When should a patient with elevated hemoglobin (Hb) and hematocrit (Hct) levels be referred to a hematologist?

When to Refer to Hematology for High Hemoglobin and High Hematocrit

Patients with hemoglobin >18.5 g/dL in men or >16.5 g/dL in women, or hematocrit >52% in men or >48% in women, should be referred to hematology for evaluation of possible polycythemia, particularly when accompanied by other abnormal blood count parameters or symptoms.

Initial Evaluation Before Referral

Before referring to hematology, primary care physicians should:

1. Confirm true erythrocytosis:

   • Repeat complete blood count to verify persistent elevation
   • Rule out relative erythrocytosis (plasma volume contraction) by checking hydration status

2. Screen for common secondary causes:

   • Smoking history
   • Alcohol use
   • Obesity (43% of young adults with erythrocytosis are obese) 1
   • Hypoxic conditions (sleep apnea, COPD, high altitude residence)
   • Medications (testosterone, anabolic steroids)

3. Basic laboratory testing:

   • Complete blood count with differential
   • Comprehensive metabolic panel
   • Arterial blood gas (if hypoxia suspected)

Referral Criteria

Urgent Referral (within 1-2 weeks):

• Hemoglobin >20 g/dL in men or >18 g/dL in women
• Hematocrit >60% in men or >56% in women 2
• Symptoms of hyperviscosity (headache, blurred vision, thrombosis)
• Abnormal blood counts in other cell lines (elevated platelets >350×10⁹/L or neutrophils >6.2×10⁹/L) 3
• Splenomegaly
• History of thrombosis

Routine Referral:

• Persistent unexplained erythrocytosis after initial evaluation
• Hemoglobin >18.5 g/dL in men or >16.5 g/dL in women
• Hematocrit >52% in men or >48% in women
• Red blood cell count >6.45×10¹²/L 3
• Family history of polycythemia or erythrocytosis

Special Populations

High-Altitude Residents:

• Higher thresholds apply: hemoglobin >21 g/dL in men or >19 g/dL in women and hematocrit >61% in men or >56% in women at 4000m altitude 2

Elderly Patients:

• Lower thresholds may be appropriate due to increased risk of thrombosis

Smokers:

• Consider smoking cessation and reassessment before referral, as smoking is a common cause of secondary erythrocytosis 1

Diagnostic Approach by Hematologists

When patients are referred to hematology, the typical workup includes:

1. JAK2 mutation testing:

   • Consider using the JAKPOT prediction rule to guide testing decisions 3
   • JAK2 testing is indicated when red blood cell count >6.45×10¹²/L, platelets >350×10⁹/L, or neutrophils >6.2×10⁹/L

2. Serum erythropoietin level:

   • Low in primary polycythemia
   • Normal or elevated in secondary causes

3. Bone marrow examination:

   • When primary polycythemia is suspected

Alternative Management Options

For selected cases with mild elevations and no concerning features, written advice from a hematologist to the referring physician may be an acceptable alternative to formal consultation. This approach has been shown to be safe and effective for 26% of hematology referrals 4.

Pitfalls to Avoid

1. Failure to distinguish between relative and absolute erythrocytosis

   • Dehydration can cause falsely elevated hemoglobin/hematocrit

2. Missing secondary causes

   • Thorough history and physical examination are essential
   • Consider sleep studies for suspected sleep apnea

3. Unnecessary JAK2 testing

   • Using the JAKPOT prediction rule can reduce unnecessary testing by over 50% 3

4. Delayed diagnosis of polycythemia vera

   • Can lead to increased risk of thrombotic events

By following these guidelines, primary care physicians can appropriately identify patients who require hematology referral for elevated hemoglobin and hematocrit, while avoiding unnecessary referrals for those with easily identifiable secondary causes.