3a.) Measurement of Kidney Function Flashcards
What do we use to measure kidney filtration function?
GFR
GFR is different for each individual; state 5 factors that affect GFR
- Age
- Gender
- Size of individuals
- Size of kidneys
- Pregnancy
State average:
- GFR
- Total glomerular filtrate per day
- GFR= 90-120 mL/min/1.73m^2
- TGF= 140 - 180 L/day
What is the GFR of a baby at birth?
By what age is their GFR normal?
At birth= 20ml/min/1.73m^2
Normal by 18 months
When is nephron devlopment finished in fetus?
35th - 36th week
Hence why premature babies can have lower nephron numbers. Low body weight babies may also have fewer nephrons
Why does GFR decrease once people are over 30?
- Loss of functioning nephrons
After 30 years of age people get loss of functional nephrons; describe how the kidenys try to compensate for this
Kidney cortex size decrease due to loss of functioning nephrons and most of nephrons in cortex (90%). Medulla increases in size to try and compensate but it doesn’t work as most of nephrons are cortical
What happens to GFR in pregnancy and why?
GFR increases by about 50%. Kidney size increases, but nephron number stays same. GFR most likely increases due to an increased fluid volume (both vascular and interstitial) in pregnancy. Back to normal 6 months post partum
Define clearance and give the equation
Clearance (C)= volume of plasma cleared of a substance per unit of time
If someone’s kidney function is declining slowly why might you not see a decrease in GFR until later down the line?
When kidney function declines, individual nephrons hypertrophy so actual GFR may not occur until significant kidney damage when hypertrophy cannot make up for loss of functioning nephrons
Why do we need to use renal clearance of a substance/marker?
We cannot measure GFR directly
Clearance compromises both renal excretion (urine) and extra-renal excretion; true or false
True
Clearance includes both renal excretion and extra-renal excretion hence we need a separate calculation to calculate renal clearance; state the equation for renal clearance
The amount of substrate in the ultrafiltrate can be altered by reabsorption and/or direct secretion into the tubules; therefore we have to ensure that, when using a substrate to measure GFR, we use a substrate with what 4 properties?
- Freely filtered across glomerulus
- No secretion by renal tubule
- No reabsorption by renal tubule
- Be produced at a constant rate
If all these are true then the amount of substanc excreted in urine per minute is equal to the amount filtered by kidney per minute
What is the ‘gold standard’ substance for measurement of renal clearance?
Why isn’t it used?
Inulin. It is an exogenous substance that is injected. It is:
- Freely filtered
- Not secreted
- Not absorbed
… therfore the amount excreted in urine per minute is equal to renal clearance.
Not used as measuring the inulin clearance is impractical: requires continous IV to maintain steady state and requires catheter and timed urine collections
Cr-EDTA, a radioactive labelled marker, can also be used to measure renal clearance. Describe:
- Procedures required in order to use it as measurement of renal clearance
- Why not as good as inulin
- When it is used
- Timed injection with blood samples taken 2,3 and 4 hours after
- Approx 10% lower clearance than inulin, maybe reabsorption?
- Used in children or prior to and post kidney transplant
Discuss the use of creatinine as an endogenous measurement of kidney function
End product of muscle breakdown which is:
- Freely filtered at glomerulus
- Not reabsorbed
- Small amount secreted into tubules (and small amount secreted via gut)
- Plasma concentration relatively stable
Timed urine collection over 24 hours and blood sample to determine plasma concentration.
Not as accurate as inulin but is very easy to measure. However, creatine production differs betwen individuals and also differs in an individual based on lifestyle at time e.g. exercise, diet…
Give an example of when creatinine would be used to measure GFR and why?
Pregnancy as non-radioactive
Does creatinine clearance tend to over or under-estimate GFR?
Over-estimate
(due to secretion into tubule- make top of equation larger)
Creatinine can be measured in umol/L or mg/dL; how do you convert between the two?
What is eGFR?
Equation used to estimate GFR using:
- Serum creatinine
- Age
- Sex
- Caucasian or black
How do you convert from mL to dL?
/100
Why do we used eGFR alongside creatinine clearance?
Creatinine clearance has disadvantages such as:
- Numerous factors can impact on the amount of creatinine in someone’s blood (not just their renal clearance)
- Patients must remember to collect all urine over 24 hour period
Will the eGFR for a black african be greater than the eGFR for a white person?
Yes, as black africans have higher plasma creatinine levels than white people with the equivalent renal function
