The Core Concept: Relative vs. Absolute
To understand how dehydration affects red blood cell counts, it's crucial to distinguish between two types of erythrocytosis (the medical term for an increase in red blood cells).
- Relative Polycythemia: This is the type caused by dehydration. It is not a true increase in the number of red blood cells. Instead, the red blood cells are simply more concentrated because the volume of blood plasma—the liquid component—has decreased. When the body loses a significant amount of water through sweating, urination, or illness, the remaining blood has a higher proportion of red blood cells per volume of fluid. A blood test will therefore show an elevated count, but once rehydrated, the levels will return to normal.
- Absolute Polycythemia: This involves an actual increase in the number of red blood cells being produced by the body. This can be caused by conditions like polycythemia vera (a bone marrow disorder), chronic low oxygen levels (due to lung or heart disease), or certain tumors that produce erythropoietin (EPO), the hormone that stimulates red blood cell production. These conditions represent a true medical issue that needs to be addressed, unlike the temporary state caused by dehydration.
The Mechanism of Hemoconcentration
When a person becomes dehydrated, the total volume of their blood decreases. The number of red blood cells, however, remains constant. This creates a disproportionate ratio, where the blood cells are more densely packed within the available plasma. This phenomenon, known as hemoconcentration, is what causes blood tests like a Complete Blood Count (CBC) to indicate a high red blood cell count, as well as elevated hemoglobin and hematocrit levels. The hematocrit, which measures the percentage of red blood cells in the blood, is a key indicator that can be artificially inflated by dehydration.
How Dehydration Appears on a Blood Test
If you go for a blood test while dehydrated, the results may present a misleading picture of your health. Your healthcare provider will look for several indicators on a CBC:
- Elevated Hematocrit: The ratio of red blood cells to total blood volume will be higher than normal.
- Elevated Hemoglobin: The protein in red blood cells that carries oxygen will appear more concentrated.
- Potentially Elevated RBC Count: The raw number of red blood cells per microliter of blood can also be elevated.
Because these can also be signs of more serious conditions, it is critical to inform your doctor if you suspect you were dehydrated before your blood draw. Often, the doctor will recommend rehydration and a follow-up test to confirm if the elevated levels were temporary or represent an underlying medical issue.
Comparing Relative and Absolute Polycythemia
| Feature | Relative Polycythemia (Due to Dehydration) | Absolute Polycythemia (Due to Underlying Cause) |
|---|---|---|
| Actual RBC Count | Total number of red blood cells is normal. | Total number of red blood cells is elevated. |
| Underlying Cause | Lack of sufficient blood plasma volume due to dehydration. | Overproduction of red blood cells, often from bone marrow issues or chronic hypoxia. |
| Effect on Blood | Higher concentration of red blood cells per volume of fluid. | Increase in the overall red blood cell mass. |
| Treatment | Rehydration by drinking fluids. | Varies depending on the cause (e.g., phlebotomy, medication). |
| Prognosis | Excellent; resolves with proper hydration. | Depends on the underlying condition; requires ongoing management. |
Other Symptoms and Risks of Dehydration
High red blood cell concentration isn't the only effect of dehydration. The thickened blood can increase its viscosity, or stickiness, which can lead to other issues. Symptoms of dehydration can include:
- Headaches and dizziness
- Fatigue and low energy
- Dark-colored urine
- Extreme thirst and dry mouth
- Reduced urination
More severe dehydration can lead to serious complications such as kidney problems, seizures, or even shock. In cases of significant and prolonged dehydration, the increased blood viscosity can potentially increase the risk of blood clots, although this is more concerning in cases of true polycythemia.
Diagnosing and Treating Dehydration-Induced High RBCs
Diagnosis of relative polycythemia often starts with a routine CBC test that shows elevated hematocrit and hemoglobin levels. To differentiate from absolute polycythemia, a doctor will consider the patient's medical history, other symptoms of dehydration, and their current lifestyle and medication use (e.g., diuretics). The simplest and most effective treatment is rehydration. For mild cases, this can be achieved by drinking plenty of water and other fluids. In severe cases, a hospital visit for intravenous (IV) fluid administration may be necessary to quickly restore proper blood volume. It's crucial for anyone with consistently elevated RBC counts to work with their healthcare provider to identify and address the root cause.
For more information on different types of polycythemia and their causes, you can refer to authoritative medical resources such as the Cleveland Clinic, which provides detailed explanations on erythrocytosis and its origins.
Steps to Take if Your Blood Test Shows High RBCs
If you receive a blood test result indicating high red blood cell counts, here are the steps you should take:
- Reflect on Your Hydration: Think about your fluid intake in the days leading up to the test. Were you more active, in a hotter climate, or feeling unwell with fever or vomiting? This context is vital.
- Speak with Your Doctor: Discuss your lifestyle and recent health with your physician. They can help determine if the result is likely due to dehydration.
- Ensure Proper Rehydration: If dehydration is suspected, make a conscious effort to increase your fluid intake. Follow your doctor's recommendations for how much to drink.
- Schedule a Follow-Up Test: Your doctor will likely order a follow-up CBC after you've been properly hydrated for a period. This will show if your levels have returned to normal, confirming a relative polycythemia diagnosis.
- Address Underlying Causes: If the retest still shows elevated levels, further investigation will be necessary to rule out more serious conditions like polycythemia vera or chronic lung disease.
Conclusion
While the sight of a lab result indicating high red blood cell counts can be alarming, it's important not to panic. Dehydration is a common and easily treatable cause of temporarily elevated RBC levels. Through a process called hemoconcentration, the loss of plasma fluid makes the blood appear thicker and denser on a blood sample, but it doesn't represent a true increase in cell production. By understanding the difference between relative and absolute polycythemia, and by communicating openly with your healthcare provider, you can ensure an accurate diagnosis and receive the appropriate treatment, which often just means a simple rehydration protocol. Always consult a medical professional to interpret blood test results and determine the proper course of action for your health.
