Measurement of Kidney Function

Question 1

What is kidney function measured in terms of

Answer 1

Kidney function is measured in term of GFR

Question 2

What is the noraml GFR

Answer 2

90-120 ml/min/1.73m²

Question 3

Why might premature babies and LBW infants have a lower GFR than normal

Answer 3

Premature babies and LBW infants often have lower nephron numbers as the nephron develpoment finishes by the 35-36th week of foetal development

This causes a lower GFR

Also few nephrons develop after birth

Question 4

How is GFR affected with advancing age

Answer 4

GFR decreases with age as the kidney shrinks with age

Renal cortex decreases in size -> less nephrons -> decreased GFR

Question 5

How is GFR affected by pregancy

Answer 5

Pregnancy causes there to be an increased fluid volume resulting in increased GFR and increased kidney size

Returns to normal post-partum

Question 6

What is compensatory hypertrophy

Answer 6

Where decreased nephorn number leads to the remaining nephrons working harder -> increase in kidney size

Common in children - especially if child only has one functioning kidney in utero

Compensatory hypertrophy leads to greater risk of kidneys wearing out as nephrons are working harder

Question 7

Why might GFR decline

Answer 7

Decline in nephron number

Decline of GFR within individual nephrons

Question 8

What is the equation used for renal clearance

Answer 8

Question 9

What factors should a substance have to be used to measure kidney clearance

Answer 9

Be produced at a constant rate

Be freely filtered across the glomerulus

Not be reabsorbed in the nephron

Not be secreted into the nephron

Question 10

Name some substances that are used to measure kidney clearance

Answer 10

Inulin

Radioactive labelled marker - 51 Cr-EDTA

Creatinine

Question 11

Why is inulin not used as a marker for kidney clearance

Answer 11

It is not continuously produced in the body

It requires catheter insertion and timed urine collection

Question 12

How is 51 Cr-EDTA used as a marker of kidney clearance

Answer 12

It is given in timed injections and then blood samples are taken after specific times

Can then work out how quickly the marker has been filtered out of the blood to give the GFR

Has some reabsorption

Question 13

Why does creatinine give an overestimate of the GFR

Answer 13

Some creatinine is secreted directly into the nephron

Question 14

How is serum creatinine used to calculate the GFR

Answer 14

Serum creatinine is compared to the normal values for that patient to see if their serum creatinine is normal or has changed from the previous one

Changes in serum creatinine away from the normal value/patients normal value suggests the GFR has changed

Question 15

What factors affect the serum creatinine level

Answer 15

Intake of protein/diet

Metabolism of the patient

Muscle bulk of the patient

Gender, age and ethnicity (blacks have more)

Renal excretion and secretion

Changes in muscle use - increased muscle use will increase amount of creatinine in the body

Extra-renal excretion - normally very low
Certain drugs affect creatinine levels

Question 16

Why is serum creatinine lower than the true GFR

Answer 16

Because some creatinine is secreted into the nephron

Question 17

What models are used to estimate the GFR from serum creatinine levels

Answer 17

MDRD eGFR - based on only a few patients with CKD -> inaccurate. Caucasians only

CKD-EPI - based on more patients than MDRD eGFR from more diverse backgrounds

Question 18

MDRD eGFR model is inaccurate in which patients

Answer 18

People without kidney disease - transplant donors

Children

Pregnancy

Old age - labelled as having kidney disease

Other ethnicities

Amputees/those with reduced muscle mass

Patients with high level of kidney function

Patients whose GFR changes quickly - AKI

Question 19

Why is the eGFR less accurate in those with mild kidney disease

Answer 19

Reduced GFR

Reduced nephron number leads to nephron hypertrophy so no change in GFR

Reduced filtration of creatinine due to reduced GFR - increases serum creatinine -> increased secretion into nephron so will maintian relatively stable state of serum creatinine - look at patients history to see if creatinine or eGFR has changed

Question 20

What are urodynamics used for

Answer 20

Urodynamics are used to determine the aetiology of voiding dysfunction and lower urinary tract symptoms

Question 21

What is a urodynamic study split into

Answer 21

Voiding phase - voiding pressure flow study. Allows diagnosis of obstruction as detrusor pressure and urinary flow rate can be measured and outlet resistance calculated

Filling and storage phase - gives information on bladder contractility

Question 22

What types of studies are done in a urodynamic study

Answer 22

Flow rate - volume of urine through urethra per unti time

Peak flow rate - maximum measured flow rate

Voided volume - total volume expelled through the urethra

Flow time - total time over which measurable flow occurs

Average flow rate - voided volume/flow time

Time to maximum flow

Question 23

What are the types of urinalysis

Answer 23

Basic urinalysis - gross examination or urine and dipstick analysis

Microscopic urinalsys - detects cellular elements, casts and crystals

Question 24

When is the best time to do a urinalysis

Answer 24

First voided morning urine as it is the most concentrated