Lecture 7: Measurement of kidney function Flashcards
What does the GFR depend on?
- age (nephron development finished by 35-36th week of fetal development)
- gender
- size of individual
- size of kidneys
- pregnancy
What happens to the nephrons in premature and low birth weight infants?
They often have lower nephron numbers
How do fetuses excrete?
Via the placenta, so until they are born the nephron number isn’t particularly important
What is the GFR at birth?
20 mL/min
Develop a normal GFR around 18 months
What happens to your GFR after you turn 30 years old?
GFR starts declining
6-7ml/min per decade
Due to loss of functioning nephrons , however we don’t see this right away due to compensatory hypertrophy
What happens to the volume of the medulla as you age?
Because the cortex volume decreases, the volume of the medulla increases to compensate for that
What happens to the GFR in pregnancy?
GFR increases by 50% (130-150 ml/min)
-number of nephrons stay the same
-kidney size increases by 1 cm: increased ECF volume
GFR drops back to normal at around 6 months post-partum
What should a substance be to ensure that their GFR=excretion rate?
- produced at a constant rate
- be freely filtered across the glomerulus
- not be reabsorbed in the nephron
- not be secreted into the nephron
e. g. inulin
What substance do we use to give an estimate of GFR and what are the drawbacks?
Inulin: plant polysaccharide that is ingested into the body
- requires continuous IV to maintain a steady state
- requires catheter and timed urine collections
What marker is used instead of inulin to measure the GFR?
51 Cr-EDTA (exogenous)
- radioactive labelled marker
- cleared exclusively by renal function
- timed injections with blood samples taken 2,3,4 hours afterwards
What is an endogenous substance used to measure GFR?
Creatinine: end product of muscle breakdown
-freely filtered
-not reabsorbed in nephron
-producing at a constant rate is variable
But it is secreted into the nephron, therefore it tends to overestimate the GFR by 10-20%
-used in pregnancy
How is creatinine levels measured?
Serum (blood) creatinine sample
Urine creatinine over 24 hours
What is a normal serum creatinine?
70-150 micromoles/L
What affects creatinine levels in an individual?
- muscle mass
- intake of meat
- renal excretion
What increases serum creatinine levels?
- large muscle bulk
- young
- black
- male
- creatine supplements
- high intake of meat
- certain drugs e.g. trimethoprim
What reduces serum creatinine levels?
- reduced muscle mass
- old
- female
- hispanic/indo-asian
- vegetarian
What is a drawback with using serum creatinine?
Have to consider the patient as a whole
Serum creatinine could be the same for some people, however they could be very different people, making it abnormal in some of those people
-serum creatinine can reflect very different glomerular filtration rate in different individuals
-creatinine varies greatly between different individuals
What happens to serum creatinine when GFR declines?
When GFR is normal, you have normal levels of serum creatinine
When GFR is lower, you have large variations in serum creatinine
How do you estimate the eGFR from serum creatinine?
MDRD
-based on serum creatinine, age, sex, caucasian/black
-standardised to body surface area of patient
CKD-EPI
-uses same variables as above but equation is quite different
-as accurate at MDRD when eGFR<60 ml/min, but more accurate when eGFR>60 ml/min
In who is MDRD eGFR inaccurate?
- people without kidney disease
- children
- pregnancy
- old age
- other ethnicities
- amputees
Why is eGFR inaccurate in mild kidney disease?
- reduced GFR, causing an increase in blood flow to increase GFR
- reduced nephron number leading to hypertrophy so no change in GFR
- reduced filtration of creatinine results in increased serum creatinine, so therefore there is an increased secretion into the tubule
