Urine sodium levels are sometimes used to help explain abnormal blood sodium results (natremia) and to assist in the investigation and treatment of certain kidney problems. Urine sodium levels are expressed in millimoles of sodium per litre of urine (mmol/L) on a single urine specimen and in millimoles per day (mmol/d) on a 24-hour urine collection. Urine sodium levels must be interpreted in parallel with blood sodium levels.
The body normally eliminates excess sodium. A high urine sodium level accompanied by normal or high blood levels may therefore reflect too much dietary intake. A high level of urine sodium accompanied by a low normal or very low blood sodium level indicates that the body is probably losing too much sodium. When both blood and urine sodium levels are low, it may mean there is insufficient dietary intake of sodium.
Low urine sodium levels can be caused by dehydration (lack of water in the body), heart failure, liver disease or kidney disease (nephrotic syndrome). High urine sodium levels may indicate the use of a diuretic medication or adrenal insufficiency (Addison’s disease).