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Difficulty: MediumInterpreting Laboratory Results

A 62-year-old man comes to the clinic reporting severe, generalized itching that typically occurs immediately after taking a warm shower. He also notes intermittent headaches and a "fullness" in his head over the past three months. His medical history includes hypertension. He has a 30-pack-year smoking history but quit 10 years ago. His current medications include lisinopril. Blood pressure is 155/92 mmHg, and pulse is 82/min. Physical examination reveals a plethoric appearance of the face and a spleen tip palpable 3 cm below the left costal margin. Laboratory studies show:

Laboratory ParameterResultReference Range
Hemoglobin19.5 g/dL19.5 \text{ g/dL}13.5−17.5 g/dL13.5-17.5 \text{ g/dL}
Hematocrit59%59\%41−53%41-53\%
Leukocyte count14,500/mm314,500/\text{mm}^34,500−11,000/mm34,500-11,000/\text{mm}^3
Platelet count550,000/mm3550,000/\text{mm}^3150,000−400,000/mm3150,000-400,000/\text{mm}^3
Serum erythropoietin2 mU/mL2 \text{ mU/mL}4−26 mU/mL4-26 \text{ mU/mL}
Arterial oxygen saturation (room air)96%96\%≥95%\ge 95\%

Which of the following best explains this patient's laboratory profile?

  1. Constitutive activation of a non-receptor tyrosine kinaseAnswer
  2. B
    Compensatory response to chronic systemic hypoxia
  3. C
    Autonomous paraneoplastic hormone secretion
  4. D
    Reduction in circulating plasma volume
  5. E
    Impaired oxygen offloading by a hemoglobin variant

Answer

Constitutive activation of a non-receptor tyrosine kinase
The patient's clinical presentation (aquagenic pruritus, splenomegaly, plethora) and laboratory findings (panmyelosis with an elevated hematocrit, leukocytes, and platelets) are classic for polycythemia vera (PV). The critical distinguishing laboratory value is the suppressed serum erythropoietin (EPO) level. In PV, a primary myeloproliferative neoplasm, red blood cell production is autonomous and independent of EPO. This autonomous production leads to increased oxygen delivery, which exerts negative feedback on the kidneys, suppressing endogenous EPO synthesis. Over 95% of PV cases are caused by a V617F point mutation in the JAK2 gene. The JAK2 gene encodes a non-receptor tyrosine kinase that normally mediates intracellular signaling downstream of the EPO receptor. The mutation results in constitutive, ligand-independent activation of this kinase, driving unchecked myeloid proliferation.

Step-by-Step Solution

1
Analyze the hematologic parameters.
The patient has elevated hemoglobin, hematocrit, leukocytes, and platelets, indicating panmyelosis (hyperproliferation of all three myeloid lineages).
Identifying panmyelosis narrows the differential diagnosis toward a primary bone marrow disorder rather than an isolated red blood cell issue.
2
Evaluate the serum erythropoietin (EPO) level in the context of the elevated hematocrit.
The serum EPO level is suppressed (low).
A low EPO level differentiates primary polycythemia (EPO-independent red blood cell production) from secondary polycythemia (where EPO is appropriately or autonomously elevated).
3
Correlate the laboratory findings and clinical presentation with the underlying pathophysiology.
The combination of panmyelosis, low EPO, aquagenic pruritus, and splenomegaly confirms Polycythemia Vera, which is driven by a JAK2 mutation (a non-receptor tyrosine kinase).
Understanding the molecular mechanism of the confirmed diagnosis is necessary to identify the correct answer choice.

Key Concept

Polycythemia vera is a primary myeloproliferative neoplasm characterized by panmyelosis and a suppressed erythropoietin level, predominantly driven by a constitutively active JAK2 mutation.
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Interpreting Laboratory Results

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