Understanding the Distinction: MCV vs. Hemoconcentration
It's important to differentiate between mean corpuscular volume (MCV) and hemoconcentration. MCV measures the average size of red blood cells, while hemoconcentration is an increased ratio of blood cells to plasma volume, often due to fluid loss. Dehydration leads to hemoconcentration, which affects various blood test values but doesn't change the size of individual red blood cells enough to be considered true macrocytosis.
When dehydrated, plasma volume decreases, but the number of red blood cells remains constant. This results in more concentrated blood and a higher hematocrit (the percentage of blood volume made up of red blood cells). While this can elevate some blood metrics, MCV, which calculates the average cell volume, is generally unaffected. Any slight, temporary MCV change is more likely due to test interpretation than a real physiological shift.
Primary Causes of a High MCV (Macrocytosis)
True macrocytosis (high MCV) is a significant clinical finding indicating underlying medical conditions, not hydration status. It means red blood cells are larger than normal (over 100 fL in adults).
Nutritional Deficiencies
The most frequent cause is a lack of vitamin B12 or folate, vital for DNA synthesis and proper red blood cell maturation. Insufficient levels prevent proper cell division, resulting in abnormally large cells. This leads to megaloblastic anemia.
Alcohol Use Disorder
Heavy, chronic alcohol consumption is another common cause. Alcohol is toxic to bone marrow and hinders nutrient absorption like folate. Macrocytosis often resolves within months of stopping alcohol, making MCV useful for monitoring recovery.
Liver Disease
The liver is crucial for red blood cell health. Liver disease can cause abnormal lipid buildup on red blood cell membranes, enlarging them. High MCV is common in liver conditions, particularly those linked to alcohol abuse.
Other Medical Conditions
Various other health issues can contribute to macrocytosis:
- Hypothyroidism: Can affect red blood cell production.
- Myelodysplastic Syndromes (MDS): Bone marrow cancers impacting blood cell production.
- Reticulocytosis: An increase in larger, immature red blood cells.
- Certain Medications: Some drugs interfere with red blood cell development.
Understanding Your Blood Test Results: High MCV
Compare the effects of dehydration and macrocytosis on blood metrics:
| Feature | Dehydration (Hemoconcentration) | High MCV (Macrocytosis) |
|---|---|---|
| Effect on Plasma Volume | Decreased | Normal |
| Effect on Red Blood Cell Size | Unchanged | Increased |
| Effect on Hematocrit | Elevated | Can be high, normal, or low |
| Underlying Cause | Fluid loss | Vitamin deficiency, alcohol, liver disease, MDS, etc. |
| Reversibility | Normalizes with rehydration | Requires treatment of underlying condition |
What to Do If You Have a High MCV
If a blood test shows a high MCV, consult a healthcare provider. They will review your history, lifestyle, and other blood test results. They may inquire about alcohol intake, diet, and symptoms like fatigue.
Further tests might be necessary, including:
- Serum Vitamin Levels: Measuring B12 and folate.
- Peripheral Blood Smear: Examining blood under a microscope.
- Reticulocyte Count: Checking for immature red blood cells.
- Liver Function Tests: Assessing liver health.
These results guide diagnosis and treatment. Deficiencies can be treated with supplements, and lifestyle changes may be advised for alcohol-related issues.
Conclusion: Separating Dehydration from True Macrocytosis
Dehydration causes hemoconcentration and can elevate hematocrit, but it does not cause a high MCV. A high MCV indicates enlarged red blood cells due to other medical conditions like vitamin deficiencies, liver disease, or bone marrow disorders. Interpreting a high MCV requires a full evaluation and considering all blood test results, not just assuming it's due to fluid loss. For more information, read about macrocytosis from the Cleveland Clinic.
