Assessment of Renal Function 1 Flashcards
(33 cards)
What is the best measure of kidney function? What is the average? How does this change with age?
Glomerular filtration rate
120ml/min normal (7.2L/hour)
Age related decline ~ 1ml/min per year
What is clearance? and what is the association between clearance and GFR?
Volume of plasma that can be completely cleared of a marker substance in unit time.
Can use clearance to calculate GFR.
What 3 criteria are necessary for a marker to be used to measure GFR via clearance?
Marker not bound to serum proteins
Freely filtered at the glomerulus
Not secreted/ reabsorbed by tubular cells
C = GFR.
What is the equation of clearance?
C = (U x V)/P
U: Urinary conc. mg/ml
P: Plasma conc. mg/ml
V: Urine flow rate in ml/min
What is the gold standard measurement for GFR? Give 5 features of this?
Inulin clearance
5.2kD fructose polymer
Neutral charge
Freely filtered
Not processed by tubular cells
Not endogenous- steady state infused required
When can you use endogenous markers to measure GFR?
Only at steady state (when renal function not changing rapidly)
What was the first endogenous marker of GFR?
Blood urea
Give 5 features of urea as a marker of GFR?
By-product of protein metabolism synthesised in liver
Freely filtered at glomerulus
Variable (30-60%) reabsorption by tubular cells
Dependent on nutritional state, hepatic function, GI bleeding
Very limited clinical value
Why is urea a bad marker of GFR when urine flow is low?
Tubular reabsorption increases at low rates of urine flow
So in fluid depletion, plasma urea conc increases even though renal function is normal
Why is serum creatinine useful as a marker of GFR?
Derived from muscle cells (small amount from intestinal absorption)
Freely filtered
Actively secreted into urine by tubular cells
Daily production is relatively stable
Describe the relationship between creatinine and GFR
Non-linear
GFR can decrease by 50% before plasma creatinine rises above ref range
Normal plasma creatinine: not necessarily normal renal function
Raised plasma creatinine: impaired renal function
What is the rate of generation of creatinine affected by?
Not equivalent in different individuals
Muscularity: proportional to mass
Age: muscle decreases with age, but so do does GFR- so creatinine conc fairly stable
Sex (M>F)
Ethnicity (higher in Afro-Caribbean)
Changes in muscle mass can change plasma creatinine conc, give examples of what may cause an increase or decrease?
Increase: refeeding, ingestion of meat rich diet
Decrease: Starvation, wasting diseases, post-surgery
What is the equation for creatinine clearance?
eCCr = (1.23 x (140-age) x weight) / serum creatinine
Adjust by 0.85 if female
What are the limitations for the equation for creatinine clearance?
Estimates creatinine clearance (not GFR)
May overestimate GFR, esp. when <30ml/min
Not widely used or validated in screening for CKD
What is eGFR?
Complex equation derived from cohort studies
Requires age, sex, serum creatinine
What is the equation for eGFR?
eGFR = 186 x (Creat x 0.0113) ^-1.154 x Age ^-0.203
Adjust by 0.742 if F
What are the limitations for eGFR?
May underestimate GFR if above average weight + young
Derived from studies on patients with CKD, may be less applicable in those with normal function/ pregnant/ children
Less accurate at high GFRs, tends to underestimate
What is a clinical alternative for serum creatinine? Give 6 features
Cystatin C
Low molecular mass peptide
Cysteine protease inhibitor
Constitutively produced by all nucleated cells at constant rate
Freely filtered
Serum conc. correlates inversely with GFR
Almost completely reabsorbed + catabolised by tubular cells
After centrifuging the urine, what should you look for in microscopy?
Crystals
Red blood cells
White blood cells
Casts
Bacteria
What can be performed instead of a 24hr urinary collection to quantify proteinuria?
Spot urine measurement
What clinical test can reliably exclude haematuria?
Urine dipstick -ve for blood
How is proteinuria used as a marker of renal damage?
Glomerular BM does NOT usually allow passage of albumin + large proteins
Impairment of integrity allows these large molecules through
Significant quantity of protein = significant damage to BM
How is proteinuria measured to correct for urine concentration?
Measure creatine
Protein : Creatinine Ratio (PCR)
