Measurement Of Renal Function

Question 1

What are 5 things the GFR of an individual depends on

Answer 1

• Age
• Gender
• Size of person
• Size of kidneys
• Pregnancy

Question 2

Nephron development usually finishes by week 35/36 of fetal development.

Premature and Low Birthweight babies usually have less nephrons.

What is the usual GFR at birth?
At what age the GFR become normal

Answer 2

• 20ml/min/1.73 metres squared

- 18 months

Question 3

At what age does GFR start declining? (Loss of functioning nephrons)
At what rate does this happen?

What may accompany this?

Answer 3

• After age of 30 (not usually noticeable till 50)
• Declines by 6/7 ml/min per decade

Compensatory Hypertrophy

Question 4

How does Pregnancy affect GFR, Kidney Size and Nephron number?

Answer 4

• GFR increases (by 50%) to 130-180 ml/ min (Drops to normal 6 months after birth)
• Kidney size increases (by 1 cm)
• Same nephrons number

Question 5

What may be happening when kidney function declines slowly?

Answer 5

• Single nephrons may Hypertrophy so actual GFR may not fall until kidney is significant damaged (Makes it harder to treat)

Question 6

What are 4 ideal properties of a substance that is used to measure kidney clearance?

What can we say if all are true

Suggest a substance

Answer 6

• Produced at constant rate
• Be freely filtered across glomerulus
• Not be secreted
• Not be reabsorped

GFR= Excretion rate

Inulin (A plant polysaccharide)

Question 7

What are 2 Cons of using Inulin

Answer 7

• Needs continuous IV

- Needs Catheter and timed urine collections

Question 8

What are 2 alternatives to using Inulin

Answer 8

• Creatinine

- 51 CR-EDTA (Chromium-EDTA)

Question 9

Describe the use of 51 CR-EDTA

Answer 9

• Radio active labelled marker
• Cleared exclusively by renal filtration
• Timed injection with blood samples after 2,3 and 4 hours

(10% Lower clearance than insulin, may be due to reabsorption)

Question 10

What are 2 clinical uses of 51 CR-EDTA

Answer 10

• In children

- Where indication of renal function is needed (transplants)

Question 11

What are 3 pros of using Creatinine?

Name 1 time we use to measure GFR

Answer 11

• Freely filtered across glomerulus
• Not reabsorbed in nephron
• In most, produced at constant rate
• In pregnancy

Question 12

What are 3 Cons of using Creatinine?

Answer 12

• Secreted into nephron, hence overestimates GFR by 10-20%
• Cumbersome (carrying a bottle of urine)
• Frequently inaccurate

Question 13

What are 2 methods we use Creatinine to measure GFR?

Answer 13

• Serum creatinine

- Urine creatinine over 24 hours

Question 14

State the range of normal Serum Creatinine in the UK

Answer 14

70-150 micromoles/ Litre

Question 15

Name 7 factors that raise serum creatinine

Answer 15

• Muscle bulk
• Young
• Male
• Black
• Meat intake
• Creatinine supplements
• Some drugs (tripmethoprim)

Question 16

Name 5 factors that decrease serum creatinine

Answer 16

• Low muscle mass
• Old
• Female
• Hispanic
• Vegetarian

Question 17

Creatine is easy to measure but varies greatly between individuals.

What can we use Serum Creatinine to measure?

Answer 17

Estimated GFR (eGFR)

Question 18

What are 2 methods of getting an eGFR

Answer 18

• MDRD eGFR

- CKD-EPI

Question 19

Describe the features of MDRD eGFR formula used

Answer 19

Variables used;

• Serum Creatinine (sCr)
• Age
• Sex
• Caucasian or Black
• Standardised to 1.73metres squared (height and weight don’t matter)

Question 20

Compare the CKD-EPI method to MDRD eGFR

Answer 20

CKD-EPI:

• Same variables used
• As accurate when eGFR< 60ml/min
• More accurate when eGFR> 60ml/min

Question 21

List some problems with MDRD eGFR

Answer 21

Inaccurate in;

• People WITHOUT kidney disease
• Children
• Old age
• Pregnancy
• Ethnicities other than white/ black
• Amputees/ Significantly reduced muscle mass
• Patients with GFR> 60ml/min

Question 22

Why is eGFR inaccurate in Mild Kidney disease

Give 3 reasons

Answer 22

• Reduced GFR causes increased blood flow-> Increased GFR
• Reduced nephron number-> Nephron Hypertrophy so no GFR change
• Reduced filtration of creatinine->Increased serum creatinine + Increased secretion into tubule (Thus abnormal results “hidden”)