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)
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
- freely filtered
- 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.
What is urine protein: creatinine ratio?
- Quantitative assessment of proteinuria
- Measurement of creatinine corrects for urine concentration
albumin:creatinine used in diabetes, better for small amounts of protein
How is proteinuria estimated?
- Spot urine protein: creatinine ratio (PCR)
NOTE: this has superseded 24-hour urine collection
Aside from blood, what else can cause a urine dipstick to be positive for blood?
Myoglobinuria (from rhabdomyolysis)
What is specific gravity?
A measure of urine concentration
What can urine microscopy be used to look for?
- Crystals
- RBCs
- WBCs
- Casts
- Bacteria
How can ethylene glycol poisoning cause AKI?
It gets converted to oxalic acid which precipitates with calcium to form calcium oxalate stones
ACEi effect on creatinine in patient with diabetes
initally elevates creatinine (decreases eGFR)
but is protective in long term –> prevents end stage renal failure
What people with diabetes should NOT be on ACEi?
What is done to monitor this
if they have diabetes AND renal artery stenosis
ACEi will make them anuric, dramatic drop in EGFR
Therefore need to check renal function soon after starting ACEi
