Can Dehydration Affect Hemoglobin Count? Understanding Hemoconcentration

September 28, 2025 •

2 min read

According to the National Institutes of Health, hemoglobin and hematocrit values depend on plasma volume. A reduction in this fluid component due to insufficient water intake can lead to skewed results. This confirms that, yes, can dehydration affect hemoglobin count, and it’s a critical factor for anyone preparing for a blood test.

Quick Summary

Dehydration creates a concentrated blood environment by reducing plasma volume, causing hemoglobin and hematocrit values to appear higher than they are. This phenomenon, known as hemoconcentration, highlights why proper hydration is essential for accurate blood test readings.

Key Points

Falsely Elevated Hemoglobin: Dehydration causes a reduction in blood plasma volume, which makes the hemoglobin concentration appear higher than its actual value.
Hemoconcentration is the Cause: The term for this phenomenon is hemoconcentration, where the non-fluid components of the blood become more concentrated.
Misleading Lab Results: An elevated hemoglobin count from dehydration can lead to incorrect diagnoses or unnecessary medical investigation.
Rehydration Reverses the Effect: Restoring proper fluid balance will cause the concentrated blood to return to a normal state, and a retest will show accurate hemoglobin levels.
Hydration is Key for Accuracy: To ensure the most accurate blood test results, it is critical to be well-hydrated, unless a specific test requires otherwise.
Timing Matters: The time of day a test is conducted can influence results, as most people are less hydrated in the morning.
Other Values are Affected: Hemoconcentration also impacts other blood test parameters like hematocrit, BUN, and electrolytes.

The Mechanism Behind Hemoconcentration

When the body is dehydrated, it loses fluids, which decreases the total blood volume while the number of red blood cells and hemoglobin remains constant. This leads to a higher concentration of red blood cells and hemoglobin within the reduced plasma, resulting in falsely high readings on laboratory tests. This effect on hemoglobin and hematocrit levels is temporary and resolves once the body is rehydrated.

How Dehydration Impacts Your Blood Test Results

Dehydration's effect on hemoglobin can lead to misinterpretation of test results. A falsely high hemoglobin count might suggest conditions like polycythemia vera, when the issue is simply a lack of fluids. Proper hydration before blood tests, unless specific fasting is required, is often recommended by healthcare professionals to prevent this. The time of day a test is taken can also play a role, as hydration levels fluctuate.

Distinguishing Hydration Status from Medical Conditions

Elevated hemoglobin can have causes other than dehydration, including more serious medical conditions. Doctors evaluate the complete clinical picture to make an accurate diagnosis. It's important to inform your doctor about any recent fluid intake or dehydration symptoms before a blood test.

Other Blood Parameters Affected by Dehydration

Hemoconcentration from dehydration can influence other blood test results besides hemoglobin and hematocrit, including electrolytes and indicators of kidney function. This can result in elevated hematocrit, higher BUN levels, and skewed electrolyte readings. There is also ongoing research into dehydration's potential impact on blood glucose tests like HbA1c.

Steps to Ensure Accurate Blood Test Results

To help ensure accurate blood test results:

Hydrate Appropriately: Drink sufficient water in the day before your test. Water is typically allowed during fasting periods unless specifically restricted.
Adhere to Fasting Rules: Follow your doctor's specific instructions regarding fasting from food and other substances.
Report Symptoms: Inform your healthcare provider if you have experienced vomiting, diarrhea, or excessive sweating recently.
Maintain Consistency: If you have regular blood work, try to schedule it at the same time to minimize the impact of natural bodily variations.
Share All Information: Discuss all symptoms, including thirst or fatigue, as they can provide clues about your hydration status.

Hydrated vs. Dehydrated Blood Test Comparison


Blood Test Parameter	Properly Hydrated	Dehydrated (Hemoconcentrated)
Hemoglobin	Within normal range	Falsely elevated
Hematocrit	Within normal range	Falsely elevated
Plasma Volume	Normal	Decreased
BUN (Blood Urea Nitrogen)	Normal	Potentially elevated
Electrolytes	Balanced	Can be imbalanced
Blood Viscosity	Normal	Increased

Conclusion

Understanding how hydration influences hemoglobin is crucial for accurate health assessments. Dehydration causes hemoconcentration, leading to misleadingly high hemoglobin and other blood test values. This can complicate diagnosis. Proper hydration, particularly before medical tests, helps ensure that blood work accurately reflects health status. Always discuss hydration concerns or abnormal lab results with your healthcare provider. For additional information on hydration, refer to the Mayo Clinic website.

Frequently Asked Questions

The effects of dehydration on blood parameters can occur relatively quickly, with measurable changes appearing even with mild fluid loss. The resulting hemoconcentration can cause a falsely elevated reading on a blood test conducted soon after a period of fluid restriction.

Yes, even mild to moderate dehydration can cause a noticeable increase in your hemoglobin and hematocrit levels. The change may be enough to shift a reading from the normal range into the elevated range, which is why proper hydration is so important for test accuracy.

Inform your doctor about your hydration status leading up to the test. They may recommend retesting after you have properly rehydrated. The false elevation should correct itself once your fluid balance is restored.

Drinking a normal amount of water to stay well-hydrated is helpful, as it ensures your blood plasma volume is at a standard level. However, excessive fluid intake right before a test could dilute the blood and potentially lead to a falsely low reading.

No, for most blood tests, including a complete blood count (CBC) with hemoglobin, you are encouraged to drink water. Fasting instructions typically refer to food and certain beverages, but water intake is generally advised to prevent dehydration and ensure accurate results. Always follow your doctor's specific instructions, though.

Many factors can influence your hemoglobin count. Other causes of high hemoglobin include smoking, living at high altitude, and certain heart or lung diseases. Low hemoglobin can be caused by conditions like anemia, blood loss, chronic kidney disease, and nutritional deficiencies.

No, dehydration does not cause a low hemoglobin count. In fact, it typically has the opposite effect, causing a falsely elevated reading due to hemoconcentration. A low hemoglobin count is usually indicative of an underlying medical issue, such as anemia.

Yes, in theory. If a patient is anemic (low actual hemoglobin) but also severely dehydrated, the hemoconcentration could temporarily raise the measured hemoglobin count into the normal range, masking the underlying anemia. Rehydrating the patient would then reveal the true, lower count.