Do you recommend obtaining a spot urine or 24-hour urine magnesium measurement when evaluating patients with persistent hypomagnesemia of unknown etiology?  

I try to avoid 24-hour collections for just about everything short of stone evaluations (Litholink). They are hard to do properly and bad information is worse than no information. I would use a FeMg and from UpToDate a FEMg > 3% in the setting of hypoMg and nl renal function usually indicates uri...

I concur with Dr. @Rodby. One caveat/reminder, however: when entering P Mg into the calculation for FE Mg, it must be multiplied by 0.7 as approximately 30% of plasma Mg is bound to albumin and only 70% is freely filtered. Failure to do this may result in a spuriously low FE Mg.

Agree with Dr. @Rodby. I order 6 or 8-hour collections if I want a 24-hour urine, then multiply the result by 4 or 3. There is a slight error due to diurnal variation but you are in the ballpark. I get the result by the next morning. Does it matter whether it’s 6 or 8 grams of proteinuria? Bot...