Urinalysis, Renal ‘Tubular Function, and Urine Flow Rate



A careful urinalysis should be performed to de­termine the presence or absence of red blood cell casts. The presence of red blood cell casts indi­cates inflammation of the glomerulus. The pres­ence of red blood cells but not red blood cell casts is less diagnostic and may indicate disease in the kidney itself or in the collecting system. White blood cell casts reflect inflammation (but not nec­essarily infection) of the renal parenchyma. Po­lymorphonuclear leukocytes may derive from the kidney and/or the collecting system. The presence of lymphocytes and/or eosinophils may suggest the diagnosis of acute interstitial nephritis. The urinalysis of a patient with obstruction to flow in the collecting system is variable. An important di­agnostic point is that the presence of azotemia in the face of a totally normal appearing urinalysis is highly suggestive of obstruction. The presence of red or white blood cells, however, does not rule out the diagnosis of obstruction. The urine of pa­tients with acute tubular necrosis usually con­tains protein and, on microscopic examination, granular casts. Glucose may be detected in the urine in the absence of hyperglycemia, a finding that reflects the tubular damage. In acute tubular necrosis, the urinalysis is almost always abnormal but nondiagnostic. It is, however, an important test to rule out other etiologies of acute renal in­sufficiency.

An important diagnostic issue is the determi­nation of tubular function. The most widely used and convenient tests are measurements of the con­centration of electrolytes and osmolality of the urine and the calculation of the fractional excre­tion of sodium. To be of optimal clinical value, such measurements must be obtained prior to the administration of diuretics and considered in con­cert with other assessments of the state of hydra­tion of the individual. The utility of these urinary indices is highest in patients with oliguria and azotemia. The urine1 indices are not diagnostic in and of themselves. In general, the acute tubular necrosis is characterized by defects in tubular ab­sorptive function as expressed by the findings of a urine concentration of sodium of greater than 40 mEq/L on a spot sample, a fractional excretion of sodium of greater than 1 per cent, and a urine os­molality that approaches that of the serum.

Prerenal azotemia is characterized by a decrease in the glomerular filtration rate as manifest by a rise in the BUN and creatinine and by normal tu­bular function as evidenced by a low urine con­centration of sodium, a high urine osmolality, and a fractional excretion of sodium of less than 1 per cent. Patients with acute renal insufficiency sec­ondary to acute glomerulonephritis may also have well-preserved renal tubular function early in the course of their disease, as may patients with the acute onset of partial urinary tract obstruction.

The major value of determining the urine con­centration of sodium and the fractional excretion of sodium is in formulating a differential diag­nosis in a patient with decreased renal function. Interpretations of these tests, however, must be made in conjunction with other assessments of the patient. While patients with renal tubular dis­ease frequently have high urine concentrations and fractional excretions of sodium and patients with prerenal disease manifest sodium avidity, there are clinically important exceptions to these generalizations. Certain types of ATN such as ra­diographic dye-induced renal injury may present with all the clinical characteristics of ATN, but with fractional excretions of sodium of less than 1 per cent. Conversely, a patient with depletion of the extracellular fluid volume may have a high urine concentration and fractional excretion of so­dium if diuretics have been administered, during the generation phase of metabolic alkalosis, and when there was pre-existing renal tubular disease with renal salt wasting. In these latter conditions, the urinary excretion of sodium does not reflect the state of hydration of the patient.
Determination of the urine flow rate is not help­ful in formulating a differential diagnosis of the etiology of the acute renal insufficiency but is im­portant in clinical management. Oliguria is con­sidered to be present when the urine volume is less than 500 ml/day. Patients with acute tubular necrosis may be oliguric or polyuria The distinc­tion between these two forms may reflect differ­ences in the etiology and/or the severity of the insult. Acute partial obstruction to urine flow may be associated with a decreased or variable urine output. However, the presence of a normal urine volume does not exclude the diagnosis. Obstruc­tion of modest duration may result in loss of con­centrating ability by the kidney and, as a consequence, the excretion of an apparently normal volume of urine.

The presence of total anuria provides an im­portant diagnostic clue in the evaluation of a pa­tient with acute renal insufficiency and somewhat reduces the differential diagnostic list. Acute ar­terial or venous catastrophes, total urinary tract obstruction, severe cortical necrosis, severe acute glomerulonephritis and, occasionally, severe acute tubular necrosis may present with anuria.