The fractional excretion of sodium (FENa) is a test that examines the functioning of the tubules in your kidney. The estimated glomerular filtration rate (eGFR) examines the functioning of glomeruli in your kidney. The FENa should normally be less than 1% to indicate normal functioning of the tubules. When the kidney is injured, the tubules are responsible for sorting out the good stuff that your body needs from the bad stuff that must be eliminated into the urine. When the FENa is increase, as for example the 3% that you mention, then the eGFR declines as a result of the tubular injury. The FENa is calculated by examining the creatinine and sodium in your urine and comparing it to the sodium and the creatinine that is in your blood. As more sodium is leaked into the urine and less creatinine is eliminated into the urine, this makes the FENa increase.