The most specific marker of renal function is-
Creatinine is a waste product that is produced continuously during normal muscle breakdown. ... Creatinine clearance in a healthy young person is about 95 milliliters per minute for women/120 milliliters per minute for men. This means that each minute, that person's kidneys clear 95-120 mL of blood free of creatinine. Ref Harrison20th edition pg 293
Creatinine clearance
Insulin clearance
Blood area
Serum creatinine
Kidney