At what hematocrit level do you suspect Obstructive Sleep Apnea (OSA)?

Hematocrit Levels That Should Raise Suspicion for Obstructive Sleep Apnea (OSA)

A hematocrit level above 44.6% in men should raise suspicion for obstructive sleep apnea, particularly when accompanied by clinical symptoms of OSA such as loud snoring, witnessed apneas, or excessive daytime sleepiness. 1

Relationship Between OSA and Hematocrit

OSA causes intermittent hypoxia during sleep, which theoretically should lead to secondary polycythemia through hypoxia-induced erythropoiesis. However, the relationship between OSA and elevated hematocrit is more complex than initially expected:

• Studies show that less than 10% of OSA patients develop polycythemia 1
• The prevalence of polycythemia varies by OSA severity:
  • 2% in mild-to-moderate OSA
  • 6% in severe OSA 2

Gender Differences

• Men with severe OSA show significantly higher hemoglobin, hematocrit, and rates of polycythemia compared to those without OSA 1
• Women show weaker correlations between OSA severity and hematocrit levels 1

Key Diagnostic Indicators

When evaluating hematocrit levels in the context of suspected OSA:

• Mean nocturnal oxygen saturation (SpO2) is a stronger predictor of polycythemia than the apnea-hypopnea index (AHI) 1
• Cutoff values that should raise suspicion for OSA in men:
  • Hemoglobin > 155.5 g/L
  • Hematocrit > 44.6% 1

Why Many OSA Patients Don't Develop Polycythemia

Despite intermittent hypoxia, most OSA patients don't develop the expected secondary polycythemia due to:

1. Neocytolysis (destruction of newly formed red blood cells when returning to normoxia) 3
2. Increased reactive oxygen species from expanded reticulocytes' mitochondria 3
3. Inflammation-induced hepcidin limiting iron availability for erythropoiesis 3

Clinical Approach

When evaluating patients with elevated hematocrit:

1. Screen for OSA symptoms:

   • Loud snoring
   • Witnessed apneas
   • Excessive daytime sleepiness 4
   • Motor vehicle accidents
   • Neck circumference >17 inches in men or >16 inches in women 4

2. Consider polysomnography when:

   • Hematocrit > 44.6% in men (especially with symptoms) 1
   • Patient has resistant hypertension, especially if obese 5
   • BMI ≥ 35 kg/m² 5

Effect of CPAP Treatment

• CPAP treatment reduces hemoglobin by approximately 3.76 g/L 2
• CPAP reduces hematocrit by approximately 1.1% in patients without baseline polycythemia 2
• In some patients, CPAP treatment may paradoxically restore anemia of aging that was previously masked by hypoxic stimulation of erythropoiesis 6

Common Pitfalls

• Relying solely on AHI rather than mean nocturnal oxygen saturation when assessing the relationship between OSA and hematocrit levels 1
• Failing to consider gender differences in the relationship between OSA and hematocrit 1
• Not recognizing that the absence of polycythemia does not rule out OSA, as most OSA patients (>90%) do not develop polycythemia despite intermittent hypoxia 3, 1
• Overlooking that a single night without CPAP treatment does not significantly worsen blood viscosity or platelet activity in patients on long-term CPAP therapy 7

Remember that while elevated hematocrit can be a clue to underlying OSA, the majority of OSA patients will not present with polycythemia, making it important to evaluate other clinical symptoms and risk factors when considering this diagnosis.