Pseudohypokalemia from extreme leukocytosis

Question

A 58-year-old man with a past medical history of chronic myeloid leukemia presents to the hospital for new-onset weakness. He denies nausea and vomiting but reports poor oral intake for the last 3 days. Three weeks ago, he saw his primary care physician, and his laboratory studies were unchanged from his baseline. Repeat laboratory data: WBC count 310,000/µL (reference 4000–11,000); blasts 25% (0%); serum potassium 2.8 mEq/L (3.5–5.0); total CO₂ 26 mEq/L (23–28); creatinine 1.3 mg/dL (baseline) (0.7–1.3); uric acid 6.5 mg/dL (3.0–7.0); magnesium 2 mEq/L (1.6–2.6); phosphorus 3.5 mg/dL (3.0–4.5). Which ONE of the following is the most likely cause of his abnormal potassium value?

Accepted Answer

B. Leukocytosis — The most likely cause of this patient’s low potassium value on laboratory testing is leukocytosis and does not represent true hypokalemia.

Profound leukocytosis may be associated with both hypokalemia and hyperkalemia. High serum potassium values can represent true hyperkalemia, such as in tumor lysis syndrome, or pseudohyperkalemia, due to the lysis of WBC in vitro after sample collection, prolonged incubation times, and tourniquet use, causing cell lysis of fragile leukemic cell membranes. These findings are classically seen in patients with chronic lymphocytic leukemia. Pseudohypokalemia in the setting of leukocytosis is thought to be related to the high metabolic activity of leukocytes in vitro, especially with prolonged incubation time, and may be seen in patients with acute and chronic myeloid leukemia. The key to diagnosing spurious potassium values is a normal arterial or venous blood gas or plasma potassium level drawn at the same time as the serum sample. Given clinical suspicion, treatment of these entities should not be undertaken until a true electrolyte abnormality is confirmed, although it may be prudent to obtain an EKG until true potassium disorders are excluded.

Tumor genesis syndrome, or the rapid growth of tumor cells, may lead to true hypokalemia and hypophosphatemia, as the growing cells utilize serum substrate in their division and growth. The normal serum phosphorus level makes this diagnosis less likely.

Metabolic alkalosis may lead to hypokalemia, as cells exchange hydrogen for potassium ions across the cell membrane in response to the high serum pH. This effect causes an intracellular shift of potassium, with resulting hypokalemia. However, this patient's metabolic alkalosis is mild and unlikely to cause significant cell shifts.

Renal potassium wasting may cause hypokalemia. A variety of mechanisms have been postulated to cause renal potassium wasting in the setting of leukemia, including increased lysozyme activity causing proximal tubule cell damage and paraneoplastic renin activity by the leukemic cells. These patients typically exhibit renal magnesium wasting as well, so the normal serum magnesium level makes this diagnosis less likely.

Pseudohypokalemia from extreme leukocytosis

Explanation

Why each option

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