Question

Mention two equations for determination of GFR value.

Answer 1

Measurement of the clearance is predominantly a test of
glomerular filtration rate (GFR). Measurement of glomerular filtration rate (GFR) provides the most useful general index for the assessment of the severity of renal
damage. A decrease in the renal function is due to the loss
of functional nephrons, rather than a decrease in the func-
tion of individual nephron.

Clearance is defined as the volume of blood or plasma completely cleared of a substance per unit time.

If the substance is freely filtered across the capillary wall, and neither secreted nor reabsorbed, then its clearance
is equal to glomerular filtration rate. A substance which
meets these requirements is an ideal filtration marker. If
the substance is also secreted by the tubules, the clearance exceeds GFR. For those which are reabsorbed by tubules, clearance is less than GFR.

Two formulas to estimate GFR -

1. Cockcroft Gault equation (determining creatinine clearance thus measures GFR)

CrCl = (140 - age) X body weight in Kg (X 0.85 in case of females) / (72 X Serum creatinine concentration)

Factor 0.85 is used in females taking consideration that females have 15% less muscle mass than man.

2. Estimated GFR formula used in MDRD -

eGFR = 186 X (Creatinine/88.4)-1.154 X Age-0.203 X (0.742 if female) X (1.210 if black).

eGFR can be used for staging of patients with chronic kidney disease.