Normal Serum Potassium Levels in Children
The normal serum potassium range in children is 3.5–5.0 mEq/L (or 3.5–5.0 mmol/L), which is essentially the same as the adult reference interval. This range applies to children from infancy through adolescence, with only minor variations in the neonatal period 1.
Age-Specific Considerations
Neonates and Infants
- Newborns and young infants may have slightly higher potassium levels in the first days of life, but these rapidly normalize to the standard pediatric range of 3.5–5.0 mEq/L 1.
- Breast milk contains approximately 546 mg/L (14 mmol/L) of potassium, while standard commercial infant formulas contain 700–740 mg/L (18–19 mmol/L) 2.
Children and Adolescents
- The reference interval remains consistent at 3.5–5.0 mEq/L throughout childhood and adolescence 1.
- Children on peritoneal dialysis are at increased risk of hypokalemia due to greater potassium filtration, and 90% of pediatric dialysis patients under age 8 are on peritoneal dialysis 3.
Important Clinical Context
Specimen Type Matters
- Plasma potassium is the preferred specimen type because serum potassium exhibits a positive shift of approximately 0.35 ± 0.308 mmol/L relative to plasma due to variable potassium release during clotting 4.
- The serum reference interval (3.5–5.1 mmol/L) is slightly higher than the plasma reference interval (3.4–4.5 mmol/L) to account for this clotting-related release 4.
- Serum potassium should be considered an inferior substitute marker for plasma potassium, with sensitivity for detecting true hypokalemia being only 45.7% and for hyperkalemia only 56.6% 4.
Special Populations
- Children with familial hyperkalemia and hypertension (FHHt) may have serum potassium levels of 5.98 ± 0.42 mmol/L, which is approximately 0.5 mmol/L higher than affected adults 5.
- In FHHt children, serum potassium ≥6.0 mmol/L occurred in nearly half of samples, representing a pathologic elevation rather than a normal variant 5.
Clinical Pitfalls to Avoid
- Always verify potassium levels with a repeat sample to rule out spurious results from hemolysis during phlebotomy, which can falsely elevate serum potassium 2.
- Do not assume that serum potassium accurately reflects plasma potassium in individual patients, as the variable clotting-related shift can cause misclassification of hypokalemia or hyperkalemia 4.
- Recognize that children on peritoneal dialysis require careful monitoring and may need dietary potassium supplementation to maintain normal levels 3.