On the Differential Diagnosis of Acute Renal Failure

D istinguishing between vasomotor nephropathy (acute tubular necrosis, acute renal failure) and functional renal insufficiency (prerenal renal failure) is perhaps the most common diagnostic exercise encountered in dealing with patients with acute azotemia. In principal, it is not difficult to separate these two entities on the basis of history, physical examination, clinical course and the very different urinary characteristics with which they present. In practice, however, hoth conditions may occur in the same clinical setting and present with comparable physical findings. Thus, differences in urinary characteristics have proved valuable as aids in differential diagnosis (Table I) as long as the subject is not under the influence of diuretics or experiencing an osmotic diuresis. Unfortunately, the urinary features separating functional renal failure from vasomotor nephropathy become blurred in elderly patients and younger patients who have hypertensive and diabetic nephrosclerosis or other chronic parenchymal renal diseases. In my experience, moreover, this same population seems prone to the development of unusually severe renal failure in response to a hemodynamic challenge, their serum creatinine concentration at times reaching preuremic levels if the ‘underlying problem remains unrecognized. In such patients, the Inability to concentrate urine normally despite marked volume depletion is well documented [ 11; their urine:plasma (U/P) creatinine concentration ratio (U/P,,) is apt to be significantly lower than is usual for functional renal failure, and the urinary sodium (Na) concentration may far exceed that considered typical of functional renal failure. Furthermore, elderly patients who have had serious depletion for some time do not necessarily respond promptly to the administration of fluid [2]. Even after full restoration of circulating volume is assured by a normal or high pulmonary capillary wedge pressure, such patients may remain oliguric for up to 24 hours [ 21. After volume has been restored, however, they often will respond promptly to a large dose of a loop diuretic or mannitol, again ‘signaling the functional origin of their sustained renal insufficiency. Thus, although urinary characteristics will most often distinguish patients with functional renal insufficiency from those with vasomotor nephropathy, that happy circumstance does not always prevail. With the existence of a “grey zone” in which the urinary Na concentration, urinary osmolality and U/P,, ratio may not be absolutely diagnostic of either vasomotor nephropathy or functional renal failure, more definitive diagnostic tests have been sought in recent years. Most notable among these has been the renal failure index (RFI) of Handa and Morrin [3] and the fractional excretion of sodium (FE&. In the former, the urinary sodium concentration (UNa) is divided by the simultaneous U/P,, ratio and the quotient is multiplied by 100. Thus, RFI = UNa 4 U/P,, X 100. The FEN, is a minor modification of the renal failure index in which the U/P sodium concentration ratio (U/P&) is substituted for the urinary Na concentration value, so that FEN, = U/P& + U/P,, X 100. With the degree of variation in serum Na concentration generally encountered, the two indexes give results that are little different in their diagnostic significance. A fractional excretion of Na below 1 percent is suggested to rule out the possibility of vasomotor nephropathy [ 451. Values between 1 and 3 percent are considered indeterminate by Miller et al. [ 51 and indicative of vasomotor nephropathy by Espinel and Gregory [ 41. Both groups of researchers have declared the fractional excretion of Na to be superior to classically employed urinary indices as a diagnostic marker. Indeed, in one of these reports [ 41, 11 of the 40 patients diagnosed as having vasomotor nephropathy had a urinary Na concentration of 7 to 20 meq/liter, 16 had U/P,, ratios between 15 and 40, and 13 had urinary osmolalities of 350-500 mOs/kg of HzO. Such values would be considered distinctly unusual by traditional criteria. The fractional excretion of Na of all 40 patients was greater than 1 percent, but 22 values fell between 1 and 3 percent. Of 55 cases of acute oliguric and nonoliguric renal failure attributed to vasomotor nephropathy by Miller et al. [ 51, 11 reportedly had urinary