What laboratory value change should indicate to a nurse that a patient with renal failure has entered the oliguric stage?

Choice A rationale In the oliguric phase of renal failure, serum calcium levels actually tend to decrease rather than increase. This hypocalcemia occurs because the failing kidneys cannot activate vitamin D, which is necessary for calcium absorption from the gut. Furthermore, the reciprocal rise in serum phosphate levels leads to the binding and precipitation of calcium. Normal total calcium is 8.5 to 10.5 mg/dL. An increase in calcium is not characteristic of the acute metabolic disturbances seen during oliguria. Choice B rationale Blood volume typically increases during the oliguric stage because the kidneys are unable to excrete water and sodium. This leads to fluid volume excess, which can manifest as hypertension, edema, and weight gain. The lack of urine output (typically less than 400 mL per day) means that the fluid consumed remains in the vascular and interstitial spaces. A decrease in blood volume would only occur if there were a separate cause, such as hemorrhage or severe dehydration. Choice C rationale In the oliguric stage of acute tubular necrosis, the urine osmolality usually stays fixed at around 300 mOsm/kg, which is similar to the osmolality of plasma. This happens because the damaged tubules lose their ability to concentrate or dilute the urine. An increase in urine osmolality would suggest that the kidneys are still functioning well enough to concentrate waste products in a small volume of water, which is the opposite of what happens during true renal failure. Choice D rationale The blood urea nitrogen level rises significantly during the oliguric stage because the kidneys are failing to clear urea, a primary metabolic waste product. This state is known as azotemia. As the glomerular filtration rate drops and urine output diminishes to less than 400 mL daily, toxins accumulate rapidly in the bloodstream. The normal BUN range is 7 to 20 mg/dL, and during this stage, values can exceed 100 mg/dL, indicating severe impairment of the renal system's excretory function.