Chem path 9 - Renal part 1

Question 1

What is the best measure of kidney function?

Answer 1

GFR

Question 2

What is the normal GFR?

Answer 2

120ml/min or 7.2L/hr

Question 3

What other measure can be uesd to calculate GFR?

Answer 3

Clearance

Question 4

What is clearance?

Answer 4

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

Question 5

What are the three criteria that need to be fulfilled for a marker to be used to measure GFR?

Answer 5

1) Not bound to serum proteins
2) Freely filtered by the glomerulus
3) Not secreted or reabsorbed by tubular cells

If these criteria are fulfilled then clearance = GFR

Question 6

What is the equation for clearance?

Answer 6

C= (UXV)/P U= urinary concentration P= plasma concentration

Question 7

Clearance of what substance is the gold-standard measure of GFR?

Answer 7

Inulin (not endogenous, difficult to measure its concentration and therefore only used as a research tool)

Question 8

Name some clinically viable/exogenous markers of GFR and when are these used?

Answer 8

51Cr-EDTA, 99Tc-DTPA, Iohexol
Can measure using a gmama counter to urine (direct) or indirectly by taking blood samples and look at the progressive reduction in radioactivity.
This is only used in special circumstances e.g. before starting chemo

Question 9

Name 2 endogenous markers of GFR

Answer 9

Serum urea and creatinine

Question 10

What are the limitations of urea as a marker of GFR?

Answer 10

30-60% reabsorption by tubular cells
Dependent on nutrional state, hepatic function, GI bleeding
Very limited clinical value

Question 11

What are the limitations of creatinine as a marker of GFR?

Answer 11

Related to pt muscle mass.
Actively secreted in to the urine by tubular cells
creatine; GFR relationship is not linear i.e. creatinine is less accurate at predicting GFR when GFR at lower values

Question 12

What are the names given to the equations which adjust the creatinine measurement?

Answer 12

Cockcroft-Gault and Estimated GFR adjusted equation

Question 13

What does the Cockcroft-Gault equation measure?

Answer 13

Estimates creatinine clearance (not GFR directly)

Question 14

In what scenario may the Cockcroft-Gault equation overestimate GFR?

Answer 14

When GFR <30ml/min

Question 15

What are the components in the estimated GFR adjusted equation? (MDRD study)

Answer 15

Age, sex, ethnicity, creatinine

Got rid of weight so equation more practical

Question 16

In what scenario may the eGFR adjusted equation underestimate GFR?

Answer 16

If young and overweight

Question 17

What is the current recommendation to measure GFR?

Answer 17

CKD-EPI (CKD-Epidemiology collaboration)

Based on same 4 variables as MDRD

Question 18

What are the limitations of the CKD-EPI?

Answer 18

Imprecise at higher GFRs

Question 19

What is a common lmitation of both CKD-EPI and MDRD?

Answer 19

Both seem to underestimate GFRs but CKD-EPI less so

Question 20

What is a clinical alternative to serum creatinine?

Answer 20

Cystatin C. CKD NICE guidelines have included it but not used that frequently. (values may be falsely elevated in pts with hypothyroidism)

Question 21

What is the most robust value of serum creatinine measurement?

Answer 21

to determine change in kidney function within an individual over time

Question 22

How is proteinurea now quantified in clinical practice?

Answer 22

By using a spot urine measurement. This has superseded 24hr urinary collection.

Question 23

If a dipstick measurement is -ve for blood, does it reliably exclude haematuria?

Answer 23

YES

Question 24

What other reason may a dipstick urine come back as +ve for blood?

Answer 24

Myoglobinuria e.g. in rhabdomyolysis

Question 25

Is a urine dipstick sensitive to bence jones proteins?

Answer 25

No, it is sensitive to albumin

Question 26

In which physiological state is glycosuria normal and not indicative of diabetes?

Answer 26

in pregnancy, there is a lower renal threshold for glucose

Question 27

In urine microscopy, you centrifuge the sample at 3,000rpm for 5 minutes. What do you then look at the sediment for?

Answer 27

Crystals (calcium oxalate stones, RBCs, WBCs, casts, bacteria

Question 28

Case scenario: 50yo, alcoholic
o Presents unwell, seemingly intoxicated with AKI
o Urine microscopy = calcium oxalate crystals

What is the diagnosis?

Answer 28

ETHYLENE GLYCOL POISONING (ANTI-FREEZE)

It gets converted to oxalate which will then precipitate with calcium in the renal tubules and ureters

Question 29

What is the first line renal imaging?

Answer 29

CTKUB (commonly used to look for stones)

Question 30

What are some other methods of kidney imaging?

Answer 30

US KUB - 2nd line (can differentiate between AKI and hydronephrosis)
Plain KUB films (staghorn calculi)
IV urogram (done more in paediatrics to see anatomical defects)
MRI KUB
Functional imaging (static and dynamic renograms)
Renal biopsy