A blood creatinine value of 5.0 mg/dL (442.0 μmol/L) is most likely to be found with which of the following blood values?

• A. Osmolality: 292 mOsm/kg
• B. Uric acid: 8 mg/dL
• C. Urea nitrogen: 80 mg/dL
• D. Ammonia: 80 μg/dL

Answer: (C)

When kidney function declines, waste products that are normally cleared from the blood accumulate. Creatinine is a steady marker of glomerular filtration rate (GFR); as GFR falls, serum creatinine rises. Urea nitrogen (BUN) is another waste product cleared by the kidneys, and it often rises in parallel with reduced renal clearance. A creatinine of 5 mg/dL indicates substantial loss of kidney function, so you’d expect the BUN to be markedly elevated as well—80 mg/dL is a plausible value in this context, reflecting impaired clearance of urea.

Other values don’t show as direct a link to this level of renal impairment. Osmolality around the normal range can occur because it’s influenced by multiple solutes, not just waste products; uric acid can be elevated in kidney disease but isn’t as tightly tied to the degree of creatinine elevation in isolation; and ammonia is mainly a concern with liver function or specific metabolic disorders, not a typical companion of isolated renal failure.