Renal 1

Question 1

What is the best measure of kidney function?

Answer 1

GFR

Question 2

Normal range of GFR

Answer 2

120mL/minute; 7.2L/hour

Question 3

How does GFR change with age?

Answer 3

Declines (by approx 1 ml/min per year)

Question 4

Define clearance

Answer 4

Volume of plasma that can be completely cleared of a marker substance per unit time

Question 5

What are the 3 criteria that must be fulfilled for a marker to be used to measure GFR? i.e. for clearance to = GFR

Answer 5

Marker is NOT bound to serum proteins
Freely filtered by the glomerulus
NOT secreted or reabsorbed by tubular cells

Question 6

Equation to calculate clearance

Answer 6

C = (U x V) / P

U = urinary concentration
P = plasma concentration

Question 7

Clearance of which substance is the golden standard to measure GFR and why?

Answer 7

Insulin

Because freely filtered and not processed by the tubular cells

Question 8

Name 3 exogenous markers of GFR

Answer 8

51Cr-EDTA
99Tc-DTPA
Iohexol

Question 9

Describe the direct vs indirect routes of measuring clearance

Answer 9

Direct method: urine collection to a gamma counter

Indirect method: take blood samples and look at the progressive reduction in radioactivity

Question 10

Name 3 endogenous markers of GFR

Answer 10

Blood urea
Serum creatinine
Cystatin C

Question 11

Is blood urea a good marker of GFR? Why/why not?

Answer 11

Not really

Variable (30-60%) reabsorption by tubular cells (you DON’T want any reabsorption…)
Dependent on nutritional state, hepatic function, GI bleeding
Very limited clinical value

Question 12

What factors affect the rate of generation of creatinine?

Answer 12

Muscularity (proportional to mass)
Age
Sex (higher in men)
Ethnicity (higher in Afro-Caribbean)

Question 13

What is the Cockcroft-Gault adjustment and what is the problem with it?

Answer 13

Estimate creatinine clearance

Not the GFR directly (just creatinine)

May overestimate low GFRs (<30mL/min)

Age + weight + gender + creatinine

Question 14

What is the estimated GFR adjusted equation / MDRD and what is the problem with it?

Answer 14

Complex equation derived from cohort studies

Requires information about age, sex, serum creatinine and ethnicity –> got rid of weight so equation more practical

May underestimate GFR if above average weight and young

Age + gender + creatinine

Question 15

What is the CKD-Epidemiology Collaboration (CKD-EPI) and what is the problem with it?

Answer 15

The equation is based on the same four variables as MDRD but models the relationship between GFR and serum creatinine, age, sex and race differently

It is an improvement on MDRD, but it is still imprecise at higher GFRs

Reduces bias at GFRs >60mL/min (but imprecise at higher GFRs)

i.e. Accurate at LOW GFRs and less accurate at HIGH GFRs

Question 16

Is serum creatinine a good marker of GFR? Why/why not?

Answer 16

No. Insensitive marker.

Question 17

How would you describe the exogenous markers of GFR (generally)?

Answer 17

Single injection plasma clearance measurements

Question 18

What is the most robust value of serum creatinine measurement?

Answer 18

To determine change in kidney function within an individual over time i.e. change in creatinine (look at previous measurements)

Question 19

What is PCR in renal terms?

Answer 19

Urine protein: creatinine ratio (PCR):

This is a quantitative assessment of the amount of proteinuria
Measurement of creatinine corrects for urinary concentration

Two methods to do PCR:
24hr urine collection (cumbersome and messy; highly inaccurate without patient education)
Spot urine measurement