# Situation: A 45-year-old man with bulky diffuse large B-cell lymphoma is admitted for his first cycle of rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone. Thirty-six hours after chemotherapy, his results are: Potassium: 6.4 mEq/L (3.5–5.0) Phosphate: 6.9 mg/dL (2.5–4.5) Calcium: 7.9 mg/dL (8.5–10.5) Uric acid: 11.8 mg/dL (3.5–7.2) He has no numbness, tingling, or muscle twitching, and his urine output is 45 mL/h. Which result poses the MOST immediate threat to his life?

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## 문제

Situation: A 45-year-old man with bulky diffuse large B-cell lymphoma is admitted for his first cycle of rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone.

Thirty-six hours after chemotherapy, his results are:
Potassium: 6.4 mEq/L (3.5–5.0)
Phosphate: 6.9 mg/dL (2.5–4.5)
Calcium: 7.9 mg/dL (8.5–10.5)
Uric acid: 11.8 mg/dL (3.5–7.2)
He has no numbness, tingling, or muscle twitching, and his urine output is 45 mL/h. Which result poses the MOST immediate threat to his life?

## 보기

1. The phosphate level
2. The calcium level
3. The potassium level **✔ 정답**
4. The uric acid level

**정답: 3**

## 해설

High potassium, phosphate, and uric acid with low calcium after chemotherapy for a bulky lymphoma is the pattern of tumor lysis syndrome. Of these, hyperkalemia can cause fatal dysrhythmias within minutes to hours, so it is the most immediate threat. His hypocalcemia causes no symptoms, and the high uric acid and phosphate threaten the kidneys more gradually while urine output is still adequate.

## 심화 해설

Clinical situation A 45-year-old man with bulky diffuse large B-cell lymphoma (DLBCL) is admitted for his first cycle of R-CHOP (rituximab, cyclophosphamide, doxorubicin, vincristine, prednisone). Thirty-six hours after chemotherapy, laboratory results show potassium 6.4 mEq/L, phosphate 6.9 mg/dL, calcium 7.9 mg/dL, and uric acid 11.8 mg/dL. He has no neuromuscular symptoms and urine output is 45 mL/h.

Core interpretation This combination of hyperkalemia, hyperphosphatemia, hyperuricemia, and hypocalcemia after cytotoxic chemotherapy for a bulky, high-grade lymphoma is the classic metabolic signature of tumor lysis syndrome (TLS). TLS occurs when massive tumor cell breakdown releases intracellular contents into the bloodstream faster than the body can clear them [1][2][3][4]. In bulky DLBCL, the large tumor burden and high proliferative rate make spontaneous or chemotherapy-induced cell lysis particularly dangerous within the first 24–72 hours after treatment begins.

Why potassium is the most immediate threat Among the four abnormalities, hyperkalemia carries the highest short-term mortality risk because elevated serum potassium directly destabilizes the resting membrane potential of cardiac myocytes. As extracellular potassium rises, the resting membrane potential becomes less negative, bringing cardiac cells closer to threshold. This initially increases excitability but then paradoxically depresses conduction, leading to peaked T waves, widened QRS complexes, and ultimately ventricular fibrillation, asystole, or sudden cardiac death within minutes to hours [1][2][3]. The patient’s potassium of 6.4 mEq/L is already above the threshold where fatal dysrhythmias become a realistic concern, even in the absence of current symptoms. The expert TLS panel consensus explicitly identifies hyperkalemia as a driver of cardiac arrhythmias and potentially sudden death [1]. Watch out! A patient with TLS-related hyperkalemia can appear hemodynamically stable one moment and arrest the next; the absence of ECG changes or symptoms does not reduce the urgency of treatment.

