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

Background: Fractional excretion of magnesium (FE) 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 FE to diagnose Mg wasting in subgroups with different SMg and eGFR (estimated glomerular filtration rate) in pediatric nephrology practice.

Methods: One hundred and nineteen patients (59 males and 60 females, median 15 years) seen in our pediatric clinic were investigated for FE, SMg, eGFR, and urine Mg-to-creatinine ratio (Mg/Cr). Normal eGFR was defined as ≥ 90 ml/min/1.73 m or for infants SCr < chronic kidney disease stage 2. Urine Mg/Cr was compared with age-specific reference values.

Results: Sixteen of all patients (13 %) had hypomagnesemia. All had FE greater than the cut-off value of 2 %. Only 4 patients had elevated urine Mg/Cr. Of 65 patients with normal SMg and eGFR, 19 had FE above the cut-off value of 4 %. Of these, 13 patients had elevated urine Mg/Cr i.e. Mg wasting (sensitivity and specificity of FE, 93 % and 88 %, respectively). Among 38 patients with normal SMg and low eGFR, 30 had FE > 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 %).

Conclusions: FE 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.