Measurement Of Renal Function Flashcards
What are 5 things the GFR of an individual depends on
- Age
- Gender
- Size of person
- Size of kidneys
- Pregnancy
Nephron development usually finishes by week 35/36 of fetal development.
Premature and Low Birthweight babies usually have less nephrons.
What is the usual GFR at birth?
At what age the GFR become normal
- 20ml/min/1.73 metres squared
- 18 months
At what age does GFR start declining? (Loss of functioning nephrons)
At what rate does this happen?
What may accompany this?
- After age of 30 (not usually noticeable till 50)
- Declines by 6/7 ml/min per decade
Compensatory Hypertrophy
How does Pregnancy affect GFR, Kidney Size and Nephron number?
- GFR increases (by 50%) to 130-180 ml/ min (Drops to normal 6 months after birth)
- Kidney size increases (by 1 cm)
- Same nephrons number
What may be happening when kidney function declines slowly?
- Single nephrons may Hypertrophy so actual GFR may not fall until kidney is significant damaged (Makes it harder to treat)
What are 4 ideal properties of a substance that is used to measure kidney clearance?
What can we say if all are true
Suggest a substance
- Produced at constant rate
- Be freely filtered across glomerulus
- Not be secreted
- Not be reabsorped
GFR= Excretion rate
Inulin (A plant polysaccharide)
What are 2 Cons of using Inulin
- Needs continuous IV
- Needs Catheter and timed urine collections
What are 2 alternatives to using Inulin
- Creatinine
- 51 CR-EDTA (Chromium-EDTA)
Describe the use of 51 CR-EDTA
- Radio active labelled marker
- Cleared exclusively by renal filtration
- Timed injection with blood samples after 2,3 and 4 hours
(10% Lower clearance than insulin, may be due to reabsorption)
What are 2 clinical uses of 51 CR-EDTA
- In children
- Where indication of renal function is needed (transplants)
What are 3 pros of using Creatinine?
Name 1 time we use to measure GFR
- Freely filtered across glomerulus
- Not reabsorbed in nephron
- In most, produced at constant rate
- In pregnancy
What are 3 Cons of using Creatinine?
- Secreted into nephron, hence overestimates GFR by 10-20%
- Cumbersome (carrying a bottle of urine)
- Frequently inaccurate
What are 2 methods we use Creatinine to measure GFR?
- Serum creatinine
- Urine creatinine over 24 hours
State the range of normal Serum Creatinine in the UK
70-150 micromoles/ Litre
Name 7 factors that raise serum creatinine
- Muscle bulk
- Young
- Male
- Black
- Meat intake
- Creatinine supplements
- Some drugs (tripmethoprim)
Name 5 factors that decrease serum creatinine
- Low muscle mass
- Old
- Female
- Hispanic
- Vegetarian
Creatine is easy to measure but varies greatly between individuals.
What can we use Serum Creatinine to measure?
Estimated GFR (eGFR)
What are 2 methods of getting an eGFR
- MDRD eGFR
- CKD-EPI
Describe the features of MDRD eGFR formula used
Variables used;
- Serum Creatinine (sCr)
- Age
- Sex
- Caucasian or Black
- Standardised to 1.73metres squared (height and weight don’t matter)
Compare the CKD-EPI method to MDRD eGFR
CKD-EPI:
- Same variables used
- As accurate when eGFR< 60ml/min
- More accurate when eGFR> 60ml/min
List some problems with MDRD eGFR
Inaccurate in;
- People WITHOUT kidney disease
- Children
- Old age
- Pregnancy
- Ethnicities other than white/ black
- Amputees/ Significantly reduced muscle mass
- Patients with GFR> 60ml/min
Why is eGFR inaccurate in Mild Kidney disease
Give 3 reasons
- Reduced GFR causes increased blood flow-> Increased GFR
- Reduced nephron number-> Nephron Hypertrophy so no GFR change
- Reduced filtration of creatinine->Increased serum creatinine + Increased secretion into tubule (Thus abnormal results “hidden”)
