ChemPath: Assessment of Renal Function 1

Question 1

What is glomerular filtrate

Answer 1

Ultrafiltrate of plasma without protein (bigger than albumin) and positively charged proteins

Question 2

What is normal GFR?

Answer 2

120 mL/min (7.2 L/hour)

Question 3

At what rate does age affect GFR?

Answer 3

Declines by 1ml/min per year

Question 4

Define clearance.

Answer 4

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

Question 5

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

Answer 5

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

Question 6

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

Answer 6

C = (U x V)/P

U = urinary concentration
V = urine flow rate
P = plasma concentration

NOTE: Only valid for steady state

Question 7

What is inulin and what is its main purpose?

Answer 7

• 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 8

Name three single injection plasma clearance markers/

Answer 8

• ⁵¹Cr-EDTA
• ⁹⁹Tc-DTPA
• Iohexol

Question 9

Describe how single injection plasma clearance markers are used.

Answer 9

The injection is administered followed by blood samples 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 10

Describe the key features of plasma urea.

Answer 10

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

Question 11

Where does serum creatinine come from?

Answer 11

Muscle cells

Question 12

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

Answer 12

• Freely filtered
• Produced at a constant rate

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

Question 13

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

Answer 13

• Muscularity
• Age
• Sex
• Ethinicity

Question 14

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

Answer 14

• 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 15

Name an alternative endogenous marker of GFR.

Answer 15

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 16

What is urine protein: creatinine ratio?

Answer 16

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

Question 17

How is proteinuria estimated?

Answer 17

• Spot urine protein: creatinine ratio (PCR)

NOTE: this has superseded 24-hour urine collection

Question 18

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

Answer 18

Myoglobinuria (from rhabdomyolysis)

Question 19

What is specific gravity?

Answer 19

A measure of urine concentration

Question 20

What can urine microscopy be used to look for?

Answer 20

• Crystals
• RBCs
• WBCs
• Casts
• Bacteria

Question 21

What are casts in urine

Answer 21

Cells in proteinaceous matrix

Question 22

How can ethylene glycol poisoning cause AKI?

Answer 22

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