Kidney Clearance Flashcards
What does the kidney regulate?
BP Blood volume pH Electrolytes Osmolality
What does the kidney excrete?
Waste products and drugs
Drugs, hormones and proteins are …. by the kidney
Metabolised
What is the endocrine function of the kidney?
1-alpha calcidol
renin
erythropoetin
Which area of the nephron is primarily assessed clinically?
Glomerulus
What is GFR?
Glomerular filtration rate
Why does the GFR drop after age of 30?
GFR deteriorates by 6-7ml/min per decade because there is a drop in nephron number
With age what happens to the size of the kidney?
Gets smaller - the cortex thins (smaller glomerulus so reduced GFR)
In what type of person is a smaller kidney more problematic?
Big people
What happens in compensatory hypertrophy?
If one kidney functions poorly the other kidneys nephrons will hypertrophy to compensate- get bigger and work harder
What is the side effect of a compensatory hypertrophy?
Nephron has to work harder and gets worn out
Before 36 weeks gestation if there is a problem with one kidney what happens to the other?
The nephrons will hypertrophy and undergo hyperplasia - tends to function normally and not be predisposed to wearing out
In what physiological state does GFR increase and renal glucose threshold decrease?
Pregnancy
What does GFR increase to in pregnancy?
130-180ml/minute- up to 50% increase
90-110ml/min is normal in a non-pregnant woman
Does the nephron number change in pregnancy?
No but the kidney will increase in size by about 1cm to deal with the increased fluid in the woman.
How long after delivery will a woman kidneys take to return to normal size and function?
6 months
Why might a persons GFR decrease?
Reduced nephron number
GFR in each nephron is reducing.
Why in a slowly deteriorating kidney is GFR change not detectable till late stages of disease?
Compensatory hypertrophy
What is the formula for clearance?
C=A/P (conc of sunstance eleiminated / conc of substance in plasma)
Clearance is the amount of a substance cleared in a given unit of time
What is renal clearance?
excretion rate/ plasma conc
How do you calculate excretion rate?
Amount in urine/ urine flow rate
We can use substances to measure GFR, what qualities should a substance we use to measure GFR have?
- constant production
- 100% filtration in glomerulus
- no reabsoprtion in the distal tubule
- not secreted
What is inulin and how is it used?
Inulin is a drug given IV to assess GFR. It is good, but needs continuous IV delivery and urine needs to be collected via catheters.
What radioactive labelled marker may be used to assess GFR?
51 Cr-EDTA
List the advantages and disadvantages of 51 Cr-EDTA for GFR assessment
Advantages- cleared exclusively by renal filtration. No urine collection. Suitable for children pre kidney transplant.
Disadvantages- Radioactive labelled marker- not to be used in pregnancy. 10% lower than clearance (reabsorbed?). 2,3,4 hourly blood tests after injection/
What endogenous marker is a good estimate of GFR?
Creatinine (it is produced at a rate proportional to muscle mass - unless other pathology, its freely filtered and not reabsorbed. There is some secretion however)
What does secretion of creatinine mean interns of the eGFR it gives?
The secretion means the eGFR is overestimated when looking at creatinine (10-20%)
How do we assess creatinine clearance?
Urine collection over 24 hours
Who is creatinine useful for and why is it cumbersome?
Pregnant women - endogenous substance
Carrying urine bottles
What is a normal serum creatinine measurement?
132micromol/L
How do we get and get rid of creatinine?
Intake (dietary) Muscle mass/ Metabolism Renal secretion Renal excretion Extra renal secretion
If renal excretion declines with a GFR decline what will take over creatinine removal?
Renal SECRETION will increase
Extra renal excretion
What factors increase serum creatinine?
Muscle break down Muscle Mass Male Black Supplementation Meat in diet Drugs like trimethoprim
An elderly hispanic lady presents with reduced muscle mass and is vegetarian what could you predict about her serum creatinine?
Likely to be lower
If we use serum creatinine to assess GFR what else must we consider?
The person (a big muscly black man will have higher creatinine irrespective of GFR than when compared to an elderly indoasian woman)
At birth what is the creatinine of the neonate?
The same as the mother (this will rapidly drop and then gradually increase again)
What is a maternal creatinine at birth?
Increased GFR means creatinine in pregnant woman is low (she’s filtering more out)
Is serum creatinine a sensitive marker for GFR deterioration?
No, 50% loss of GFR before creatinine effected. Compensatory hypertrophy and increased secretion hides a subtle problem.
If you look at inulin clearance (true GFR) and compare it with serum creatinine why is the GFR predicted by GFR lower?
Creatinine secretion
Serum creatinine is used for what in terms of renal function
eGFR
not GFR, its just a best guess
What does the MDRD eGFR used by UHL consider when calculating eGFR?
Serum Creatinine
Age
Sex
Caucasian or Black
What body surface area do we standardise to and what does this mean?
1.73m2
we don’t need to consider height and weight —- really big or really small people get an inaccurate result
Who is an MDRD eGFR less accurate in?
People without kidney disease (transplant donors) Children Pregnancy Old Age Non Black or caucasians Reduced muscle mass/ amputees High renal function patients >60mls/min
Acute Kidney injury (needs a gradual change to detect )
Why should you be careful not to label a patient with chronic kidney disease on the basis of eGFR alone?
It underestimates true GFR if its close to normal range
You need to look at if an individuals GFR changes and always consider what else could be effecting a serum creatinine
What is CKD-EPI?
Like MDRD with a slightly different calculation used in other hospitals. Better when eGFR>60mls/min
Serum creatinine will …. estimate GFR in normals.
UNDER >60mls/min
Why does eGFR get OVER estimated in CKD patients?
Compensatory hypertrophy with increased renal secretion and a reduction in GFR will initially cause increase in renal blood flow to compensate
Why do bigger people have higher GFRs?
Bigger kidneys in bigger people.
Remember that they are likely to have higher serum creatinine from larger muscle mass
