A prospective study indicating that fractional excretion of sodium is a good marker for fluid loss

Abstract

INTRODUCTIONPre-renal failure, a reversible form of acute renal failure (ARF), accounts for 60-70% of all cases of ARF.1,2 The early diagnosis and management of pre-renal failure assists in preventing ARF. Sodium is the key component of the extracellular fluid volume. In a patient following sodium-restricted diet (40 to 50 mEq/d), urinary sodium level decreases to <10 mEq/L within 3-5 days. If the person was earlier on normal sodium diet (150 to 200 mEq/d), the resultant decrease in sodium is sensed by the kidney and thereby tries to conserve sodium.3 These modest changes in total body sodium and thus in extracellular fluid volume would not be reflected on physical examination and in the assessment of sodium in serum. Fractional excretion of sodium (FENa), a measure of the percentage of the sodium filtered by the kidney, is one of the helpful tools for assessing the same. It is also used to distinguish pre-renal failure from acute tubular necrosis (ATN). 4 A FENa <1% indicates pre-renal azotemia, and >1% indicates ATN.3 Since FENa works on the principle that sodium reabsorption is enhanced with volume depletion; use of diuretics that decrease the sodium reabsorption can elevate the level of FENa, thereby producing misleading values. 4 In addition, inaccurate results of FENa has been reported in patients with metabolic alkalosis. 5Fractional excretion of urea nitrogen (FEUN), another diagnostic tool used for differentiating pre-renal failure from acute tubular necrosis, is less influenced by diuretic therapy. 1,4 There are studies showing improved sensitivity, specificity, and overall accuracy of FEUN in differentiating pre-renal failure from acute tubular necrosis than FENa. Moreover, studies indicate good correlation between FEUN and FENa, and a weak ABSTRACT Background: Pre-renal failure, a reversible form of acute renal failure (ARF), accounts for 60-70% of all cases of ARF. To study the factors affecting fractional excretion of sodium (FENa) in patients with pre-renal failure. Methods: The study involved patients with pre-renal failure, admitted in a multi-speciality hospital in south India for a period of two years. The demographic and clinical data were collected using a standard pro forma. The correlation between FENa and factors such as age, diabetes mellitus, fluid loss, fever, urine output and creatinine of pre-renal failure were statistically evaluated. Results: The prospective study involved 24 patients diagnosed as pre-renal failure with a mean age of 52.75±18.78. The subjects included 14 males and 10 females, with a median FENa of 0.55 (0.10-0.90). A moderate negative correlation was observed between FENa and fluid loss in pre-renal failure patients (r -0.646, P=0.0007). Conclusions:The level of FENa may assist in estimating fluid loss in patients with pre-renal failure.