Review Question - QID 220677

This patient who presents with lightheadedness in the setting of decreased oral intake and diuretic use most likely has hypovolemia. This would cause a prerenal source of kidney injury that is consistent with a BUN:Cr ratio of > 20:1.

Causes of acute kidney injury can be divided into prerenal, intrarenal, and postrenal etiologies. Prerenal etiologies occur when perfusion to the kidney is impaired, leading to a decreased glomerular filtration rate. Since the kidney reacts appropriately in these cases, prerenal patients will have a highly concentrated urine due to vasopressin activity, a low excreted sodium due to aldosterone activity, and a high BUN/creatinine ratio because of increased urea reabsorption. Intrarenal etiologies occur when the renal tubules or ducts are damaged such that proper filtrate processing does not occur. Finally, postrenal etiologies occur when the urinary collecting system is obstructed, causing a build-up of pressure proximal to the obstruction. The important tests that will distinguish between these causes are urine osmolality, urine sodium, serum BUN/creatinine ratio, and the fraction of excreted sodium.

Moore et al. review the evidence regarding the diagnosis and treatment of acute kidney injury. They discuss how this disease is associated with poor outcomes, including chronic kidney failure. They recommend understanding how to diagnose the cause of the kidney injury in these patients.

Figure/Illustration A is a clinical photograph demonstrating urine that has a dark color (red circle). This indicates that the patient has highly concentrated urine, which is consistent with prerenal azotemia.

Incorrect Answers:
Answer 1: Detectable urine hemoglobin would be seen in patients with hematuria. These patients would present with dark urine as well; however, patients typically present with hypertension rather than hypotension in a nephritic syndrome. In addition, patients will not have other signs of volume depletion such as lightheadedness or orthostasis.

Answers 2, 4, & 5: Urine osmolality < 350 mOsm/kg, urine sodium > 40 mEq/L, and a fraction of excreted sodium > 2% would all be seen in patients with intrarenal acute kidney injury. Intrarenal etiologies occur when the renal tubules or ducts are damaged such that proper filtrate processing does not occur. This means that the urine cannot be concentrated appropriately and sodium is wasted in the urine. Patients with prerenal azotemia have appropriate concentrating abilities but have decreased intravascular volume.

Bullet Summary:
Prerenal acute kidney injury will present with urine osmolality > 500 mOsm/kg, urine sodium < 20 mEq/L and BUN:Cr ratio of > 20.