Urinary Na concentration is a very useful urinary test that we use in clinical practice to decide the volume status of the patient. It is also very useful in helping decide pre renal vs intrinsic renal disease.
There are several causes of AKI where urine Na concentration and fractional excretion of Na may be initially low, only to increase later. They are listed below
1. Radiocontrast agent induced AKI- due to AngII increase and ischemic damage
2. Sepsis- activation of AngII leads to initial vasoconstriction and low levels of urinary Na
3. NSAIDs- unopposed vasoconstriction from Ang II
4. Rhabdomyolysis- mechanism unclear
5. Acute obstruction( initial phase)- again activation of AngII
6. Acute GN- cytokines that lead to decrease GFR causing lowering of filtered Na load
7. Acute rejection- similar to GN
A recent article in CJASN reviews the use of urinary studies in diagnosis of kidney diseases.