Renal Pathophysiology

CHAPTER 8 Urinalysis and Approach to the Patient With Renal Dysfunction

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Quantity of sodium excreted Quantity of sodium fil tered

100

FENa (%)

Sodium excretion is equal to the product of the urine sodium concen tration and the urine flow rate (V), whereas the quantity of sodium filtered is equal to the product of the GFR (estimated from the creatinine clearance; See Chapter 1; Eq. 2) and the plasma sodium concentration:

UNa V PNa UCr V PCr ( / )

FENa %

100

UNa PCr PNa UCr

100

Patients with prerenal disease and a decline in GFR generally have a FENa that is < 1%, indicating that the patient is sodium avid with over 99% of the fil tered sodium being reabsorbed. In comparison, the FENa is generally > 2% when tubular reabsorption is impaired in acute tubular necrosis. The overlap is much less than that seen with the urine sodium concentration alone because the lat ter is also influenced by the rate of water reabsorption (see Chapter 11; Table 2). A patient with AKI has a plasma creatinine concentration that is continuously rising due to the fall in GFR and is now 3.2 mg/dL. The following additional values are obtained: Urine sodium concentration is 35 mEq/L, plasma sodium concentration is 140 mEq/L, and the urine creatinine concentration is 160 mEq/L. Calculate the FENa. There is, however, an important potential problem with using the FENa in patients with normal GFR. Both the FENa and the urine sodium concentra tion are generally obtained in an effort to determine if a patient is effectively volume depleted. A urine sodium concentration < 25 mEq/L is usually indic ative of hypovolemia at any level of renal function; as noted earlier, however, somewhat higher values do not exclude this diagnosis because there may also be a high rate of water reabsorption. In comparison, there is no absolute value for the FENa in volume depletion because this parameter is greatly in fluenced by the filtered sodium load, which in turn is dependent on the GFR. This principle is illustrated in the following example. A patient with hyponatremia and normal renal function is evaluated. The patient is taking no medications. The urine sodium concentration is 67 mEq/L, and the urine volume is ∼ 1,500 mL on the first day. The plasma sodium concentration is 120 mEq/L, the plasma creatinine concentration is 1.0 mg/dL, and the urine creatinine concentration is 67 mg/dL. Calculate the FENa. From the urinary findings, is the patient volume depleted or normovolemic? 4 5

CHAPTER 8 Urinalysis and Approach to the Patient With Renal Dysfunction

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