McMahon Score for Rhabdomyolysis

Why use it

Rhabdomyolysis-induced AKI requiring dialysis occurs in approximately 10–15% of hospitalized patients with rhabdomyolysis and carries significant mortality. Creatine kinase alone is a poor predictor of renal outcomes — many patients with CK >10,000 U/L never develop AKI, while some with lower CK values do. The McMahon Score integrates multiple risk factors to provide a more accurate individual risk estimate, enabling targeted intensity of monitoring, fluid resuscitation, nephrology consultation, and ICU admission.

About

The McMahon Score is a validated clinical prediction tool developed by McMahon and colleagues and published in the American Journal of Kidney Diseases in 2013. It was derived from a retrospective cohort of 2,371 patients with rhabdomyolysis admitted to a large US academic medical center. The score uses 7 weighted clinical and laboratory variables available at the time of initial evaluation to predict a composite outcome of death or acute kidney injury (AKI) requiring renal replacement therapy (RRT/dialysis) during hospitalization. Variables include initial creatine kinase (CK) level, age, cause of rhabdomyolysis, initial phosphate and calcium levels, initial creatinine, and presence of dehydration. The score stratified patients into low risk (score <6) and high risk (score ≥6), with a c-statistic of 0.85. The tool is particularly useful for resource allocation and early identification of patients who benefit from aggressive IV fluid resuscitation and nephrology consultation.

When to use

• Any patient diagnosed with rhabdomyolysis (elevated CK with or without myoglobinuria, elevated creatinine)
• At initial presentation to determine need for ICU admission vs. ward monitoring
• Identifying patients who require aggressive IV fluid resuscitation (high-risk McMahon ≥6)
• Guiding frequency of monitoring labs (CK, creatinine, electrolytes)
• Informing nephrology consultation decisions
• Stratifying risk in clinical trials and quality improvement initiatives for rhabdomyolysis management

Formula

Pearls & pitfalls

• The McMahon Score cut-off of ≥6 is associated with approximately 46% risk of death or dialysis-requiring AKI, compared to ~2% for scores <6.
• CK alone is an unreliable predictor of AKI — the McMahon Score integrates initial CK as one of 7 factors, providing much better discrimination (AUC 0.85 vs 0.73 for CK alone).
• Creatinine elevation at presentation is a strong predictor — even mild creatinine elevation (1.4–1.9 mg/dL) at admission adds significant risk.
• Dehydration (clinical or laboratory) is a modifiable risk factor — early and aggressive IV fluid resuscitation is the cornerstone of treatment regardless of score.
• Hyperphosphatemia and hypocalcemia reflect the degree of muscle breakdown and are independently associated with worse outcomes — monitor and correct carefully.
• The most common causes of rhabdomyolysis are: trauma/crush injury, vigorous exercise, alcohol and drug intoxication, medications (statins, antipsychotics), seizures, infections, and immobility.
• Target urine output in high-risk rhabdomyolysis is 200–300 mL/h (3 mL/kg/h) — may require 6–12 L/day of IV fluids. Monitor for fluid overload.
• Urine alkalinization (IV bicarbonate to target urine pH >6.5) remains controversial — not consistently shown to reduce AKI incidence but commonly used in severe myoglobinuria.
• Hyperkalemia is a life-threatening complication — monitor potassium every 4–6 hours in high-risk patients. Patients with established AKI and hyperkalemia may require emergency dialysis.
• Hypocalcemia in rhabdomyolysis is typically caused by calcium sequestration in injured muscle — do NOT aggressively correct unless symptomatic (tetany, seizures), as reperfusion may cause hypercalcemia.

Next steps

McMahon Score <6 (Low Risk): AKI/death risk ~2–3%. IV fluid resuscitation. Monitor CK, creatinine, electrolytes, urine output. Ward admission generally appropriate unless other indications for ICU. McMahon Score ≥6 (High Risk): AKI/death risk ~46–58%. Aggressive IV fluid resuscitation (target UO 200–300 mL/h until myoglobinuria resolves). ICU or monitored setting. Nephrology consultation. Electrolyte monitoring and correction (hyperkalemia, hypocalcemia, hyperphosphatemia). Consider Foley catheter for strict UO monitoring.

Evidence

McMahon GM et al. A risk prediction score for kidney failure or mortality in rhabdomyolysis. JAMA Intern Med. 2013;173(19):1821-1828. PMID:23978964. Derivation cohort: 2,371 patients. Validation cohort: 1,186 patients. C-statistic 0.85 (score) vs 0.73 (CK alone). Score ≥6: death/dialysis in 46% vs 2% for score <6. Validated in multiple external cohorts.

References

1. McMahon GM et al. A risk prediction score for kidney failure or mortality in rhabdomyolysis. JAMA Intern Med. 2013;173(19):1821-1828. — link
2. Bosch X et al. Rhabdomyolysis and acute kidney injury. N Engl J Med. 2009;361(1):62-72. — link
3. Chavez LO et al. Beyond muscle destruction: a systematic review of rhabdomyolysis for clinical practice. Crit Care. 2016;20(1):135. — link