Anisocytosis: What It Means for Your Blood Test
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Introduction
Seeing the word “anisocytosis” on a blood test can be confusing. It means red blood cells differ in size more than expected7,9. It is a lab finding, not a disease by itself7,10.
The blood may contain cells that are too small or too large. It may also contain a mix of both sizes7,9. A healthcare provider must review this finding with your symptoms and health history2.
Understanding your blood test results often starts with red cell distribution width, or RDW. Mean corpuscular volume, or MCV, adds more useful details. Hemoglobin, hematocrit, other cell measures, and a blood smear also provide context2,3.
No single value explains the whole picture. A high RDW blood test may point toward a problem. However, it cannot show the cause on its own1,2.
Overview: What Anisocytosis Means
A simple definition of anisocytosis
Anisocytosis means that red blood cells in a blood sample have unequal sizes9,10. Healthy red blood cells are often close to the same size1. They normally have a round, disc-like shape6,9.
Doctors use more specific terms to describe red blood cell size:
Microcytosis means that the red blood cells are smaller than normal9.
Macrocytosis means that the red blood cells are larger than normal9.
Mixed anisocytosis means that both large and small cells are present7,9.
These words describe the cells. They do not explain why the size difference began.
Anisocytosis is not a diagnosis by itself7,10. It can appear with anemia or other health conditions7. Some people may have an abnormal result without knowing the cause yet.
Why red blood cell size matters
Red blood cells contain a protein called hemoglobin. Hemoglobin carries oxygen from the lungs to cells across the body1. Red blood cells form in the bone marrow1.
The size and shape of these cells can help identify types of anemia2. Anemia occurs when the body lacks enough healthy red blood cells1. It may reduce the amount of oxygen-rich blood reaching body tissues7,10.
Changes in cell size can give clues about the cause of anemia1,2. For example, some forms of anemia produce small cells. Other forms produce large cells2.
Anisocytosis can also appear with a lack of key nutrients. It may occur with a bone marrow disorder as well7. Still, the finding does not prove that one specific condition is present.
Anisocytosis versus poikilocytosis
Anisocytosis describes changes in red blood cell **size**7,9. Poikilocytosis describes changes in red blood cell **shape**7,9.
Altered shapes can include round, long, broken, or crescent-shaped cells6,7. The same blood sample may show one or both findings7.
A peripheral blood smear helps the lab assess size and shape6,9. The lab examines a thin blood sample under a microscope9,10. The results can then be compared with the complete blood count.
How Anisocytosis Appears on Blood Tests
RDW and red blood cell size variation
Red cell distribution width measures differences in red blood cell size and volume1,2. RDW is often part of a complete blood count, or CBC1.
Blood testing machines can place cell sizes on a graph called a histogram1. Cells with similar sizes appear close together on that graph1. Cells with a wider range of sizes spread farther apart1.
A high RDW blood test means the cells vary in size more than expected1. The sample may contain small cells, large cells, or both7. The RDW value is often shown as a percentage1.
A high result may offer a helpful clue. However, it cannot diagnose anemia by itself2,8. It also cannot show the exact cause of the size change2.
Reading RDW together with MCV
MCV means mean corpuscular volume. It measures the average size of red blood cells2. Doctors use MCV to group anemia by average cell size3.
The three main MCV patterns are simple to understand:
A low MCV means that the red blood cells are small on average3,5.
A normal MCV means that the average size is within the lab’s range3,5.
A high MCV means that the red blood cells are large on average3,5.
A low-MCV pattern is called microcytic3. A normal-MCV pattern is called normocytic3. A high-MCV pattern is called macrocytic3.
MCV is an average, so it has limits. It may look normal when both small and large cells are present4. The two groups can balance each other in the average4.
RDW may reveal the size gap between those cells. Reading RDW and MCV together gives more information than either value alone3. The rest of the CBC is still needed for proper review.
When a peripheral blood smear is used
A peripheral blood smear uses a thin layer of blood on a slide9,10. A lab worker views the sample under a microscope9. The smear shows details about cell size, shape, color, and appearance6.
A smear can confirm red blood cell size variation6,9. It may also show unusual shapes or young red blood cells6. These details can help explain an abnormal CBC.
A blood smear can also help check odd automated results3. Testing machines are fast, but some factors can cause false readings3,4.
