The degree to which the kidney concentrates the glomerular filtrate can be determined by:

• A. Urine creatine
• B. Serum creatinine
• C. Creatinine clearance
• D. Urine to serum osmolality ratio

Answer: (D)

The ability of the kidney to concentrate the glomerular filtrate is best assessed by how concentrated the urine is relative to the blood, which is captured by the urine to serum osmolality ratio. Urine osmolality shows how concentrated the urine is, while serum osmolality reflects the blood’s solute load. When antidiuretic hormone is acting and the kidneys are concentrating urine, urine osmolality rises while serum osmolality remains around a normal range, leading to a higher ratio. If concentrating ability is impaired, the urine stays dilute even as serum osmolality is higher, giving a lower ratio.

Urine creatinine and serum creatinine mainly indicate filtration rate (GFR) rather than concentrating power, and creatinine clearance estimates GFR; they don’t directly measure the kidney’s water-reabsorption work.