A 6-year-old girl is brought to the office by her mother due to abdominal pain and vomiting. The patient has Down syndrome but has otherwise been healthy. Abdominal examination reveals the liver edge 3 cm below the costal margin and the spleen tip extending past the level of the umbilicus. Leukocyte count is 104,000/mm³, and subsequent peripheral blood smear reveals numerous lymphoblasts. The patient is diagnosed with acute lymphoblastic leukemia and combination cytotoxic chemotherapy is planned. Which of the following laboratory changes are most likely to be seen in this patient following treatment initiation?

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11ec160b_3ccc_ab57_9249_71b8b271974c_MD0001_00 Acute lymphoblastic leukemia (ALL) is a hematologic malignancy that is generally associated with a high tumor cell burden and a rapid turnover rate. Therefore, initiation of cytotoxic chemotherapy often leads to massive lysis of tumor cells, which spills intracellular contents into the blood (tumor lysis syndrome), resulting in the following: Hyperphosphatemia: Intracellular phosphate concentration is much higher than serum phosphate levels. Therefore, serum phosphate levels typically spike following chemotherapy initiation. Because excess phosphate is excreted via the kidneys, patients are at risk for calcium-phosphate deposition in the renal tubules and collecting system, which can lead to hypocalcemia and acute renal failure. Hyperuricemia: Purine nucleic acids released from lysed tumor cells are metabolized by xanthine oxidase into uric acid. Because uric acid is excreted through the kidneys and is poorly soluble, patients are at risk for obstructing uric acid stones, which can lead to acute renal injury. Hyperkalemia: Potassium is primarily an intracellular ion. Therefore, significant tumor lysis is generally associated with elevations of serum potassium, which can lead to cardiac arrhythmia. Elevated lactate dehydrogenase (LDH): Lactate dehydrogenase, an enzyme involved in cellular respiration, is found in almost all human cells. It is an important laboratory marker for cell lysis in a wide range of disease states (eg, hemolytic anemia). Levels are typically significantly elevated in tumor lysis syndrome. Educational objective: Patients with hematologic malignancies that have high tumor cell burdens or rapid turnover rates are at high risk for electrolyte abnormalities following the initiation of cytotoxic chemotherapy due to the massive lysis of tumor cells. Patients generally develop hyperphosphatemia, hyperkalemia, hyperuricemia, and elevated lactate dehydrogenase.

A 6-Year-Old Girl Is Brought to the Office by Her