GFR and clearance Flashcards
When would GFR change?
kidney function changes
- decreased number of nephrons- (diseases or nephrotoxic drugs)
- decline of function of each individual nephron (diabetes and hypertension causing scarring and fibrosis)
What happens if kidney function decreases slowly?
-hypertrophy of nephrons may occur so actual GFR may not fall until significant kidney damage has occured
What is GFR?
the amount of filtrate that is produced from the blood flow per unit time
Why is it difficult to measure GFR accurately?
ignoring active reabsorption and active secretion that occurs in the nephrons
What is clearance?
vol of plasma cleared of a substance per unit time OR volume of plasma that the body needs to get rid of substance X (could be through faeces, urine, sweat exhaling)
What is renal clearance?
vol of plasma cleared of a substance per unit time OR volume of plasma that the body needs to get rid of substance X - purely by the KIDNEYS!!!
What is the formula for renal clearance?
excretion rate / plasma conc where excretion rate is the amount of the substance in the urine multiplied by how fast the urine is flowing
What happens if flow rate increases?
renal clearance will increase by the same amount
Why does renal clearance increase with increased flow rate?
if urine is leaving the body at a faster rate, substance X is being removed from the body faster hence increased clearance
What happens if plasma concentration increases?
renal clearance will decrease proportionally
Why does renal clearance decrease with increase plasma concentration?
If there is more of substance X in the plasma then the body is not removing enough substance X that it should
What should the characteristics be of the substance used to measure kidney clearance?
-produced at a constant rate
-be freely filtered across the glomerulus
-not be reabsorbed in the nephron
-not be secreted in the nephron
SOOO if all these are true then excretion rate is exactly the GFR but there is not a molecule like that in the body
What are exogenous substances that can be used to measure GFR?
inulin
51 Cr EDTA
What are endogenous substances that can be used to measure GFR?
creatinine clearance
What is inulin?
a plant polysaccharide
-not produced by body at all as needs to be injested
What are the benefits of inulin in measuring GFR?
-if you are hooked up to an IV supplying inulin at CONSTAMT RATE then the excretion of inulin is equal to your GFR
What are the negative of inulin in mesauring GFR?
- not clinically used as you would need to be hooked to an IV to maintain the constant rate
- also required catheter and timed urine collections
What is 51 Cr EDTA?
- it is a radioactive labelled marker
- cleared exclusively by renal filtration
- can be given in 1 injection shot and monitor using blood samples not urine output
When is 51 Cr EDTA used clinically?
-used in children where indication of renal function is required e/g/ kidney transplant or kidney donation
Why do you need to know GFR before cancer treatment?
- need to know if kidney will work and function well after treatment / removal
- cancer treatments are nephrotoxic
What are the negatives associated with using 51 Cr EDTA?
it is radioactive - patient needs to go to nuclear medicine until no radiation is being radiated from their body
-shoudlnt be used in pregnant people
What is creatinine?
-end product of muscle breakdown
Is creatinine a good measure of GFR?
-yes however some creatinine is still being secreted into urine by the nephron
So how is creatinine clearance measured?
- serum creatinine AND
- urine creatinine over 24 hrs
What are the disadvantages of using creatinine to measure GFR?
- cumbersome (carrying a bottle of urine)
- frequently inaccurate
- overestimates GFR by 10-20% due to creatinine secretion
What are the advantages of using creatinine to measure GFR?
can be used in pregnancy
What affects the serum creatinine?
- high muscle mass
- lots of meat intake
- black male
Highest creatinine
What factors contribute to reduced serum creatinine?
- old age
- women
- hisospanic/indo-asian
- vegetarian
Why do babies have low serum creatinine?
kidneys don’t work in foetus so placenta takes over role - initially VERY high as takes into account mothers creatinine levels BUT when the baby is born, the babies kidneys have to work and remove the creatinine created by mum so there is a DRASTIC fall in serum creatinine
What is eGFR?
ESTIMATED GFR (best guess)
How is eGFR calculated?
blood test - no need for uric output
What is MDRD eGFR?
formula that takes into account; -age -sex -ethnicity -serum creatinine (is standardised to body surface area so don't need height and weight)
What types of people is MDRD eGFR inaccurate in?
- people without kidney disease e.g. donors
- children
- pregnancy
- older
- other ethnicities to black or caucasian
- amputees (weigh reduction)
- patietns with higher levels of kidney function
What is CKD-EPI?
uses the same variable as MDRD but the formula is different - more accurate when eGFR > 60mls/min
Why is eGFR less accurate with mild kidney disease?
- reduction in GFR causes increases in blood flow
- reduced nephron number leads to nephron hypertrophy so no change in GFR
- reduced filtration of creatine due to reduced GFR results in increases serum creatinine and increased secretion into the tubule to maintain steady state of serum creatinine
