Acute and Chronic Renal Failure 2 Flashcards
How can we measure kidney function?
GFR (Gold Standard- eGFR is used in clinical practice)
Creatinine and Urea
What is renal function?
A - controlling ACID-base balance.
W - controlling WATER balance.
E - maintaining ELECTROLYTE balance.
T - removing TOXINS and waste products from the body.
B - controlling BLOOD PRESSURE.
E - producing the hormone ERYTHROPOIETIN.
D - activating vitamin D
What is an normal GFR?
120ml/min normal (7.2L/hour)
What is the yearly age related decline of eGFR (and when does this start)?
1ml/min/year
How do we calculate GFR from clearance?
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
What is the formula for clearance?
At any one time:
C = (U x V)/P
U – urinary conc
P – plasma conc
V – volume
What is the gold standard measure of GFR?
inulin clearance
the perfect marker- steady state infusion is required and it’s not easy so it’s only used in research
What is inulin?
5.2kD fructose polymer
Neutral charge
Freely filtered
Not processed by tubular cells
What are the clinically viable measures of GFR?
Single injection plasma clearance measurements:
51Cr-EDTA
99Tc- DTPA
Iohexol
What is direct and indirect measure of clearance?
Direct: Clearance calculated from urine collection
Indirect: Clearance calculated from plasma regression curve
What are the ideal characteristics of endogenous markers of clearance?
- Not plasma protein bound
- Freely filtered at glomerulus
- Not modified by tubules
If there is steady state production of an endogenous marker, what is the formula for clearance?
Clearance = (U x V)/P
Why is blood urea a bad marker of GFR?
Variable (30-60%) reabsorption by tubular cells
Dependent on nutritional state, hepatic function, GI bleeding
Very limited clinical value
Why might urea be a good GFR marker?
By-product of protein metabolism
Freely filtered at glomerulus
What are the characteristics of creatinine as an endogenous GFR marker?
Derived from muscle cells (small amount from intestinal absorption)
Freely filtered
Creatinine is actively secreted into urine by tubular cells
Which demographics have variable creatinine?
- Muscularity
- Age
- Sex
- Ethnicity
What is the Cockcroft Gault equation?
eCCr = (1.23 x (140-age) x weight) / serum creatinine
Adjust by 0.85 if female
Estimates creatinine clearance (not GFR)
May overestimate GFR, especially when <30ml/min
What is the Complex equation derived from cohort studies (MDRD) for eGFR?
eGFR = 186 x ( Creat x 0.0113) -1.154 x Age -0.203
Adjust by 0.742 if female
underestimates GFR if above average weight and young
What is the current recommendation of estimation of GFR?
CKD-Epidemiology Collaboration (CKD-EPI) 2009 : The equation is based on the same four variables as the MDRD Study equation but models the relationship between GFR and serum creatinine, age, sex, and race differently.
Improves on the MDRD equation by reducing bias at GFRs >60ml/min but still imprecise at higher GFRs
What is cystatin C?
13.6kD protein
Cysteine protease inhibitor
Constitutively produced by all nucleated cells
Why can Cystatin C be used for eGFR?
Constant rate generation
Freely filtered
Almost completely reabsorbed and catabolised by tubular cells
How is renal function measured in practice?
Serum creatinine is an insensitive marker of GFR
Other endogenous blood markers (ie Cystatin C) are better
Constant rate infusion GFR measurement is a research tool
Single injection GFR measurement is reserved for specific situations
In practice, estimated GFR / CCr is the best compromise
What is the urine protein:creatinine ratio?
Quantitative assessment of amount of proteinuria
Measurement of creatinine corrects for urinary concentration
What are the negative aspects of 24 hour urine collection?
Cumbersome and messy
Highly inaccurate without specific patient education
Estimation of proteinuria superceded by urinary PCR
What can you do on a single sample urine examination?
Dipstick testing
Microscopic examination
Proteinuria quantification
Electrolyte estimation
What can you do with a 24 hour urine collection?
Creatinine clearance estimation
Stone forming elements
(Proteinuria quantification)
(Electrolyte estimation)
What can be measured on a dipstick?
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
How do you do urine microscopy?
Centrifuge at 3000rpm
5-10 minutes
What are you looking for in sediment in urine microscopy?
- Crystals
- Red blood cells
- White blood cells
- Casts
- Bacteria
What forms of renal imaging exist?
Plain KUB films Intravenous urogram (IVU) KUB ultrasound Cross-sectional imaging (CT and MRI) Functional imaging (static and dynamic renograms)
How can a renal biopsy be done?
US or CT guided percutaneous renal biopsy
What is the best measure of kidney function?
GFR is best measure of kidney function, but it is not easy to measure
What is the functional unit of the kidney?
The nephron
What can be excluded if there are no nitrites in urine?
You can reliably exclude bacteriuria if there are no nitrites in the urine
Other than blood what may give positive blood on dipstick?
Myoglobin
A 50 year old known alcoholic presents generally unwell and seemingly intoxicated with AKI. Urine microscopy reveals calcium oxalate crystals, what diagnosis do you suspect?
Antifreeze poisoning (Ethinyl glycol)
You admit a 28 year old man who you suspect has a renal stone, what is your first choice of imaging?
CT KUB