Some smear reports use grades from 1+ through 4+6. These grades show the share of cells affected6. Ask the ordering provider how the grade fits with the other results.
Understanding Your RDW, MCV, and CBC Results
What high, normal, and low RDW may mean
An adult RDW range is often about 11.5% to 15%3. Other sources give a common range of 12% to 15%8,9. The correct range is the one printed on your lab report.
Reference ranges differ between laboratories1,3. Testing methods may also differ3. A result from one lab should not be judged by another lab’s range.
Each RDW category has a narrow meaning:
A high RDW means there is more cell size variation than expected1.
A normal RDW means size variation falls within the lab’s expected range1.
A low RDW means most red blood cells are similar in size1,8.
A low RDW is not often a sign of anemia1,8. It is not usually a reason for concern by itself1.
A normal RDW does not rule out anemia or another condition1,8. The cells could all be smaller than normal1. They could also all be larger than normal1.
This point is key when understanding your blood test results. RDW measures the range of sizes. It does not measure the average size.
Common RDW and MCV patterns
RDW and MCV patterns can guide the next step. They are not final diagnoses.
Common patterns that a provider may consider include:
High RDW with low MCV may appear with iron deficiency anemia4,10.
High RDW with high MCV may appear with vitamin B12 or folate deficiency7,10.
Low MCV may also occur with thalassemia, an inherited blood disorder2,5.
Normal MCV may occur with sudden blood loss or long-term disease5.
Normal MCV may also occur with kidney failure or aplastic anemia5.
High MCV may occur with vitamin B deficiency or liver disease2,5.
High MCV may also occur during chemotherapy5.
Different health problems can produce similar patterns. More than one issue may also affect the blood at the same time. This is why a pattern serves as a clue, not a conclusion.
Other CBC values that provide context
A standard CBC contains several blood measurements. It includes red blood cell count, hemoglobin, and hematocrit3. It also includes red cell measures, platelets, and white blood cells3.
The following values help explain CBC results:
Hemoglobin is the oxygen-carrying protein measured in a standard CBC1,3.
Hematocrit reflects the share of blood made up of red blood cells3.
MCH measures the average amount of hemoglobin in one red blood cell2.
MCHC measures the concentration of hemoglobin within red blood cells2.
Red blood cell count measures the number of red cells in the blood3.
These values add context to RDW and MCV. A provider looks for patterns across the full report.
Unusual groups of results may need more review. Some lab factors can affect automated blood counts3,4. A blood smear may help check whether the results reflect real cell changes3.
Symptoms Associated With the Underlying Condition
Anisocytosis is a description from a lab test. Any symptoms often come from anemia or another health problem9.
Symptoms linked with anemia can include:
Fatigue or weakness may occur with anemia1,2.
Shortness of breath may occur with anemia1,2.
Headache or dizziness may occur with anemia1,2.
Pale skin may occur with anemia2.
A fast or uneven heartbeat may occur with anemia8,9.
Cold hands and feet may occur with anemia2.
Some people first learn about a high RDW during a routine CBC1,2. They may not have had symptoms before the test. Symptoms alone cannot explain the cause of anisocytosis.
Possible Causes of Anisocytosis
Nutritional deficiencies and anemia
Iron deficiency anemia is the most common form of anemia2. It may cause red blood cells to become smaller than normal2. It often produces a microcytic anisocytosis pattern10.
Iron deficiency can result from blood loss10. It may also follow low iron intake or poor iron absorption10.
Vitamin B12 or folate deficiency can cause megaloblastic anemia10. This type of anemia involves cells that are larger than normal10. It may cause macrocytic anisocytosis and a high RDW7,10.
Pernicious anemia can also produce a macrocytic pattern10. This immune system disorder prevents enough vitamin B12 from being absorbed from food10.
Blood loss, inherited disorders, and chronic illness
Blood loss can lead to iron deficiency anemia10. Sudden blood loss can also occur with a normal MCV pattern5. A smear may show young red blood cells after bleeding6.
Inherited blood disorders can affect cell size, shape, or hemoglobin. Thalassemia can cause small red blood cells2. It also causes the body to make less healthy hemoglobin2.
Sickle cell disease causes crescent-shaped red blood cells7,10. It may also appear with anisocytosis7,10.
