ChemPath: Assessment of Renal Function 1 Flashcards
What is glomerular filtrate
Ultrafiltrate of plasma without protein (bigger than albumin) and positively charged proteins?
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)
V – urine flow rate
P – plasma concentration
NOTE: Only valid for steady state
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
- 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 measurement of urine collection using a Geiger counter or blood samples can be taken 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 absorption (30-60%) by tubular cells
- Dependent on nutritional state, hepatic function and GI bleeding
- 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 by taking into account weight, age and sex (may overestimate when GFR <30ml/min)
- MDRD - estimates GFR from creatinine clearance and takes into account age, sex, serum creatinine and ethnicity (may underestimate in overweight and young people)
- CKD-EPI - improvement of MDRD and currently recommended
Name an alternative endogenous marker of GFR.
Cystatin C
NOTE: it is constitutively produced by all nucleated cells, is generated at a constant rate and freely filtered. It is almost completely reabsorbed and catabolised by tubular cells.
