8. Measurement of kidney function Flashcards
What does GFR depend on?
- Age
- Gender
- Size of individual
- Size of kidneys
- Pregnancy
Why is low GFR in babies not a concern?
- Nephron development finished by 35th-36th week of fetal development
- Premature and LBW infants often have lower nephron numbers
- Fetal excretion predominantly via placenta
- At birth GFR – 20ml/min/1.73m2
- Normal GFR by ~ 18 months
Why does GFR start declining after 30 years of age?
- Loss of functioning nephrons
* Some compensatory hypertophy
How does pregnancy affect GFr?
- GFR increases (~ 50%)
- 130 – 180 ml / minute
- Kidney size increases (~ 1 cm)
- Increased fluid volume (vascular & interstitial)
- Nephron number the same
- Back to pre-pregnancy levels ~ 6 months post-partum
What does a decrease in GFR in an individual suggest?
kidney function has worsened
• Decline in number of nephrons
• Decline of GFR within individual nephrons
What does a increase in GFR in an individual suggest?
kidney function has recovered
In decline of kidney function, why is a decrease in GDR not seen initially?
When kidney function declines slowly, individual nephrons may hypertrophy so actual GFR may not fall until significant kidney damage has occurred
Conditions for a substance to be used to measure GFR?
- Be produced at a constant rate
- Be freely filtered across the glomerulus
- Not be reabsorbed in the nephron
- Not be secreted into the nephron
Why is inulin not used to measure GFR?
- It requires continuous IVI to maintain steady state
* Requires catheter and timed urine collections
What is 51 CR-EDTA?
- Radio-active labelled marker
- Cleared exclusively by renal filtration
- Timed injection with blood samples taken 2,3,4 hours afterward
used to measure GFR
When is 51 CR-EDTA used to measure GFR?
- In children
* Where indication of renal function required e.g. kidney transplant or work up to donate kidney for transplant
What is creatinine?
- Endogenous substance
* End product of muscle breakdown
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 (more with more severe renal impairment)
When is creatinine used to measure GFR?
pregnancy
What factors can increase serum creatinine levels in an individual?
- Large muscle bulk (Muscle cell breakdown)
- Young
- Male
- Black
- Creatine supplements
- High intake of meat
- Certain drugs e.g. trimethoprim
What factors can decrease serum creatinine levels in an individual?
- Reduced muscle mass
- (Muscle cell breakdown)
- Old
- Female
- Hispanic / Indo-Asian
- Vegetarian
Is there variation in GFr between individuals?
yes
- Serum creatinine (sCr) stable in an individual (in steady state)
- But serum creatinine can reflect very different glomerular filtration rates in different individuals
What is • MDRD eGFR?
• estimated GFR Standardised to body surface area of
1.73 m2 therefore don’t need patient height and weight
What factors affect eGFR?
- Serum creatinine
- Age
- Sex
- Caucasian or Black
- Standardised to body sur
When is eGFR inaccurate?
- People without kidney disease (e.g. transplant donors)
- Children
- Pregnancy
- Old age
- Other ethnicities
- Amputees / significantly reduced muscle mass
- Patients with higher levels of kidney function (GFR > 60 mls/min)
- When true GFR changes quickly (e.g. AKI)***
Why is eGFR inaccurate in mild kidney
disease?
- Reduction in GFR (e.g. if glomerular surface area reduced) causes increases in blood flow)
- Reduced nephron number leads to nephron hypertrophy so no change in GFR
- Reduced filtration of creatinine (due to reduced GFR) results in increased serum creatinine and increased secretion into the tubule (in order to maintain relatively steady state of serum creatinine)
