How do I interpret a hematocrit result?

How to Interpret Hematocrit

Hematocrit interpretation requires understanding normal reference ranges adjusted for sex, age, and altitude, recognizing that values vary significantly based on these factors and clinical context.

Normal Reference Ranges

Standard Sea-Level Values

• Men: Normal range 42-54% 1
• Women: Normal range 38-46% 1
• Anemia thresholds: Hematocrit <39% in men, <36% in women corresponds to hemoglobin <13 g/dL in men and <12 g/dL in women 1

Altitude-Adjusted Values

• High altitude (4000m): Normal ranges shift dramatically higher 2
  • Men: 45-61% (mean 52.7%) 2
  • Women: 41-56% (mean 48.3%) 2
• Altitude correction formula: Hematocrit increases approximately 0.003% per meter of elevation 3

Clinical Interpretation Framework

Step 1: Assess for Relative vs Absolute Changes

• Dehydration is the most common cause of falsely elevated hematocrit 4
• Ensure adequate hydration status before interpreting elevated values 5, 4
• Normal within-subject biological variation is ±3%, allowing up to 12% relative change between measurements taken 1 day to 2 months apart 6

Step 2: Identify Elevated Hematocrit Requiring Investigation

Investigate if any of the following are present:

• Hematocrit >60% in men or >55% in women at sea level 1
• Sustained increase ≥2% from baseline, even within normal range 7
• Values above 95th percentile adjusted for sex, race, and altitude 1
• Borderline-high hematocrit with associated features: thrombocytosis, leukocytosis, splenomegaly, unusual thrombosis, or erythromelalgia 1

Step 3: Differentiate Primary vs Secondary Causes

For elevated hematocrit after confirming adequate hydration:

1. Order serum erythropoietin (EPO) level 1, 5

   • Low EPO: >90% specific for polycythemia vera 1, 7
   • Normal EPO: Does not exclude polycythemia vera (sensitivity <70%) 1
   • High EPO: Suggests secondary erythrocytosis 1, 7

2. JAK2 mutation testing if polycythemia vera suspected 5, 7

   • JAK2V617F present in >95% of polycythemia vera cases 7
   • If negative, proceed to JAK2 exon 12 testing 7

3. Bone marrow examination when diagnosis remains unclear 1, 7

   • Look for hypercellularity with trilineage growth (panmyelosis) 7

Relationship with Kidney Function

Hematocrit decreases progressively with declining kidney function:

• Significant decline begins when GFR <60 mL/min/1.73 m² (CKD stage 3) 1
• At GFR 40-30 mL/min/1.73 m²: Women show 2.1% decrease, men show 3.9% decrease from baseline 1
• At GFR <20 mL/min/1.73 m²: Women show 5.3% decrease, men show 10.0% decrease 1
• Patients with diabetes develop anemia at earlier CKD stages than non-diabetics 1

Management Thresholds

Polycythemia Vera

• Target hematocrit <45% through phlebotomy to reduce thrombotic risk 1, 5
• Phlebotomy protocol: Remove 300-450 mL weekly until target achieved 5
• Add low-dose aspirin 100 mg daily unless contraindicated 5

Secondary Erythrocytosis

• Phlebotomy only indicated when 5:
  • Hematocrit >65% AND
  • Symptomatic hyperviscosity (headache, poor concentration) AND
  • Adequate hydration confirmed AND
  • No iron deficiency present
• First-line treatment is hydration, not phlebotomy 5, 4

Common Pitfalls to Avoid

• Do not perform routine phlebotomy in secondary erythrocytosis without clear indications—this causes iron deficiency and paradoxically increases stroke risk 5
• Do not ignore seasonal variation: Population mean hematocrit is approximately 3% lower in summer than winter 6
• Do not overlook iron deficiency: Check ferritin and iron studies if mean corpuscular hemoglobin concentration (MCHC) <32% 7
• Do not assume single elevated value is pathologic: Repeat after ensuring 48-72 hours of adequate hydration 4
• Do not use fixed "critical hematocrit" values universally: Optimal levels differ between patients based on clinical context 8