6 - GFR and Clearance Flashcards
What is GFR and what is the normal values for this?
- Amount of filtrate produced from the blood flow per unit time
- 90 to 120 mL/min
125 mls/min
What does GFR depend on?
- Babies have lower GFR due to maturing of kidney. Normalises from 20ml/min to normal by 18 months
- Over 30 GFR starts to decline as loss of functioning nephrons, there is compensatory hypertrophy so increase in medulla volume
- Pregnancy GFR increases as hypertrophy of kidney 130-180ml/min
How is GFR a measure of kidney function?
- Decrease: function declined as either drop of nephrons (e.g nephrotoxic drugs) or drop of GFR in individual nephrons (e.g glomerulonephritis)
- Increase: kidney function recovered
- If kidney function declines slowly may be no drop in GFR due to nephron hypertrophy so won’t show on GFR until significant damage
What is clearance?
- Volume of plasma cleared of a substance per unit of time
- Encompasses renal and extra-renal clearance
What is renal clearance?
If urine flow or plasma concentration increases, what happens to renal clearance?
- Urine flow: clearance would increase
- Plasma concentration: clearance will decrease
What properties does a substance need to have to measure GFR?
What substance is gold standard for measuring GFR?
- Inulin injection that fits all the criteria apart from being produced at a constant rate
What other exogenous marker can be used to measure eGFR but not everyday?
- 51 Cr-EDTA
- Only need one injection and blood not urine samples
- Can’t be used every day due to radiation
What endogenous substance is used to measure GFR?
Why does using creatinine only provide us with an estimate of GFR?
- Overestimate by 10-15% as there is secretion into tubule
- Different factors affect your creatinine levels e.g age, diet, ethnicity and activity levels
How is creatinine clearance measured?
- Used in pregnancy as no radiation
- May forget to fill urine bottle every time
What is the normal levels of creatinine in the body and how do you convert between American and UK units?
How direct is the relationship between true GFR and serum creatinine?
Serum 130 creatinine would be anywhere from 20 to 90 mL/min so we call it eGFR
What is the method to calculate eGFR in UHL and Lincolnshire?
- UHL MDRD (inaccurate in children, pregnancy, old age, other ethnicities to black and caucasian, amputees, patients with higher levels of kidney function)
- CKD-EPI (more accurate than MDRD when eGFR over 60mls/min)
Quick change in GFR may be AKI
Why is eGFR less accurate in mild kidney disease?
Poor clearance means poor renal function
Label the following diagram
What can be some causes of acute kidney injury?
- Common in hospitals, look and U+Es and GFR
- Hypotension secondary to blood loss or sepsis
- Dehydration without hypotension
