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Difficulty: MediumInterpreting Laboratory Results

An asymptomatic 72-year-old man presents to his primary care physician for a scheduled health maintenance visit. His past medical history is significant for polycythemia vera and essential hypertension. His daily medication regimen consists of low-dose aspirin and amlodipine. The patient states he feels completely well and denies any recent muscle weakness, palpitations, syncope, or shortness of breath.

Vital signs show a temperature of 37.0°C (98.6°F), blood pressure of 135/82 mmHg, pulse of 78/min, and respiratory rate of 14/min. Physical examination reveals a plethoric appearance of the face and mild splenomegaly. A 12-lead electrocardiogram demonstrates normal sinus rhythm with normal QRS duration and no ST-segment or T-wave abnormalities.

Laboratory results obtained from a serum sample are as follows:

TestResultReference Range
Sodium140 mEq/L136–145 mEq/L
Potassium6.7 mEq/L3.5–5.0 mEq/L
Chloride104 mEq/L98–106 mEq/L
Bicarbonate25 mEq/L22–28 mEq/L
Blood urea nitrogen16 mg/dL8–20 mg/dL
Creatinine1.0 mg/dL0.6–1.2 mg/dL
Hemoglobin17.5 g/dL13.5–17.5 g/dL
Leukocytes14,000/mm³4,500–11,000/mm³
Platelets1,250,000/mm³150,000–400,000/mm³

Which of the following is the most likely mechanism responsible for the abnormal potassium level reported in this patient?

  1. Release of intracellular potassium during in vitro blood clottingAnswer
  2. B
    Shift of potassium from the intracellular to the extracellular space
  3. C
    In vivo destruction of erythrocytes due to microvascular shear stress
  4. D
    Medication-induced impairment of the renin-angiotensin-aldosterone system
  5. E
    Decreased renal tubular secretion due to occult nephropathy

Answer

Release of intracellular potassium during in vitro blood clotting
The correct answer identifies the release of intracellular potassium during in vitro blood clotting as the cause of this patient's lab abnormality. This patient presents with an elevated serum potassium level of 6.7 mEq/L, but is completely asymptomatic and has a normal electrocardiogram (no peaked T waves, normal QRS duration). Crucially, the patient has a markedly elevated platelet count (1,250,000/mm³). Pseudohyperkalemia is an in vitro phenomenon where potassium is released from cells (platelets, leukocytes, or erythrocytes) during the clotting process in the collection tube. It is most frequently seen in patients with severe thrombocytosis or leukocytosis. Because the potassium is only elevated in the specimen tube and not in the patient's circulating plasma, the condition is benign and requires no treatment. The diagnosis can be confirmed by measuring potassium in a heparinized tube (plasma potassium), which prevents clotting and will reflect the true physiological level.

Step-by-Step Solution

1
Evaluate the patient's clinical presentation and electrocardiogram (ECG) findings in relation to the laboratory abnormality.
The patient is completely asymptomatic, and the ECG shows no classic signs of hyperkalemia (such as peaked T waves or widened QRS complexes) despite a severely elevated reported potassium of 6.7 mEq/L.
To determine whether the hyperkalemia represents a life-threatening, true physiologic state or a laboratory artifact.
2
Analyze the complete laboratory panel for underlying causes of the abnormal potassium value.
Renal function (BUN and creatinine) is normal, ruling out renal failure. Bicarbonate is normal, ruling out severe acidosis. However, the platelet count is massively elevated at 1,250,000/mm³ due to his myeloproliferative disorder.
To identify specific derangements that either explain true hyperkalemia or point toward a spurious result.
3
Synthesize the clinical and laboratory data to conclude the mechanism.
The extreme thrombocytosis leads to the massive release of intracellular potassium as platelets degranulate during the clotting process in the serum collection tube, causing pseudohyperkalemia.
To establish the diagnosis of pseudohyperkalemia, avoiding unnecessary and potentially harmful therapies like intravenous calcium or insulin.

Key Concept

Pseudohyperkalemia is a laboratory artifact typically caused by the in vitro release of intracellular potassium from cells (most commonly due to extreme thrombocytosis or leukocytosis) during specimen clotting.
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Topics

Interpreting Laboratory Results

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