In other sites than the kidneys, however, where clearance is made by membrane transport proteins rather than filtration, extensive plasma protein binding may increase clearance by keeping concentration of free substance fairly constant throughout the capillary bed, inhibiting a decrease in clearance caused by decreased concentration of free substance through the capillary. Equation "1" is derived from a mass balance:  where:   In words, the above equation states:  Since  and  Equation A1 can be rewritten as:  If one lumps the "in" and "gen." terms together, i.e. formula_16 and divides by formula_5 the result is a difference equation:  If one applies the limit formula_19 one obtains a differential equation:  Using the Product Rule this can be rewritten as:  If one assumes that the volume change is not significant, i.e. formula_22, the result is Equation "1":  The general solution of the above differential equation ("1") is:  formula_24  Where:  The solution to the above differential equation ("9") at time infinity (steady state) is:  formula_25  The above equation ("10a") can be rewritten as:  formula_26  The above equation ("10b") makes clear the relationship between mass removal and "clearance". It states that (with a constant mass generation) the concentration and clearance vary inversely with one another. If applied to creatinine (i.e. creatinine clearance), it follows from the equation that if the serum creatinine doubles the clearance halves and that if the serum creatinine quadruples the clearance is quartered. Renal clearance can be measured with a timed collection of urine and an analysis of its composition with the aid of the following equation (which follows directly from the derivation of ("10b")):  formula_27  Where:  When the substance "C" is creatinine, an endogenous chemical that is excreted only by filtration, the clearance is an approximation of the glomerular filtration rate. Inulin clearance is less commonly used to precisely determine glomerular filtration rate. Note - the above equation ("11") is valid "only" for the steady-state condition. If the substance being cleared is "not" at a constant plasma concentration (i.e. "not" at steady-state) "K" must be obtained from the (full) solution of the differential equation ("9").