What causes a temporary elevation of red blood cells, hematocrit (packed cell volume), and hemoglobin?

Temporary Elevation of Red Blood Cells, Hematocrit, and Hemoglobin

Temporary elevations in red blood cells, hematocrit, and hemoglobin are most commonly caused by dehydration and plasma volume depletion (relative polycythemia), which resolve with rehydration, or by transient hypoxic conditions such as acute altitude exposure, sleep apnea episodes, or smoking cessation. 1, 2

Relative Polycythemia (Plasma Volume Depletion)

The most frequent cause of temporary elevation is relative polycythemia, where red cell mass remains normal but plasma volume contracts, artificially concentrating the blood 1:

• Dehydration from vomiting, diarrhea, severe burns, or inadequate fluid intake causes rapid hemoconcentration that reverses with fluid replacement 1, 2
• Diuretic use acutely reduces plasma volume, temporarily elevating hemoglobin and hematocrit values 1, 2
• Capillary leak syndrome shifts fluid from intravascular to interstitial spaces, concentrating blood components 1

These conditions are clinically obvious and do not require specialized red cell mass measurements for diagnosis 1. The key distinguishing feature is that hemoglobin, hematocrit, and red blood cell count all normalize rapidly (within hours to days) once the underlying cause is corrected 1, 2.

Transient Hypoxia-Driven Erythrocytosis

Acute Altitude Exposure

Initial ascent to high altitude produces a biphasic response 3, 4:

• Immediate hemoconcentration occurs within the first 24-48 hours due to dehydration and fluid shifts from intravascular to interstitial spaces, causing hemoglobin to rise by 131% and hematocrit by 117% of baseline 3
• This initial peak is followed by a brief decrease during the first week as the body rehydrates 3
• Erythropoietin (EPO) rises temporarily (maximum +11 mU/mL) but returns toward normal as hemoglobin stabilizes at higher levels 3, 5
• Upon return to sea level, hemoglobin and hematocrit normalize within a few days, though true red cell mass may remain elevated for weeks 3, 6

Intermittent Hypoxic Exposure

Frequent altitude changes (as seen in South American and Asian populations moving between low and high elevations) cause oscillating hemoglobin and hematocrit values 6:

• Plasma volume decreases at altitude and increases again at sea level, causing hemoglobin concentration to fluctuate even after 20+ years of regular exposure 6
• These represent optimal rapid adaptations rather than pathological changes 6

Obstructive Sleep Apnea

Nocturnal hypoxemia from sleep apnea produces chronic intermittent hypoxia that stimulates erythropoietin production 7:

• Hemoglobin elevations are typically mild (e.g., 17.4 g/dL) rather than marked 7
• CPAP therapy eliminates nocturnal hypoxemia and resolves the polycythemia by addressing the underlying cause 7
• Weight loss is essential as it directly treats the pathophysiology of obstructive sleep apnea 7

Smoker's Polycythemia

Chronic carbon monoxide exposure from smoking causes tissue hypoxia that stimulates EPO production 1, 2:

• This represents true secondary polycythemia, not relative polycythemia 1
• Smoking cessation resolves the polycythemia as carbon monoxide clears and tissue oxygenation normalizes 1, 2

Stress and Emotional Factors

Stress polycythemia (Gaisböck syndrome) is poorly understood and controversial 1:

• The concept of chronic, subtle plasma volume contraction from emotional stress has little scientific foundation 1
• In a series of 109 consecutive red cell mass measurements, no patients with true relative polycythemia from stress were identified 1

Critical Distinction: Temporary vs. Persistent Elevation

Key diagnostic principle: Temporary elevations resolve when the precipitating cause is removed 1, 2:

• Dehydration corrects with rehydration within hours to days 1
• Altitude-induced changes normalize within days of descent to sea level 3, 6
• Sleep apnea-related erythrocytosis resolves with CPAP therapy 7
• Smoker's polycythemia reverses with smoking cessation 1, 2

If elevations persist despite addressing these causes, evaluation for primary polycythemia vera (JAK2 mutation testing) or other secondary causes (renal tumors, testosterone use, congenital disorders) becomes necessary 2, 7.

Common Pitfalls to Avoid

• Do not perform red cell mass measurements for clinically obvious causes of relative polycythemia like dehydration or diuretic use 1
• Do not rush to diagnose polycythemia vera without first excluding secondary causes, especially in patients with obesity, fatigue, or smoking history 7
• Do not assume "adequate sleep" rules out sleep apnea - patients are typically unaware of their sleep fragmentation and nocturnal hypoxemia 7
• Do not ignore the possibility of coexisting iron deficiency in patients with erythrocytosis, as this can cause microcytic polycythemia with elevated RBC count but paradoxically reduced hemoglobin 1, 2