Which of the following laboratory findings is most suggestive of acute tubular necrosis?

• A. BUN to creatinine ratio less than 20
• B. Blood urea nitrogen normal
• C. Decreased urine sodium concentration
• D. Urinary white blood cells present

Answer: (A)

Acute tubular necrosis (ATN) is a common cause of acute kidney injury characterized by the injury and loss of function of the renal tubules. In evaluating laboratory findings associated with ATN, the most indicative parameter is the blood urea nitrogen (BUN) to creatinine ratio.

A BUN to creatinine ratio of less than 20:1 typically suggests intrinsic renal pathology, which includes ATN. In contrast, prerenal causes of acute kidney injury often present with a higher BUN to creatinine ratio (generally greater than 20:1) due to increased urea reabsorption during states of reduced renal perfusion. The lower ratio in patients with ATN indicates a direct injury to the renal tubules, leading to a less efficient reabsorption of urea.

Additionally, normal blood urea nitrogen, decreased urine sodium concentration, and the presence of urinary white blood cells can be attributed to other conditions or forms of kidney injury, such as prerenal azotemia or interstitial nephritis, rather than being definitive markers of ATN. Therefore, the finding of a BUN to creatinine ratio less than 20 is the key indicator that points toward acute tubular necrosis.