Page 91 - Clinical Biochemistry

• GFR declines with age

Glomerular Filtration Rate (GFR)

• Accurate measurement of the GFR by Clearance Tests
• Clearance = UV/P
• U is concentration of substance in urine, V is volume of urine produced by min, and P is
concentration of substance in plasma

Accurate measurement of GRF by clearance tests requires determination of the
concentration in plasma and urine of a substance that is:

• Freely filtered at glomeruli.
• Neither reabsorbed nor secreted by tubules.
• Its concentration in plasma needs to remains constant throughout the period of urine
collection.
• Better if the substance is present endogenously.
• Easily measured.

Creatinine, cystatin as endogenous substances meets most of these criteria, Inulin as
exogenous substance.

Direct measurement of glomerular filtration rate (GFR)

Creatinine clearance

The clearance of creatinine (normal product of muscle metabolism) from plasma is directly
related to GFR.

Is calculated as follows:

Clearance (C) = U (mg/dL) X V (ml)
P (mg/dL) X24 X60

U = urine concentration of creatinine (mg/dl)

V = Volume of urine: urine volume produced by one min (ml/min)

P = plasma concentration of creatinine (mg/dl)

Measurement of creatinine clearance (crcl) through 24hr urine collection