Differential Diagnosis and Evaluation of the Patient



A careful history for exposure to nephrotoxic drugs and a physical examination with a partic­ular emphasis on the evaluation of the circulating volume are required in any patient with suspected ATN. Routine blood and urine chemistries and a urinalysis should be performed. A sonogram of the kidney should be obtained, and where indi­cated, renal blood flow should be determined. More invasive tests such as angiography and/or renal biopsy are required in special circumstan­ces. ATN is a diagnosis of exclusion. The clinical history and the determination of the functional status of the kidney remain the cornerstone of the differential diagnosis.
Prerenal azotemia is a clinical condition in which there is a rise in the plasma concentration of BUN and creatinine consequent to a reduced or ineffective circulating fluid volume and renal plasma flow. Prerenal azotemia differs from ATN by virtue of the fact that it is reversible when the circulating volume is restored and by the presence of normal tubular function. A patient with pre­renal azotemia will be characterized by an elevated BUN-to-creatinine ratio in the blood (greater than 20:1) if the patient has a source of nitrogen intake, a low concentration of sodium and chloride in the -urine on a spot sample (less than 20 mEq/L), a fractional excretion of sodium of less than 1 per cent, and a urine osmolality higher than that of plasma. These determinations must be obtained prior to the administration of diuretics. The above findings would indicate the normal renal re­sponse to conserve salt and water in the face of a decrease in the real or effective circulating blood volume. The patient may manifest other clinical signs of depletion of the extracellular fluid vol­ume. Central hemodynamic monitoring may be necessary when clinical estimates of the circulat­ing volume are unreliable, such as in the elderly, in patients with heart or liver disease, and in pa­tients receiving vasoactive drugs.

Acute partial bilateral obstruction to urine flow may also be associated with an elevated BUN-to-creatinine ratio in blood, a low urine concentra­tion of sodium, and a low fractional excretion of sodium. Thus, partial obstruction to urine flow may mimic the findings of depletion of the extra­cellular fluid volume. In both extracellular fluid volume depletion and partial obstruction, the ur­inalysis may be nearly normal, a feature differ­entiating it from ATN. With obstruction of longer duration, tubular injury occurs, the excretion of sodium increases, and the urine osmolality de­creases. The sonogram of the kidneys will usually detect the presence of dilatation of the urinary col­lecting system.

The diagnosis of ATN is suggested in a patient with a BUN-to-creatinine ratio of less than 20:1, a urinary excretion of sodium and fractional ex­cretion of sodium that is high (greater than 40 mEq/L and 1 per cent, respectively), and a urine osmolality between 250 and 400 mOsm/kg H20. The urinalysis reveals the presence of protein and granular casts.