Session 2 Flashcards
How is kidney function measured?
Glomerular filtration rate
What is the GFR?
Amount of filtrate that is produced from blood flow per unit time
What are the factors that affect the GFR?
Gender Age Size of the individual Size of the kidneys Pregnancy
How does advancing age affect the glomerular filtration rate?
- Declines after 30 years of age
- Rate of decline is 6-7mls/min per decade
- Loss of functioning nephrons
- Some compensatory hypertrophy
What is a bigger problem?
or small kidneys in a big person
Small kidneys in a small person
What is compensatory hypertrophy?
- If the nephron number decreases
- Exisiting nephrons get bigger
- Healthy kidney can also get bigger
- Occurs to much greater extent in childhood
What are the risks associated with compensatory hypertrophy?
- Nephrons have to work harder
- Greater risk of wearing out
- Cortical scarring
What happens to the kidneys and their function in pregnancy?
- GFR increases
- Kidney size increases due to increased fluid volume
- Nephrons number stays the same
GFR is not constant in an individual. True/False
False. It is relatively constant
What does a decline in GFR show?
- Decline in the number of nephrons
- Decline of GFR within individual nephrons
Overall the kidney function has worsened
A patient arrives with significant kidney damage. Their GFR has been stable but recently has decreased. Upon examination, you notice there is kidney hypertrophy. What does this tell you about the kidney function?
The kidney function has declined slowly so GFR didn’t fall until there was significant kidney damage
Why do we need a surrogate marker?
The actual GFR cannot be measured
What is clearance?
The volume of plasma cleared of a substance per unit time where the substance is denoted as ‘x’.
What is the formula for clearance?
from the body
C=A/P
C= clearnace A= amount of substance eliminated from plasma P= plasma concentration of substrate
What is the formula for Renal clearance?
C=(UxV)/Pa
C= clearance U= amount in urine V= urine flow rate Pa= arterial plasma concentration
What are the properties of substances used to measure GFR?
- Produced at a constant rate
- Be freely filtered across the glomerulus
- Not be reabsorbed in the nephron
- Not be secreted into the nephron
Why isn’t inulin used to measure GFR?
- Reuqires a continuous IV to maintain a steady state
- Reuqires catheter and timed urine collections
What is 51 Cr-EDTA?
- Radioactively labelled marker for clearance
- Cleared exclusively by renal filtration
- Approximately 10% lower clearance than inulin
When is 51 Cr-EDTA used clinically?
- In children
- Where indication renal function is required
What is creatinine?
- Endogenous substance
- End product of muscle breakdown
What are the properties of creatine in regards to being an indicator of clearance?
- Produced at a constant rate
- Freely filtered across the membrane
- Not reabsorbed along the nephron
- However it is secreted into the nephron
Creatinine is an underestimate for GFR. True/False
False. It is an overestimate
How is creatinine clearance measured?
- Urine is collected over 24 hours
- The serum creatinine is measured
What are the issues associated with using creatinine as a measurement of clearance?
- Cumbersome and frequently inaccurate
- Overestimates GFR by 10-20% due to creatinine secretion
Which population is creatinine best for in terms of indicators of clearance?
-Pregnant women
What factors lead to increased creatinine measurement?
- Muscle cell break down
- Large muscle bulk
- Black
- Male
- Creatinine supplements
- Certain drugs
What factors lead to reduced serum creatinine?
- Reduced muscle mass
- Old
- Hispanic/Indo-asian
- Female
- Vegetarian
What is the eGFR?
A ‘best guess’ of the GFR which uses models to try and better estimate the GFR from serum creatinine levels. It is not the same as GFR.
What are the variable for the equation for the MDRD eGFR based on?
- Serum creatinine
- Age
- Sex
- Caucasian or Black
When is MDRD eGFR inaccurate?
- People without kidney disease
- Children
- Pregnancy
- Old age
- Other ethnicities
- Amputees
- People with significantly reduced muscle mass
- Patient with higher levels of kidney function
Why is there a risk of patient being labelled as CKD due to the MDRD eGFR?
There is an underestimation of the true GFR when the serum creatinine is close or within normal range
What is particularly important about the individual patient in regard to kidney function?
- The GFR
- The change in the GFR
Why is eGFR less accurate with mild kidney disease?
- Reduction in GFR causes increase in blood flow
- Reduced nephron number lead to nephron hypertrophy so no change in GFR
- Reduced filtration of creatinine results in increased serum creatinine and increased secretion into the tubule
