Creatinine clearance tests are utilized to estimate the glomerular function of the kidney. The creatitine clearance calculation is defined as the volume of plasma that is cleared of creatinine by the kidney per unit of time and uses the following formula:

creatinine clearance = Urine creatinine conc x Volume / Plamsa creatinine conc

Creatinine clearance measurements have a recommended specimen requirement = 24-hour urine collection.

Which of the following tests would be useful in the assessment of glomerular filtration: