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Urinary > GFR and clearance > Flashcards

GFR and clearance Flashcards

1
Q

When would GFR change?

A

kidney function changes

  • decreased number of nephrons- (diseases or nephrotoxic drugs)
  • decline of function of each individual nephron (diabetes and hypertension causing scarring and fibrosis)
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2
Q

What happens if kidney function decreases slowly?

A

-hypertrophy of nephrons may occur so actual GFR may not fall until significant kidney damage has occured

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3
Q

What is GFR?

A

the amount of filtrate that is produced from the blood flow per unit time

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4
Q

Why is it difficult to measure GFR accurately?

A

ignoring active reabsorption and active secretion that occurs in the nephrons

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5
Q

What is clearance?

A

vol of plasma cleared of a substance per unit time OR volume of plasma that the body needs to get rid of substance X (could be through faeces, urine, sweat exhaling)

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6
Q

What is renal clearance?

A

vol of plasma cleared of a substance per unit time OR volume of plasma that the body needs to get rid of substance X - purely by the KIDNEYS!!!

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7
Q

What is the formula for renal clearance?

A

excretion rate / plasma conc where excretion rate is the amount of the substance in the urine multiplied by how fast the urine is flowing

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8
Q

What happens if flow rate increases?

A

renal clearance will increase by the same amount

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9
Q

Why does renal clearance increase with increased flow rate?

A

if urine is leaving the body at a faster rate, substance X is being removed from the body faster hence increased clearance

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10
Q

What happens if plasma concentration increases?

A

renal clearance will decrease proportionally

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11
Q

Why does renal clearance decrease with increase plasma concentration?

A

If there is more of substance X in the plasma then the body is not removing enough substance X that it should

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12
Q

What should the characteristics be of the substance used to measure kidney clearance?

A

-produced at a constant rate
-be freely filtered across the glomerulus
-not be reabsorbed in the nephron
-not be secreted in the nephron
SOOO if all these are true then excretion rate is exactly the GFR but there is not a molecule like that in the body

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13
Q

What are exogenous substances that can be used to measure GFR?

A

inulin

51 Cr EDTA

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14
Q

What are endogenous substances that can be used to measure GFR?

A

creatinine clearance

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15
Q

What is inulin?

A

a plant polysaccharide

-not produced by body at all as needs to be injested

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16
Q

What are the benefits of inulin in measuring GFR?

A

-if you are hooked up to an IV supplying inulin at CONSTAMT RATE then the excretion of inulin is equal to your GFR

17
Q

What are the negative of inulin in mesauring GFR?

A
  • not clinically used as you would need to be hooked to an IV to maintain the constant rate
  • also required catheter and timed urine collections
18
Q

What is 51 Cr EDTA?

A
  • it is a radioactive labelled marker
  • cleared exclusively by renal filtration
  • can be given in 1 injection shot and monitor using blood samples not urine output
19
Q

When is 51 Cr EDTA used clinically?

A

-used in children where indication of renal function is required e/g/ kidney transplant or kidney donation

20
Q

Why do you need to know GFR before cancer treatment?

A
  • need to know if kidney will work and function well after treatment / removal
  • cancer treatments are nephrotoxic
21
Q

What are the negatives associated with using 51 Cr EDTA?

A

it is radioactive - patient needs to go to nuclear medicine until no radiation is being radiated from their body
-shoudlnt be used in pregnant people

22
Q

What is creatinine?

A

-end product of muscle breakdown

23
Q

Is creatinine a good measure of GFR?

A

-yes however some creatinine is still being secreted into urine by the nephron

24
Q

So how is creatinine clearance measured?

A
  • serum creatinine AND

- urine creatinine over 24 hrs

25
Q

What are the disadvantages of using creatinine to measure GFR?

A
  • cumbersome (carrying a bottle of urine)
  • frequently inaccurate
  • overestimates GFR by 10-20% due to creatinine secretion
26
Q

What are the advantages of using creatinine to measure GFR?

A

can be used in pregnancy

27
Q

What affects the serum creatinine?

A
  • high muscle mass
  • lots of meat intake
  • black male

Highest creatinine

28
Q

What factors contribute to reduced serum creatinine?

A
  • old age
  • women
  • hisospanic/indo-asian
  • vegetarian
29
Q

Why do babies have low serum creatinine?

A

kidneys don’t work in foetus so placenta takes over role - initially VERY high as takes into account mothers creatinine levels BUT when the baby is born, the babies kidneys have to work and remove the creatinine created by mum so there is a DRASTIC fall in serum creatinine

30
Q

What is eGFR?

A

ESTIMATED GFR (best guess)

31
Q

How is eGFR calculated?

A

blood test - no need for uric output

32
Q

What is MDRD eGFR?

A 
formula that takes into account;
-age
-sex
-ethnicity
-serum creatinine
(is standardised to body surface area so don't need height and weight)
33
Q

What types of people is MDRD eGFR inaccurate in?

A
  • people without kidney disease e.g. donors
  • children
  • pregnancy
  • older
  • other ethnicities to black or caucasian
  • amputees (weigh reduction)
  • patietns with higher levels of kidney function
34
Q

What is CKD-EPI?

A

uses the same variable as MDRD but the formula is different - more accurate when eGFR > 60mls/min

35
Q

Why is eGFR less accurate with mild kidney disease?

A
  • reduction in GFR causes increases in blood flow
  • reduced nephron number leads to nephron hypertrophy so no change in GFR
  • reduced filtration of creatine due to reduced GFR results in increases serum creatinine and increased secretion into the tubule to maintain steady state of serum creatinine

Urinary (11 decks)

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