GLOMERULAR FILTRATION RATE (GFR) – MEASUREMENT
GFR – MEASUREMENT
METHODS:
GFR estimation usually done by,
- Clearance tests.
- Radiological methods.
- Tc99 DTPA.
1. CLEARANCE TEST:
- Clearance of certain substance is used as measure for GFR estimation.
Criteria for selection:
- Substances should pass freely through glomerular membrane.
- Should neither reabsorbed/nor secreted by tubular activity.
- Should neither metabolized nor synthesized by kidneys.
- Urinary excretion should represent only the amount filtered.
Tests:
1a) Inulin clearance test:
- Most accurate measure for GFR estimation.
Properties qualifying inulin:
- Small molecular size
- Hence, free passage through glomerular membrane
- Neither secreted/nor synthesized/nor metabolized/ & non-toxic.
- Inulin clearance = 126 ml/min (Equal to GRF)
1b) Other clearance tests:
Creatinine clearance:
- Less accurate method since gets secreted in renal tubules.
- Yet, more convenient to determine creatinine clearance
- Is a metabolic product with constant levels.
- Hence, used as “Index of GFR”.
1c) Difference between Inulin & creatinine clearance methods:
- Inulin requires exogenous administration (IV injection) for testing.
- Creatinine is available endogenously & no external administration required.
- Added advantage for creatinine over inulin.
1d) Others: Clearance of Vitamin B12, EDTA, & Polyfructosan
RADIOLOGICAL METHODS:
1. Tc99 In-labeled DTPA scan
- DTPA (Diethylenetriaminepenta acetic acid)
- No glomerular filtration/nor tubular reabsorption/nor excretion.
- Hence, its excretory rate measures GFR.
Uses of DTPA Scan:
- Useful for evaluation of renal perfusion & functionality of each kidney
- Indicated in Urinary tract obstruction
- Renovascular cause of hypertension
- Renal transplant
- Assess renal cortical function & morphology.
Tc99 DTAP:
- Functional nucleoside agent, assessing renal perfusion, plasma flow, glomerular filtration & tubular secretion
2. Other radiological methods: Chromium 51-labeled-EDTA scan.
Factors influencing measurement:
1. Substance filterability:
Types of substance filtration:
- Free filterability
- Non-filterability
- Partial filterability
a) Free filterability:
- Filterability value = 1
- Means freely filtrable through glomerular membrane barrier.
- Eg., Water, sodium, bicarbonate, inulin, Creatinine
b) Partial filterability:
- Filterability value = 0.75
- Implies only 75% substance is filtered.
- Partially filtered.
- Eg: Myoglobin
c) Substance not filtered:
- Plasma proteins not filtered.
- Eg., Albumin, Hemoglobin (since are bound inside RBC)
- Yet, significant amounts of FREE hemoglobin gets excreted.
2. Interpretation of afferent & efferent arteriole concentration:
- Afferent & efferent arterioles substances concentration evaluation implies glomerular filterability.
Interpretation:
Equal afferent & efferent concentrations:
- Implies substance unfiltered by glomerulus.
Lower efferent arteriole concentration:
- Implies partial filtration by glomerulus.
Completely absent efferent concentration:
- Positive for concentration in afferent arteriole.
- Implies complete filtration by glomerulus.
Exam Important
GFR – MEASUREMENT
GFR estimation usually done by,
1. Clearance test:
Criteria for selection:
- Substances should pass freely through glomerular membrane.
- Should neither reabsorbed/nor secreted by tubular activity.
1a) Inulin clearance test:
Most accurate measure for GFR estimation.
Properties qualifying inulin:
- Small molecular size.
- Hence, free passage through glomerular membrane.
- Neither secreted/nor synthesized/nor metabolized/ & non-toxic.
- Inulin clearance = 126 ml/min (Equal to GRF).
- 1b) Other clearance tests:
Creatinine clearance:
- Less accurate method since gets secreted in renal tubules.
- Yet, more convenient to determine creatinine clearance
- Is a metabolic product with constant levels.
- Hence, used as “Index of GFR”.
2. Radiological methods:
- Tc99 In-labeled DTPA scan
- DTPA (Diethylenetriaminepenta acetic acid)
- Excretory rate measures GFR.
Uses of DTPA Scan:
- Useful for evaluation of renal perfusion & functionality of each kidney
Factors influencing:
1. Substance filterability:
a) Free filterability:
- Filterability value = 1
- Means freely filtrable through glomerular membrane barrier
- Eg., Water, sodium, bicarbonate, inulin, Creatinine
b) Partial filterability:
- Filterability value = 0.75
- Implies only 75% substance is filtered.
- Partially filtered.
- Eg: Myoglobin
c) Substance not filtered:
- Plasma proteins not filtered.
- Eg., Albumin, Hemoglobin (since are bound inside RBC).
- Yet, significant amounts of FREE hemoglobin gets excreted.
2. Interpretation of afferent & efferent arteriole concentration:
Equal afferent & efferent concentrations:
- Implies substance unfiltered by glomerulus.
Lower efferent arteriole concentration:
- Implies partial filtration by glomerulus.
Completely absent efferent concentration:
- Positive for concentration in afferent arteriole.
- Implies complete filtration by glomerulus.