Why the other abnormalities are less immediately lethal Hyperphosphatemia and hyperuricemia primarily threaten the kidneys through intratubular precipitation of calcium phosphate and uric acid crystals, which can lead to acute kidney injury [3][4]. However, this process evolves over hours to days, and the patient’s urine output of 45 mL/h indicates that renal perfusion and tubular flow are still adequate at this point. Hypocalcemia in TLS is usually secondary to hyperphosphatemia—phosphate binds calcium and precipitates in tissues—and can cause neuromuscular irritability, tetany, seizures, or QT prolongation [1][4]. This patient has no numbness, tingling, or muscle twitching, so the hypocalcemia is currently asymptomatic and less immediately dangerous than the potassium level.

| Abnormality | Primary danger | Time course | Why not the top priority here |
| --- | --- | --- | --- |
| Hyperkalemia (6.4 mEq/L) | Ventricular dysrhythmias, cardiac arrest | Minutes to hours | This is the most immediate threat |
| Hyperuricemia (11.8 mg/dL) | Uric acid crystal nephropathy, AKI | Hours to days | Urine output still adequate |
| Hyperphosphatemia (6.9 mg/dL) | Calcium phosphate precipitation, AKI | Hours to days | Renal injury is gradual |
| Hypocalcemia (7.9 mg/dL) | Tetany, seizures, QT prolongation | Variable | No neuromuscular symptoms present |

Clinical reasoning for the licensure exam When a question presents multiple simultaneous electrolyte derangements after chemotherapy, the correct answer is almost always the one that can kill the patient fastest. Hyperkalemia is the only abnormality in this TLS panel that can cause death within minutes through a primary cardiac mechanism [1][2][3]. Hyperuricemia and hyperphosphatemia are serious but act through renal injury, which is a slower process and is still compensated here by adequate urine output. Hypocalcemia is dangerous only when it produces neuromuscular or cardiac manifestations, which are absent. Key point! In TLS, the order of immediate threat is potassium first, then uric acid and phosphate (renal), then calcium only if symptomatic. The management priority mirrors this: cardiac monitoring and urgent potassium-lowering measures precede aggressive hydration, rasburicase for uric acid, and phosphate binders [1][4].References (research sources)

- [1]Recommendations for the evaluation of risk and prophylaxis of tumour lysis syndrome (TLS) in adults and children with malignant diseases: an expert TLS panel consensus.GuidelineCairo MS, Coiffier B, Reiter A, Younes A, TLS Expert Panel (2010) · DOI: 10.1111/j.1365-2141.2010.08143.x

- [2]Managing tumor lysis syndrome.Research articleGreguska C (2021) · DOI: 10.1097/01.JAA.0000723908.23506.85

- [3]Tumor Lysis Syndrome.Research articleBarbar T, Jaffer Sathick I (2021) · DOI: 10.1053/j.ackd.2021.09.007

- [4]Tumour lysis syndrome.Research articleHoward SC, Avagyan A, Workeneh B, Pui CH (2024) · DOI: 10.1038/s41572-024-00542-w

## 임상 시나리오

Tumor Lysis Syndrome: Immediate Threat IdentificationPrioritizing hyperkalemia in post-chemotherapy metabolic derangements
In tumor lysis syndrome, hyperkalemia is the most immediate life threat because elevated potassium directly destabilizes cardiac myocyte resting membrane potential, leading to fatal dysrhythmias within minutes to hours.

The classic TLS pattern after treating bulky high-grade lymphoma includes hyperkalemia, hyperphosphatemia, hyperuricemia, and hypocalcemia, typically peaking 24–72 hours after chemotherapy initiation.

Hyperphosphatemia and hyperuricemia primarily threaten renal function through calcium phosphate deposition and uric acid nephropathy, but these develop more gradually while urine output remains adequate at 45 mL/h.

CautionAsymptomatic hypocalcemia in TLS does not require urgent correction, but hyperkalemia demands immediate ECG evaluation and cardiac monitoring even before laboratory confirmation of severity.

## 핵심 개념

- **Tumor Lysis Syndrome** — Metabolic derangement from massive tumor cell breakdown releasing intracellular contents, characterized by hyperkalemia, hyperphosphatemia, hyperuricemia, and hypocalcemia
- **Hyperkalemia** — Elevated serum potassium that destabilizes cardiac resting membrane potential, causing peaked T waves, widened QRS, and potentially fatal ventricular dysrhythmias
- **R-CHOP** — Chemotherapy regimen for DLBCL consisting of rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone
- **Diffuse Large B-Cell Lymphoma** — High-grade aggressive non-Hodgkin lymphoma with high proliferative rate and large tumor burden, placing patients at high risk for TLS
- **Cardiac Dysrhythmia** — Abnormal heart rhythm caused by electrolyte disturbances, with hyperkalemia being the most immediately life-threatening trigger

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