Fast forward a few years to another retrospective study of 28 children with a mean age of 11.1 plus or minus 5.6 years (median 12.5 years old).⁴ The most common cause of rhabdomyolysis was infection (43 percent), exertional (25 percent), trauma (10.7 percent), unknown (10.7 percent), DKA (3.6 percent), seizures (3.6 percent), and toxin from spider bite (3.6 percent). All the patients were admitted to the hospital and the authors sought to determine what factors were associated with acute renal failure requiring dialysis. CK, IV fluid bolus administration, use of sodium bicarbonate fluids, and electrolytes were among the values recorded. Those who received an IV fluid bolus were less likely to require dialysis, but the number was not statistically significant. Seven of the 28 children required dialysis. The authors did not find any significant difference in admission or peak CK levels among those who did versus did not require dialysis. They did mention, though, that all the patients who required dialysis had a peak CK level greater than 5000 U/L. While the results are interesting, they don’t really address the ED setting, as all the patients were admitted patients with variable causes of rhabdomyolysis. Perhaps a CK of 5,000 U/L is an important number and provides a hopping-off point for discussion.

A separate 2013 retrospective ED study by Chen et al., also identified 5000 U/L as a number with increased probability of developing acute renal failure.⁵ It’s difficult to draw any true conclusions related to children, as only 20 percent of the study participants were children and the overall predominant causes of rhabdomyolysis were trauma (27 percent) and infection (18 percent). There was another inpatient 10-year retrospective study in Taiwan that looked at 172 children with rhabdomyolysis.⁶ The mean age was 7.3 years old. The most common cause of rhabdomyolysis was viral myositis (72 percent). Although limited by the non-ED setting, the authors identified some factors associated with acute renal failure. They include: an elevated serum CK, elevated serum AST, and an elevated serum myoglobin. The serum CK in those with and without acute renal failure was 20,780 versus 7,124 IU/L, respectively, so a higher number seemed to be associated with renal failure, but there was no magic number.

A more recent study from 2021 by Kuok et al., retrospectively identified 54 children with a median age of 7.8 years old who were admitted for rhabdomyolysis.⁷ Again, it’s an inpatient study. Most of the rhabdomyolysis was caused by viral myositis (72 percent), though, which is good for looking at our question of interest. The peak CK in children who did versus did not develop AKI was 23,086 versus 3,960 IU/L, respectively. The peak values were recorded over the first 72 hours of admission. Specifically on admission, though, the CK level for the two groups was 2,727 and 3,014, respectively—so it was not significantly different. An AKI developed in 18.5 percent of the children and just two children warranted dialysis. In general, those that developed an AKI seemed to be more dehydrated with obvious signs of rhabdomyolysis on the urine dipstick test and the presence of metabolic acidosis. Another 10-year, retrospective 23-hospital multicenter study in South Korea found similar results. In that study of 880 inpatient admitted children with rhabdomyolysis, there was no significant difference in admission CK levels in children who did and did not develop an AKI—3,161 versus 1,986 U/L—respectively.⁸ It was more about the overall clinical picture of dehydration, associated elevation in AST, urinary abnormalities, and the presence of an AKI at presentation. It appears that a specific screening CK level is less important than the overall clinical picture of the child. Still no magic number.