ChemPath: Assessment of Renal Function 1 Flashcards
What is normal GFR?
120 mL/min (7.2 L/hour)
At what rate does age affect GFR?
Declines by 1ml/min per year
Define clearance.
The volume of plasma that can be completely cleared of a marker substance per unit time.
What are the three criteria for a marker to be used to measure GFR?
- Marker is not bound to serum proteins
- Freely filtered by the glomerulus
- Not secreted or absorbed by tubular cells
State the equation that links clearance with urine and plasma concentration.
C = (U x V/P)
U = urinary concentration
P = plasma concentration
V = urine flow rate (ml/min)
What is inulin and what is its main purpose?
- Neutral, freely filtered fructose polymer that is technically the perfect marker
- However, measurement of inulin concentrations is difficult and it requires a steady-state infusion, hence not practical
- So, it is only used as a research tool
Name three single injection plasma clearance markers/
- 51Cr-EDTA
- 99Tc-DTPA
- Iohexol
Describe how single injection plasma clearance markers are used.
The injection is administered followed by either
1. measurement of urine collection using a Geiger counter
2. take blood samples to look for a progressive reduction in radioactivity.
NOTE: this is only used under specific circumstances (e.g. when accurate estimation of GFR is necessary before chemotherapy)
Describe the key features of plasma urea.
- By-product of protein metabolism
- Variable reabsorption (30-60%) by tubular cells -> hence does not meet 3 criteria
- Dependent on nutritional state, hepatic function and GI bleeding
i.e.
urea prod increased with high protein intake, gi bleeding
urea prod decreased with low protein intake, liver disesae - Limited clinical value
Where does serum creatinine come from?
Muscle cells
Which features of serum creatinine make it a useful marker of GFR?
- Freely filtered
- Produced at a constant rate
NOTE: it is actively secreted into the urine by tubular cells
List some factors that affect the rate of generation of creatinine.
- Muscularity
- Age
- Sex
- Ethinicity
Name and briefly describe three equations that are used to estimate creatinine clearance or GFR.
- Cockcroft-Gault - estimates creatinine clearance (NOT GFR)
- takes into account age, sex, weight, creatinine
- may overestimate when GFR <30ml/min
- MDRD - estimates GFR from creatinine clearance
- takes into account age, sex, ethnicity, serum creatinine
- may underestimate in overweight and young people
- CKD-EPI
- same 4 variables as MDRD but models relationship better
- is currently recommended
Note all of these are accurate at low GFRs but are less accurate at normal/near normal kidney function, though CKD-EPI is the best
Note MDRD and CKD-EPI tend to underestimate GFR (but CKD-EPI less so)
Name an alternative endogenous marker of GFR and its pros & cons
Cystatin C
- generated at a constant rate
- freely filtered
- almost completely reabsorbed and catabolised by tubular cells
pros:
- not affected by weight, height, age, sex, muscle
- more accurate than creatinine
cons:
- increased in malignancy, hyperthyroidism, and corticosteroids use
What is urine protein: creatinine ratio?
- Quantitative assessment of proteinuria
- Measurement of creatinine corrects for urine concentration (due to diff in hydration status)
