CHEMPATH: Assessment of renal function Flashcards
What, by consensus, is the best measure of kidney function?
A. Serum creatinine itself
B. Serum creatinine and urea
C. Urine protein:creatinine ratio
D. Glomerular filtration rate
E. Cystatin C
D. Glomerular filtration rate
What is the normal GFR?
120ml/min normal (7.2L/hour)
How does the GFR change with age?
Decreases - greater decrease in males than females
Define clearance. What 3 characteristics must a marker have for its clearance to be used to estimate GFR?
Clearance = the volume of plasma that can be completely cleared of a marker substance in unit time
The marker used for clearance must be:
- not bound to serum proteins
- freely filtered at glomerulus
- not secreted/reabsorbed by tubular cells
Then a marker’s clearance = GFR
What is the equation for clearance?
C = (U x V)/P
- C = clearance of X
- U = urinary concentration of X
- V = urinary flow rate
- P = plasma concentration of X
What is the ‘perfect marker’ for clearance for GFR measurement? Why is it not used clinically?
Gold-standard = inulin (a fructose polymer, neutral charge, freely filtered and not processed by tubules)
BUT
- Steady state infusion is required
- Measurement is not simple
- So research tool only
Name 3 single-injection substances which can be used clinically to assess clearance.
- 51 Cr EDTA
- 99 Tc DTPA
- Iohexol
What is direct vs indirect clearance?
Direct - when clearance is calculated from urine collection
Indirect - when clearance is calculated from plasma regression curve
What are the pros and cons of urea being measured as a marker of GFR?
Urea was the first endogenous marker of EGFR - a by product of protein metabolism
Pros:
- Freely filtered
Cons:
- ~30-60% reabsorption by tubular cells
- depends on nutritional state, hepatic function, GI bleeding
Overall, limited clinical value.
Which factor(s) limit(s) the use of serum creatinine as a marker of GFR?
A. It is influenced by intake of fat
B. It is lower in the black population
C. It is related to muscle mass
D. It is reabsorbed by the renal tubules
E. All of the above
C. It is related to muscle mass
Where is urea vs creatinine derived from ?
Urea - protein metabolism
Creatinine - muscle cells (small amount from intestinal absorption)
What 4 factors is generation of creatinine affected by?
- Muscularity
- Age
- Sex
- Ethnicity
Does creatinine fulfil the criteria for a perfect marker to be used for GFR?
It is freely filtered BUT secreted into urine by tubular cells
Plasma creatinine concentration is inversely related to the GFR. A fall in GFR will produce a rise the the plasma creatinine concentration but GFR can decrease by a half before plasma creatinine concentration rises beyond reference range.
What is the Cockcroft Gault equation used to estimate?
Creatinine clearance (not GFR)
eCCr = (1.23 x (140 age) x weight) / serum creatinine
Adjust by 0.85 if female
Is the Cockcroft Gault equation likely to under- or over-estimate GFR?
May overestimate GFR, especially when <30ml/min
Which equation is likely to understimate GFR?
Estimated GFR equation
Complex equation derived from cohort studies (MDRD) Requires age, sex, serum creatinine and ethnicity
eGFR = 186 x ( Creat x 0.0113) -1.154 x Age -0.203
Adjust by 0.742 if female
Which equation is currently used to calculate the GFR? How is this an improvement on other equations?
Equation by CKD-EPI 2009* which is similar to the MDRD eGFR equation (using same 4 variables) but reduces bias at GFRs >60ml/min (although still imprecise at higher GFRs)
*(CKD-epidemiology collaboration 2009)
What are the 4 variables use in the MDRD eGFR equation?
- Age
- Sex
- Ethnicity
- Serum creatinine
What are the clinical alternatives to serum creatinine meaurement?
Cystatin C - a cysteine protease inhibitor constitutively produced by all nucleated cells
What are the pros and cons of Cystatin C use to measure GFR?
Pros:
- Constant rate generation
- Freely filtered
Cons:
- Almost completely reabsorbed and catabolised by tubular cells
True or false:
A spot urine measurement to quantify proteinuria can be done instead of a 24 hour urinary collection?
True - because it can be used to assess protein:creatinine ration which is a quantitative assessment of amount of proteinuria because the creatinine corrects for urinary concentration e.g. in dehydration
Define protein:creatinine ratio.
Quantitative assessment of amount of proteinuria.
Measurement of creatinine corrects for urinary concentration.
Why is PCR preferred over 24 hour urine collection?
24hr urine collection is:
- Cumbersome and messy
- Highly inaccurate without specific patient education
List three single sample and three 24hour collection urine investigations.
Single sample
- Dipstick testing
- Microscopic examination
- Proteinuria quantification
- Electrolyte estimation
24hour collection
- Creatinine clearance estimation
- Stone forming elements
- (Proteinuria quantification)
- (Electrolyte estimation)
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.
C. You can reliably exclude bacteriuria if the urine dipstick is negative for nitrites
List 5 measures on the urine dipstick.
- pH - 4.5-8 is normal
- Specific gravity - effectively the density of the urine
- Protein -sensitive to albumin, not Bence-Jones proteins
- Blood - myoglobin will also give a positive test
- Leucocyte esterase
- Nitrite - detects bacteria esp. gram negatives
How is urine microscopy done? What is the sample examined for?
Centrifuge at 3000rpm for 5- 10 minutes
Examine sediment for:
- Crystals
- Red blood cells
- White blood cells
- Casts
- Bacteria
A 50 year old, known alcoholic, presents generally unwell, seemingly intoxicated, with acute kidney injury. Urine microscopy reveals calcium oxalate crystals, what diagnosis do you suspect?
Ethylene glycol poisoning
What component of urine is shown?
Calcium Oxalate Crystals
What component of urine is shown?
RBC
What component of urine is shown?
White blood cells
What component of urine is shown? What does it indicate?
Casts - indicated glomerular damage
What component of urine is shown?
Bacteria
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
B. CT - CT is first line when you suspect kidney stones
Name 5 differrent types of renal imaging and invasive investigations.
- Plain KUB films - radiolucent stones will not show
- Intravenous urogram (IVU)
- KUB ultrasound - operator dependent
- Cross sectional CT and MRI
- Functional imaging (static and dynamic renograms)
Invasive:
- Renal biopsy - US or CT guided
