Renal Assessment and Micturition Flashcards
What are 2 clinical situations where ability to measure GFR is particularly useful?
In patients with renal disease - nephron destruction and decreased nephron function
Many drugs - if GFR falls then excretion falls so can cause toxicity
What is the total GFR?
Sum of all filtration by functioning nephrons so progression of disease would be indicated by a reduction
Describe the measurement of GFR
Plasma clearance testes are used to measure renal function - ability of kidney to clear the plasma of various substances
Plasma is what is important not urine, and volume of plasma cleared not quantity of substance
What is plasma clearance equation?
Plasma clearance of X, CX = (UX)V/ (PX)mls/min
UX - urine conc. of X
V - urine flow rate
PX - plasma conc. of X
What is the gold standard measurement of GFR?
Insulin clearance, polyfructose, loading IV dose of insulin
Allow time to equilibrate and then sample simultaneously plasma and urine
Explain insulin clearance
Insulin is freely filtered at glomerulus and neither reabsorbed nor secreted
Not metabolised by kidney and does not interfere with renal function so insulin clearance is a measure of GFR
What is the GFR of 21 year old with UIN = 285mg/dl
V = 1.1mls/min
PIN = 2.5mg/dl?
125mls/min
Which is normal for a man
What happens if substances are filtered and secreted?
Higher clearance than insulin because UX will be higher and PX will be lower
What happens if substances are filtered and reabsorbed?
Lower clearance than insulin as UX will be lower and PX will be higher
What does the magnitude of GFR correlate to?
Surface area
Values in women are 10% lower even after correlation of surface area
GFR declines by 1ml/min/year after 30
What is used in clinical practice instead of insulin?
51 Cr-EDTA - suitable radioactive substance
Takes several hours
Creatine clearance used to estimate GFR
What is GFR equal to compared with plasma creatine levels?
GFR is proportional to 1/plasma creatine conc.
Used to estimate GFR
Is GFR and Pcr a linear relationship?
No because can half GFR before elevation of Pcr
Big caution
What is GFR written as when using creatinine?
eGFR
Formulae using serum creatinine value which takes into account cofounding variables
What are factors affecting serum creatinine?
Muscle mass - athletes vs malnutrition
Dietary intake - creatinine supplements vs vegetarians
Drugs - some can lead to ketoacidosis
What is normal GFR?
Approx. 100mls/min/1.73m2
Can also be written as a percentage of normal function