Assessment of renal function

Question 1

How do we measure kidney function and what is normal?

Answer 1

• Kidney function is measured by assessment of glomerular filtration rate
• 120ml/min is normal or 7.2L/hour
• Age related decline approx 1ml/per year
  *

Question 2

Why do we need to know clearance of the kidneys for GFR?

Answer 2

• We can use clearance to calculate GFR
• The volume of plasma that can be completely cleared of a marker substance in unit time
• If marker is not bound to serum proteins, freely filtered at the glomerulus, and not secreted/reabsorbed by tubular cells, C = GFR

At any one time: C = (U x V)/P

U – urinary conc

P – plasma conc

Question 3

1. What is the gold standard measure of GFR?
2. What is it?
3. What are the difficulties with using it?

Answer 3

1. Inulin clearance
2. Inulin is:

• 5.2kD fructose polymer
• Neutral charge
• Freely filtered - hence can be used for clearance
• Not processed by tubular cells

3. It has to have a steady infusion, not a simple procedure

Question 4

What is the difference between direct and indirect clearance?

Answer 4

• Direct: Clearance calculated from urine collection
• Indirect: clearance calculated from plasma regression curve

Question 5

What properties are needed for an endogenous marker to work as a marker of GFR?

Answer 5

• Same clearance characteristics of the ideal injectable marker
  
  • Not plasma protein bound
  • Freely filtered at glomerulus
  • Not modified by tubules

Question 6

Before inulin what was the popular endogenous marker of GFR?

Answer 6

• Blood urea
  
  • By-product of protein metabolism
  • Freely filtered at the glomerulus
  • Variable reabsorption by tubular cells
  • Dependent on nutritional state, hepatic function, GI bleeding
  • Limited clinical value

Question 7

Describe serum creatinine as an endogenous marker of GFR

Answer 7

• Derived from muscle cells (small amount from intestinal absorption)
• Freely filtered
• Creatinine is actively secreted into urine by tubular cells
• Generation is not equivalent in different individuals. Dependent on:
  
  • • Muscularity
  • • Age
  • -Sex
  • -Ethnicity

Question 8

What is a downfall of estimated creatine clearance (Cockcroft Gault)?

Answer 8

Can overestimate GFR, especially whej <30ml/min

Question 9

What is the downfall of the equation derived from cohort studies to estimate GFR? Requires age, sex, serum creatinine and ethnicity

Answer 9

Esrimated GFR this way may underestimate GFR is abpve-average weight and in young people

Question 10

Besides serum creatinine, what is an alternative endogenous marker of kidney function?

Answer 10

Cystatin C

• 13.6kD protein
• Cysteine protease inhibitor
• Constitutively produced by all nucleated cells
• Constant rate generation
• Freely filtered
• Almost completely reabsorbed and catabolised by tubular cells

Question 11

When is serum creatinine most useful?

Answer 11

In clinical practice, the most useful value of serum creatinine measurement is to determine change in kidney function over time, by comparing creatinine levels taken at different times in the same person, and assess whether kidney function is stable, improving or declining

Question 12

1. What is the protein:creatinine ratio?
2. What test has it replaced?

Answer 12

1.

• Quantitative assessment of amount of proteinuria
• Measurement of creatinine corrects for urinary concentration

2. This one time measurement has replacd 24 hour urine collection

• Cumbersome and messy
• Highly inaccurate

Question 13

Describe what you can do with the following urine samples:

1. Single sample
2. 24 hour collection

Answer 13

1. Single sample

• Dipstick testing
• Microscopic examination
• Proteinuria quantification
• Electrolyte estimation

2. 24 hour collection

• Creatinine clearance estimation
• Stone forming elements
• Proteinuria quantification

Question 14

Which of the following is true regarding urine dipstick testing?

A.If the dipstick is negative for blood it reliably excludes haematuria

B.Haematuria is the only cause of a positive dipstick test for blood.

C.You can reliably exclude bacteriuria if the urine dipstick is negative for nitrites

D.The urine dipstick detects Bence Jones proteins

E.Glycosuria detected by the dipstick means the patient has diabetes.

Answer 14

A

(double check)

Question 15

What can a urine dipsick test show?

Answer 15

• pH
  
  • 4.5 to 8.0
• Specific gravity
• 1.003 to 1.035

(Bowmans space 1.007 to 1.010)

• Protein
  
  • Sensitive to albumin, not BJPs
• Zero, Trace, 1+ to 4+
• Blood
• Leucocyte esterase
  
  • Negative result is significant
• Nitrite
  
  • Detects bacteria, esp. Gm negatives
• Glucose
• Ketones

Question 16

1. How is urine microscopy done?
2. What can be examined in the sediment?

Answer 16

1. Centrifuge the specimen at 300rpm for 5-10 minutes
2. Examine sediment for

• Crytsals
• Red blood cells
• White blood cells
• Casts
• Bacteria

Question 17

1. What does this image of urine microscopy show?
2. What can this be caused by?

Answer 17

1. Calcium oxalate crystals
2. Kidney stones

Question 18

What does this image show (urine microscopy)

Answer 18

Red blood cells (haematuria)

Question 19

What does this image show (urine microscopy)?

Answer 19

White blood cells in urine

Question 20

What does this image show (urine microscopy)?

Answer 20

Red cells casts

Question 21

What does this image show (urine microscopy)?

Answer 21

Bacteria

Question 22

You admit a 28 year old man who you suspect has a renal stone, what is your first choice of imaging?

A.Plain KUB

B.CT

C.Ultrasound KUB

D.IVU

E.MRI

Answer 22

A. Plain KUB

Question 23

What are the different types of renal imaging?

Answer 23

• Plain KUB films
• Intravenous urogram (IVU)
• KUB ultrasound
• Cross-sectional imaging (CT and MRI)
• Functional imaging (static and dynamic renograms)

Question 24

What is the best measure of kidney function?

Answer 24

GFR