Hemolytic anemia occurs when red blood cells break down too fast2. The body cannot replace them at the same rate2. A smear may show young cells as the bone marrow responds6.
Other links include chronic liver disease and thyroid disorders10. Myelodysplastic syndrome is another possible cause10. This group of disorders affects blood cell production in the bone marrow7.
Pregnancy, transfusions, and medications
Anemia during pregnancy may appear with anisocytosis9,10. Iron needs rise during pregnancy to support fetal growth10. A pregnancy care provider can review the CBC and order more tests9.
A recent blood transfusion can add a second group of red blood cells4,9. Those cells may differ in size from the person’s own cells4. The mixture can raise the amount of size variation.
Some medicines can also cause anisocytosis9. Cytotoxic chemotherapy drugs are one example10. A full medicine history can therefore help explain an unexpected result.
Treatment and Next Steps After an Anisocytosis Result
Tests a healthcare provider may order
The next step depends on the full CBC pattern. Symptoms, diet, health history, and medicines may also guide testing9,10.
A healthcare provider may order some of these tests:
Ferritin measures a protein that stores iron9.
Iron testing can help assess possible iron deficiency9,10.
Vitamin B12 and folate tests can identify low levels of these nutrients9.
Liver function tests may help assess the liver9.
A peripheral blood smear can show cell size and appearance3,9.
A reticulocyte count can assess young red blood cells after blood loss or cell breakdown6.
A provider may repeat the CBC when results seem inconsistent. Some conditions can affect the values produced by automated testing machines3,4.
How the underlying cause is treated
Treatment targets the cause rather than anisocytosis itself9,10. The right plan depends on what follow-up testing finds.
Treatment may involve several different approaches:
Diet changes may help when a nutrient deficiency is present1,10.
Iron, vitamin B12, or folate may be used for a confirmed deficiency9,10.
Medicines may be needed for some types of anemia or blood disease1,10.
Blood transfusions may be used for certain forms of anemia10.
Bone marrow disorders may need care directed by a specialist10.
Some people cannot absorb enough nutrients from food. In such cases, nutrients may be given through another route10.
Do not start iron or vitamins only because RDW is high. MedlinePlus advises speaking with a provider before taking supplements1. It also advises discussion before making major diet changes1.
Testing can show whether a nutrient is low. It can also help identify which nutrient needs attention. This helps treatment match the cause.
Inherited anemia and bone marrow disease may need more complex care. Options can include medicines, transfusions, or other specialist treatments10.
When to See a Doctor
Contact the healthcare provider who ordered the test if the report lists anisocytosis. The same advice applies to a high RDW blood test. The provider can compare the result with MCV and hemoglobin1,2.
The provider may also review hematocrit and other CBC values2,3. Symptoms and medical history are part of that review2.
Contact a healthcare provider if anemia symptoms become worse8. These may include more fatigue, pale skin, shortness of breath, or an uneven heartbeat8. The sources advise contacting a clinician about worsening symptoms8.
The following questions may help during the appointment:
“Are my hemoglobin and hematocrit within the laboratory’s range?”.
“What do my RDW and MCV results mean when read together?”.
“Could a recent blood transfusion affect these results?”.
“Could one of my medicines affect my red blood cells?”.
“Do I need iron, ferritin, vitamin B12, or folate tests?”.
“Would liver testing or a peripheral blood smear help?”.
“Should the CBC be repeated before treatment starts?”.
Bring a current list of medicines and supplements. Tell the provider about diet concerns or a history of bleeding. Providers also consider pregnancy, chronic illness, and family health history1,2.
Conclusion
Anisocytosis means red blood cells differ in size more than expected7,9. The finding may involve small cells, large cells, or both7,9.
RDW measures this red blood cell size variation1,3. A peripheral blood smear allows the lab to view the cells directly6,9.
A high RDW shows increased size variation1. It does not diagnose anemia or reveal the cause by itself1,2. A normal RDW also does not rule out anemia1,8.
A low RDW means red blood cells are close to the same size1,8. It is not usually considered a sign of anemia1,8.
Understanding your blood test results requires more than one number. RDW should be read with MCV, hemoglobin, and hematocrit2,3. MCH, MCHC, symptoms, and health history also add context2,3.
A healthcare provider can decide whether more testing is needed. If treatment is needed, it should address the underlying cause9,10.
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