Fractional excretion of sodium

What the risks are:

The urine sample has no risk.
Risks of venipuncture include:

• excessive bleeding
• fainting or feeling lightheaded
• hematoma (blood accumulating under the skin)
• infection (a slight risk any time the skin is broken)
• multiple punctures to locate veins

Special considerations:

The normal value depends on consumption of fluid and salt. Consequently, the test is only of value in specific circumstances, such as sudden decreased urine output.

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.