ChemPath: Assessment of Renal Function 1

Question 1

What is normal GFR?

Answer 1

120 mL/min (7.2 L/hour)

Question 2

At what rate does age affect GFR?

Answer 2

Declines by 1ml/min per year

Question 3

Define clearance.

Answer 3

The volume of plasma that can be completely cleared of a marker substance per unit time.

Question 4

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

Answer 4

• Marker is not bound to serum proteins
• Freely filtered by the glomerulus
• Not secreted or absorbed by tubular cells

Question 5

State the equation that links clearance with urine and plasma concentration.

Answer 5

C = (U x V/P)

Question 6

What is inulin and what is its main purpose?

Answer 6

• Neutral, freely filtered fructose polymer that is technically the perfect marker
• However, measurement of inulin concentrations is difficult and it requires a steady-state infusion
• So, it is only used as a research tool

Question 7

Name three single injection plasma clearance markers/

Answer 7

• ⁵¹Cr-EDTA
• ⁹⁹Tc-DTPA
• Iohexol

Question 8

Describe how single injection plasma clearance markers are used.

Answer 8

The injection is administered followed by measurement of urine collection using a Geiger counter or blood samples can be taken to look for a progressive reduction in radioactivity.

NOTE: this is only used under specific circumstances (e.g. when accurate estimation of GFR is necessary before chemotherapy)

Question 9

Describe the key features of plasma urea.

Answer 9

• By-product of protein metabolism
• Variable absorption (30-60%) by tubular cells
• Dependent on nutritional state, hepatic function and GI bleeding
• Limited clinical value

Question 10

Where does serum creatinine come from?

Answer 10

Muscle cells

Question 11

Which features of serum creatinine make it a useful marker of GFR?

Answer 11

• Freely filtered
• Produced at a constant rate

NOTE: it is actively secreted into the urine by tubular cells

Question 12

List some factors that affect the rate of generation of creatinine.

Answer 12

• Muscularity
• Age
• Sex
• Ethinicity

Question 13

Name and briefly describe three equations that are used to estimate creatinine clearance or GFR.

Answer 13

• Cockcroft-Gault - estimates creatinine clearance by taking into account weight, age and sex (may overestimate when GFR <30ml/min)
• MDRD - estimates GFR from creatinine clearance and takes into account age, sex, serum creatinine and ethnicity (may underestimate in overweight and young people)
• CKD-EPI - improvement of MDRD and currently recommended

Question 14

Name an alternative endogenous marker of GFR.

Answer 14

Cystatin C

NOTE: it is constitutively produced by all nucleated cells, is generated at a constant rate and freely filtered. It is almost completely reabsorbed and catabolised by tubular cells.

Question 15

What is urine protein: creatinine ratio?

Answer 15

• Quantitative assessment of proteinuria
• Measurement of creatinine corrects for urine concentration

Question 16

How is proteinuria estimated?

Answer 16

• Spot urine protein: creatinine ratio (PCR)

NOTE: this has superseded 24-hour urine collection

Question 17

Aside from blood, what else can cause a urine dipstick to be positive for blood?

Answer 17

Myoglobinuria (from rhabdomyolysis)

Question 18

What is specific gravity?

Answer 18

A measure of urine concentration

Question 19

What can urine microscopy be used to look for?

Answer 19

• Crystals
• RBCs
• WBCs
• Casts
• Bacteria

Question 20

How can ethylene glycol poisoning cause AKI?

Answer 20

It gets converted to oxalic acid which precipitates with calcium to form calcium oxalate stones