What does a high hematocrit (Hct) level combined with a low mean corpuscular hemoglobin concentration (MCHC) indicate?

High Hematocrit with Low MCHC: Clinical Implications and Evaluation

A high hematocrit with low mean corpuscular hemoglobin concentration (MCHC) most commonly indicates iron deficiency in the setting of relative polycythemia, requiring evaluation for both the cause of elevated red cell mass and iron status.

Understanding the Parameters

Hematocrit (Hct)

• Represents the volume percentage of red blood cells in blood
• Elevated hematocrit (>45% in men, >42% in women) suggests:
  • Absolute polycythemia: true increase in red cell mass
  • Relative polycythemia: hemoconcentration due to dehydration or decreased plasma volume

MCHC

• Measures the concentration of hemoglobin in a given volume of packed red blood cells
• Low MCHC (<32 g/dL) indicates:
  • Hypochromia - reduced hemoglobin concentration in red cells
  • Most commonly caused by iron deficiency 1
  • Can also occur in certain hemoglobinopathies

Clinical Significance

Diagnostic Considerations

1. Iron Deficiency with Relative Polycythemia

   • Most common explanation for this combination
   • Iron deficiency leads to hypochromic cells (low MCHC)
   • Relative polycythemia from dehydration or decreased plasma volume causes high hematocrit

2. Polycythemia Vera with Iron Deficiency

   • Polycythemia vera (PV) patients often develop iron deficiency due to:
     • Repeated phlebotomy treatments
     • Increased iron utilization from excessive erythropoiesis 2
   • Microcytosis from iron deficiency is a PV-related feature 2

3. Chronic Kidney Disease with Iron Deficiency

   • CKD patients with anemia typically have normocytic, normochromic indices
   • Low MCHC suggests inadequate iron availability for hemoglobin synthesis 1

Prognostic Implications

• Low MCHC (<32 g/dL) has been associated with:
  • Increased risk of non-atherosclerotic cardiovascular disease 3
  • Left ventricular hypertrophy and left atrial enlargement 3
  • In cyanotic heart disease, increased risk of cerebrovascular accidents 4

Evaluation Algorithm

1. Confirm findings and assess for dehydration

   • Repeat complete blood count to verify high hematocrit and low MCHC
   • Assess hydration status (skin turgor, mucous membranes, BUN/Cr ratio)

2. Iron studies

   • Serum ferritin (marker of iron stores)
   • Transferrin saturation (marker of available iron)
   • Consider reticulocyte count to assess bone marrow response 2

3. Rule out polycythemia vera if hematocrit is significantly elevated

   • Check serum erythropoietin level (typically low in PV)
   • Consider JAK2 mutation testing
   • Evaluate for PV-related features: thrombocytosis, leukocytosis, splenomegaly 2

4. Assess for chronic conditions

   • Kidney function tests (CKD can cause both anemia and altered red cell indices)
   • Evaluate for chronic inflammatory conditions
   • Consider hemoglobinopathies if appropriate based on ethnicity/family history

Management Approach

1. Address dehydration if present

   • Oral or IV fluid rehydration based on severity

2. Treat iron deficiency if confirmed

   • Oral iron supplementation (if tolerated)
   • IV iron if oral not tolerated or in specific conditions like CKD

3. For polycythemia vera (if diagnosed)

   • Target hematocrit <45% through phlebotomy 2
   • Low-dose aspirin for thrombosis prevention 2
   • Consider cytoreductive therapy for high-risk patients

4. Monitor response

   • Follow hematocrit and MCHC to ensure normalization
   • Reassess iron studies to confirm repletion

Important Caveats

• Spurious results: Verify that high MCHC is not a laboratory error, as values significantly above reference range are physiologically implausible 5
• Pregnancy and menstruation: Standard definitions of anemia may not apply 2
• Altitude and smoking: Can affect normal hematocrit values 2
• Age considerations: Normal values may differ in elderly patients (>70 years) 2

Remember that this combination of findings requires thorough investigation as it may represent either a benign condition (dehydration with iron deficiency) or more serious underlying disorders that require specific management.