Measurement of Kidney function Flashcards
What does the kidney regulate and what else does it do?
BP Blood volume pH Electrolytes Osmolality
Excretion of waste products and drugs
Metabolism of drugs, hormones and proteins
Endocrine - makes 1-alpha calcidol, renin, erythropoietin.
What is GFR?
Glomerular Filtration Rate.
The amount of filtrate produced from the blood flow per unit time.
The amount of filtrate is determined by the product of the average filtration of each nephron in each kidney.
What is a normal GFR?
90-120ml/min (lower end for women)
OR
140-180L/day
What does GFR depend on?
Gender Age Size of individual Size of kidneys Pregnancy
When does nephron development finish?
35-36 weeks = premature = lower nephron number as foetal excretion is predominantly via placenta.
How long after birth does GRF become normal?
18 months
When does kidney function start declining?
After 30yrs old - decline 6-7ml per decade
Medulla stays same but cortex decreases in size. This is the important bit.
What impact does your size have on your kidneys?
Bigger person = bigger kidneys = more nephrons.
Small kidneys in a big person is worse than small kidney in a small person.
What happens if you have a reduced nephron number?
Compensatory hypertrophy. This is when your existing nephrons get bigger to compensate for the reduced amount of nephrons.
Healthy kidneys can also get bigger and this occurs to much greater extent in childhood.
What are the consequences of compensatory hypertrophy?
Nephrons work harder and are at greater risk of wearing out.
How does GFR change in pregnancy?
GRF increases (50%) - 130-180ml/min
Kidney size increases by about 1cm - increased fluid volume (vascular and interstitial).
BUt, nephron number is the same and it goes back to pre-pregnancy levels 6 months postpartum.
What do you look at when looking at GFR?
Look at if it changes day to day for that person or if normal for that person because GFR is greatly variable.
Why might GFR decline?
No of nephrons
Decline of GFR within nephrons.
What does a fall / rise in GFR mean?
Fall = worse Rise = recovered
What is clearance?
Clearance is the volume of plasma cleaned of a substance per unit of time where the substance is denoted as ‘X’
What is the formula for clearance?
C = A/P
C = clearance A= amount of substance eliminated from plasma P= plasma concentration of substate
What is the formula for renal clearance?
C = UxV(excretion rate) / P
C = Clearance U = amount of urine V = urine flow rate P= arterial plasma concentration
What characteristics must the substance measured in the urine have to be an acurate representation of 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 If all true then excretion rate = GFR
Why dont we use inulin to measure kidney function?
- Requires a constant IV to maintain a steady state
- Requires a catheter and timed urine collections.
What do we used to measure GFR as a gold standard (not used in all cases)?
51 Cr-EDTA
-This is a radio-active labelled marked that is cleared exclusively by renal filtration.
-You have timed injections with blood samples taken 2,3,4 hours afterwards.
It is used clinically in children and where indication of accurate renal function is required e.g. transplant.
What do we normally used to measure GFR in pregnancy?
Creatinine clearance (as can’t do either of the other methods)
Why do we use creatinine to measure?
Produced at a constant rate
Freely filtered
Not reabsorbed
Not secreted into nephron (not perfect as should be)
Why dont we use creatinine clearance in everyone?
Collect 24 hours of urine so inconvenient which makes the results inaccurate as it decreases compliance.
What things can affect individual levels of creatinine?
Intake (meat) Metabolism Extra-renal excretion Renal excretion (also secretion) Skin colour - black = more and hispanic / into-asian = less Age (old = less)
In who is MDRD eGFR inaccurate in?
People without kidney disease Children Pregnancy Old age Other ethnicities Amputees / reduced muscles mass Patients with higher level of kidney function When TRUE GFR changed quickly (AKI)
Inaccurate in lots of people s only based on study with 1600 people with little variation and no extremes.
What is used when GFR is better?
CKD-EPI
This uses more patients and patients from a more diverse background
Why is eGFR less accurate with mild kidney disease?
3 contributing factors
- Reduction in GFR 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 increases serum creatinine and increased secretions into the tubule.
What is inulin?
A polisaccaride in fruit and veg where none is absorbed and none is excreted.
What is normally used to measure GFR?
Serum creatinine
