Can dehydration cause elevated hematocrit (Hct) levels?

Dehydration and Elevated Hematocrit

Yes, dehydration is a common cause of elevated hematocrit levels due to hemoconcentration from reduced plasma volume.

Mechanism of Dehydration-Induced Hematocrit Elevation

Dehydration leads to elevated hematocrit through a simple but important physiological mechanism:

• When the body loses water without proportional loss of red blood cells, the blood becomes more concentrated
• This concentration effect results in a higher percentage of red blood cells per unit volume of blood (hematocrit)
• The actual red blood cell mass remains unchanged, but appears increased due to reduced plasma volume

Evidence Supporting This Relationship

Research clearly demonstrates this relationship:

• Studies show that water deprivation causes progressive reduction in blood fluidity with significant increases in hematocrit (up to 22%) 1
• Animal studies confirm that dehydration leads to hemoconcentration due to reduced plasma volume 2
• Even short-term dehydration can cause measurable increases in hematocrit values 3

Clinical Implications

Understanding this relationship has important clinical implications:

• Hydration status assessment: Elevated hematocrit may serve as an indicator of dehydration
• Interpretation of lab values: Clinicians should consider hydration status when interpreting hematocrit results
• Treatment approach: Rehydration can normalize artificially elevated hematocrit values

Distinguishing from True Polycythemia

It's crucial to differentiate dehydration-induced hematocrit elevation from true polycythemia:

• Dehydration causes temporary hematocrit elevation that normalizes with rehydration
• True polycythemia involves an actual increase in red blood cell mass
• Additional testing may be needed if hematocrit remains elevated after adequate hydration

Special Considerations

Several clinical scenarios warrant special attention:

• Patients with heart failure: May require careful fluid management as both dehydration and fluid overload can worsen outcomes 4
• Patients with cyanotic heart disease: Already have compensatory erythrocytosis; dehydration can further increase blood viscosity and risk of complications 4
• Neurosurgical patients: Dehydration can complicate management of hyponatremia; central venous pressure measurements may help distinguish volume status 4

Management Approach

When encountering elevated hematocrit:

1. Assess for clinical signs of dehydration (dry mucous membranes, reduced skin turgor, orthostatic changes)
2. Consider rehydration before additional workup if dehydration is suspected
3. Recheck hematocrit after adequate hydration
4. If hematocrit remains elevated despite hydration, investigate for true polycythemia

Pitfalls to Avoid

• Unnecessary phlebotomy: Phlebotomy is rarely necessary and can be harmful in patients with dehydration-induced hematocrit elevation 4, 5
• Misdiagnosis: Attributing elevated hematocrit to pathological conditions when dehydration is the cause
• Inadequate rehydration: Failing to correct the underlying dehydration before further diagnostic workup

Remember that proper hydration is essential before any procedure involving contrast media, especially in patients with renal impairment 5, as dehydration can exacerbate contrast-induced nephropathy.