Utility of fractional excretion of magnesium in diagnosing renal magnesium wasting in pediatric nephrology practice

[Display omitted] •In hypomagnesemic subjects fractional excretion of magnesium (FEMg) greater than 2% had 100% sensitivity and 100% specificity for renal Mg wasting.•In normomagnesemic subjects, age-specific urine Mg/creatinine (Cr) had 100% sensitivity and specificity.•In normomagnesemic subjects with normal eGFR FEMg > 4 % had 93 % sensitivity and 88 % specificity.•In normomagnesemic subjects with reduced eGFR, FEMg > 4 % had 100 % sensitivity and 25 % specificity. Fractional excretion of magnesium (FEMg) is commonly used to diagnose of renal magnesium (Mg) wasting, but it can be affected by serum Mg (SMg) and serum creatinine concentration (SCr). We investigated the sensitivity and specificity of FEMg to diagnose Mg wasting in subgroups with different SMg and eGFR (estimated glomerular filtration rate) in pediatric nephrology practice. One hundred and nineteen patients (59 males and 60 females, median 15 years) seen in our pediatric clinic were investigated for FEMg, SMg, eGFR, and urine Mg-to-creatinine ratio (Mg/Cr). Normal eGFR was defined as ≥ 90 ml/min/1.73 m2 or for infants SCr 4 %, but only 6 had elevated urine Mg/Cr (sensitivity 100 % and specificity 25 %). Overall, hypomagnesemic patients and normomagnesemic patients with elevated urine Mg/Cr were diagnosed with Mg wasting (36/119, 30 %). FEMg has variable sensitivity and specificity depending on SMg and eGFR in the diagnosis of Mg wasting. Mg wasting is not uncommon in pediatric nephrology practice.