Assessment of renal function Flashcards
How do we measure kidney function and what is normal?
- 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
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Why do we need to know clearance of the kidneys for GFR?
- 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
- What is the gold standard measure of GFR?
- What is it?
- What are the difficulties with using it?
- Inulin clearance
- Inulin is:
- 5.2kD fructose polymer
- Neutral charge
- Freely filtered - hence can be used for clearance
- Not processed by tubular cells
- It has to have a steady infusion, not a simple procedure
What is the difference between direct and indirect clearance?
- Direct: Clearance calculated from urine collection
- Indirect: clearance calculated from plasma regression curve
What properties are needed for an endogenous marker to work as a marker of GFR?
- Same clearance characteristics of the ideal injectable marker
- Not plasma protein bound
- Freely filtered at glomerulus
- Not modified by tubules
Before inulin what was the popular endogenous marker of GFR?
- 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
Describe serum creatinine as an endogenous marker of GFR
- 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
What is a downfall of estimated creatine clearance (Cockcroft Gault)?
Can overestimate GFR, especially whej <30ml/min
What is the downfall of the equation derived from cohort studies to estimate GFR? Requires age, sex, serum creatinine and ethnicity
Esrimated GFR this way may underestimate GFR is abpve-average weight and in young people
Besides serum creatinine, what is an alternative endogenous marker of kidney function?
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
When is serum creatinine most useful?
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
- What is the protein:creatinine ratio?
- What test has it replaced?
1.
- Quantitative assessment of amount of proteinuria
- Measurement of creatinine corrects for urinary concentration
- This one time measurement has replacd 24 hour urine collection
- Cumbersome and messy
- Highly inaccurate
Describe what you can do with the following urine samples:
- Single sample
- 24 hour collection
- Single sample
- Dipstick testing
- Microscopic examination
- Proteinuria quantification
- Electrolyte estimation
- 24 hour collection
- Creatinine clearance estimation
- Stone forming elements
- Proteinuria quantification
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.
A
(double check)
What can a urine dipsick test show?
- 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
- What does this image of urine microscopy show?
- What can this be caused by?
- Calcium oxalate crystals
- Kidney stones
What does this image show (urine microscopy)
Red blood cells (haematuria)
What does this image show (urine microscopy)?
White blood cells in urine
What does this image show (urine microscopy)?
Red cells casts
What does this image show (urine microscopy)?
Bacteria
