (before the average cell size shifts noticeably), chronic inflammation, kidney disease, or after a recent blood transfusion. Certain hemolytic anemias, where the body destroys red blood cells faster than it makes them, also cause this pattern.
Symptoms You Might Notice
Anisocytosis itself doesn’t cause symptoms. What you feel comes from the underlying condition driving it, most often anemia. When red blood cells vary widely in size, many of them are less efficient at carrying oxygen. Common symptoms include fatigue, weakness, shortness of breath, dizziness, and pale skin. Some people notice a fast or irregular heartbeat, headaches, or occasionally a whooshing sound in one ear.
These symptoms tend to develop gradually, which is why anisocytosis often shows up as an unexpected finding on routine blood work before a person realizes anything is wrong. If your RDW is only slightly elevated and your hemoglobin is normal, you may feel perfectly fine.
What Happens After an Abnormal RDW
A high RDW on its own isn’t a diagnosis. It’s a starting point. Your doctor will look at the full CBC, paying attention to hemoglobin, MCV, and red blood cell count to narrow down the cause. From there, follow-up testing typically targets the most likely explanation based on that pattern.
- Iron studies (ferritin, serum iron, transferrin saturation) if small cells suggest iron deficiency
- Vitamin B12 and folate levels if large cells point toward a nutrient deficiency
- Reticulocyte count to see how actively your bone marrow is producing new red blood cells
- A peripheral blood smear, where a technician examines your blood under a microscope to look at cell shapes and sizes directly
- Liver and kidney function tests if those organs are suspected contributors
In many cases, the cause turns out to be something straightforward like an iron or vitamin deficiency that responds well to dietary changes or supplements.
RDW as a Broader Health Marker
Research over the past decade has revealed that elevated RDW correlates with health risks beyond anemia. In a study of 700 cardiovascular patients followed for nearly four years, those in the highest third of RDW values were 2.7 times more likely to die from any cause compared to those in the lowest third, even after adjusting for age, kidney function, and hemoglobin levels. The same group had roughly double the risk of heart attacks, strokes, and blood clots.
This doesn’t mean a high RDW causes heart disease. Rather, it appears to reflect underlying processes like chronic inflammation, oxidative stress, and poor nutritional status that also contribute to cardiovascular problems. For people already managing heart disease, RDW is increasingly recognized as a useful prognostic marker. For otherwise healthy people with a mildly elevated RDW, it’s more of a signal to investigate the cause than a reason for alarm on its own.
What a Normal RDW Means
If your RDW falls within the normal range, your red blood cells are reasonably uniform in size. That’s generally reassuring, but it doesn’t rule out all blood disorders. Some inherited conditions like thalassemia trait can produce small, uniform cells, keeping the RDW normal even though the MCV is low. RDW is one piece of the puzzle, not the whole picture. Your doctor reads it alongside other values to get a complete understanding of what’s happening with your blood.