Chem Path: Assessment of renal function

Question 1

What is the best way to measure kidney function?

Answer 1

GFR

Question 2

What is a normal GFR?

Answer 2

120ml/min or 7.2L/hour

Question 3

What are the 3 criteria for for a marker to be used to measure GFR

Answer 3

Not bound to serum proteins
Not secreted or absorbed by the tubular cells
Freely filtered

Question 4

What is the formula for GFR?

Answer 4

Urine concentration x volume = Plasma concentration

Question 5

What is the gold standard research marker?

Answer 5

Inulin - not used as you need steady state infusion

Question 6

What are the clinical ways of measuring GFR very accurately in things like chemotherapy

Answer 6

IV injections of Cr-EDTA or Tc-DTPA or lohexol

Question 7

How do you measure the GFR from these radioactive markers

Answer 7

Direct - Collect urine and look at progressive reduction in radioactivity

Indirect - Take blood samples and look at plasma regression curve

Question 8

Discuss why urea is a good and bad endogenous marker

Answer 8

Good - It rises before creatinine in pre renal AKI. It rises much more than creatinine in post obstructive AKI
Bad - Depends on nutritional state, reabsorbed by renal tubules,

Question 9

What can alter creatinine’s usefulness as a marker?

Answer 9

It is related to muscle mass, it is secreted BY renal tubules, is high in black people, can go down in starvation, post surgery, steroid treatment and increase in refeeding, meat rich meal, strenuous exercise, pregnancy

Question 10

How is creatinine refined as a endogenous marker then?

Answer 10

Cockroft gault, eGFR

Question 11

How is eGFR adjusted for women and black people

Answer 11

X 0.742 for women x 1.21 for black

Question 12

Discuss the effectiveness of using Cystatin C as a marker

Answer 12

It is produced by all cells, absorbed and catabolised by tubular cells. Goes up in hyperthyroidism, corticosteroid use and malignancy. NICE guidelines indicate its use for those who’s eGFR is between 45-60 and no albumin in the urine.

Question 13

What is the other way of measuring kidney function?

Answer 13

Protein: Creatinine ratio - via urine

Question 14

When do you do PCR vs 24 hour collection

Answer 14

PCR:
Dipstick test
Microscopy
Proteinurea quantification
Electrolyte estimation

24 hour;
Creatinine clearance estimation - in primary hyperparathyroidism you need to know calcium creatinine ratio to exclude FHH
Stone forming elements - To know if its cystine or urate

Question 15

How does urine pH work?

Answer 15

Renal tubules excrete H+ ions. If it fails you can have renal tubule acidosis. A pH of less than 5.3 indicates that the kidneys are able to acidify urine

Question 16

Why can you have blood in the urine

Answer 16

Haematurea or myoglobinuria in rhabdomyolysis

Question 17

What is ethylene glycol

Answer 17

Non toxic substance in antifreeze

Question 18

What happens to ethylene glycol in the body

Answer 18

It is metabolised to oxalic acid and glycolic acid. Oxaclic acid collects calcium and can solidify. Patients often end up with acidosis and high osmolar gap

Question 19

What are casts in urine microscopy

Answer 19

RBC or WBC formed with other proteins. If you see RBC casts, worry about glomerular disease

Question 20

When would you use plain KUB

Answer 20

Staghorn calculi which are formed with mixed phosphates and may be related to chronic infection

Question 21

When would you use IV urogram

Answer 21

Paeds to look for anatomical defects

Question 22

When would you use KUB USS

Answer 22

To look for hydronephrosis

Question 23

When would you use CT vs MRI

Answer 23

CT - Renal mass / carcinoma

MRI - Renal arteries

Question 24

What is renal biopsy used for

Answer 24

Gold standard for renal dysfunction when underlying cause in unknown.