Urine Sodium and Fractional excretion of Sodium (FENa)

Urine Sodium and FENa:

Urine sodium is the concentration of sodium in the urine. Remember that the normal value for blood sodium is 135-145 mmol/litre. Note that the range of sodium in urine is far greater than the range in the blood.

The reference range varies according to age and sex. If also depends upon the hydration state of the person. 

  40-220 mmol/L (For male)
27-287 mmol/L (For female)

For children aged 6-10 Year:
41-115 (For male)
20-69 (For female)

Volume status and Urine Sodium:

• When you are dehydrated, your kidneys will conserve water. However, there is no “water pump” that enables them to perform this function. Therefore, they must osmotically draw the water back into the body. This is accomplished through pumping sodium back into the body via reabsorption in the renal tubules.
• As a result of reabsorbing sodium, the urine sodium is low.
• In states of pathological dehydration, the urine sodium would be expected to be <20 mmol/litre. It can be tempting to think that when you conserve water, your urine will contain little water and therefore a high concentration of sodium. This is not true. However, it will contain a high concentration of just about everything else and thus be “concentrated”.

ATN and Urine Sodium:

• When acute tubular necrosis (ATN) is present, the tubules are damaged. Therefore, they are incapable of actively reabsorbing sodium. Therefore, higher levels of sodium are seen in the urine: >40 mmol/litre.

Fractional Excretion of Sodium ( FENa ): 

The fractional excretion of sodium is a comparison between the amounts of sodium and creatinine in the blood compared to the urine. The equation requires all 4 of these values and the FENa generally has a value between less than 1% to greater than 3%. The importance of the FENa is that it provides information to help determine if situations of poor renal function and low urine output are due to volume and dehydration factors (pre-renal) or rather, due to kidney damage (acute tubular necrosis - ATN). The equation is essentially your sodium:creatinine ratio in your urine compared to your sodium:creatinine ratio in your plasma:

UNa/UCr
FENa = ----------------
PNa/PCr


In other words, it is a comparison of the sodium:creatine ratio in the urine to that of the blood. If you have a small amount of sodium in your urine then the ratio will be < 1. If you have a lot of sodium in your urine, the ratio will be > 2.

Remember what dehydration does to urine sodium? It makes it low. Therefore since the UNa factor is at the very top of the fraction equation, the  FENa  will be low if you are dehydrated. Conversely, with ATN, you will “waste” sodium and thus have a high  FENa .

Fundamentals first:

Your kidneys handle electrolytes (and other molecules) in the following way:

• the plasma is filtered (and becomes ultrafiltrate) through the glomerulus
• the tubules can then do one of three things to the electrolytes in the ultrafiltrate:

1. nothing
2. secrete more electrolyte into the ultrafiltrate
3. reabsorb electrolyte from the ultrafiltrate

• if your tubules do nothing, then the concentration of the electrolyte will remain the same as it was in the plasma. If secretion occurs, then the concentration will increase. If reabsorption occurs, then the concentration will decrease.

Logic:
If your kidneys are working well and you are severely dehydrated, then you would expect your kidneys will work very hard to conserve water; therefore:

• Your kidneys will filter less ultrafiltrate. Since you are not filtering much, your blood creatinine will rise.  
• Your kidneys will reabsorb (via the tubules) as much sodium as they can so that water will follow and your urine will have low volumes. However, your kidneys will never reabsorb creatinine.
• in this setting, you would predict that your urine will have a lower sodium compared with creatinine and in fact, a FENa of < 1 indicates pre-renal causes of renal dysfunction.

Note: a point of confusion is that during dehydration, it is tempting to conclude that you will have concentrated urine and therefore a high urine sodium. While it is true that the urine will be concentrated, it is not true that the sodium will be high. Remember that sodium is the electrolyte that is used to reabsorb the water back into the body. Therefore the concentration of sodium (but only sodium) will be low. The overall urine osmolality will be high due to all other solutes.

Advanced: 
other conditions associated with a FENa < 1% include those situations in which there is intense intrarenal vasoconstriction or vascular inflammation (within the kidney):

• Intrarenal vasoconstriction:

1. liver disease
2. CHF
3. high dose norepinephrine infusion
4. NSAIDs
5. sepsis
6. contrast dye

• Vascular Inflammation:

1. acute glomerulonephritis
2. acute vasculitis 

You can interpret the findings as follows:

Prerenal Intrinsic Renal Postrenal
UNa (mmol/L) 20 >40 >40
FENa <1% >1% >4%

(Source: 1. Jason Waechter @2007 www.teachingmedicine.com
2. http://www.mdcalc.com/fractional-excretion-of-sodium-fena/)