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)
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
High sg -> more concentrated (means more dehydrated)
What can urine microscopy be used to look for?
- Crystals
- RBCs
- WBCs
- Casts
- Bacteria
How can ethylene glycol poisoning cause AKI?
ethylene glycol = antifreeze
It gets converted to oxalic acid which precipitates with calcium to form calcium oxalate stones
Calcium oxalate crystals seen on microscopy
What is the most robust way of assessing kidney function?
Look at the CHANGE in kidney function (serum creatinine) with an individual over time
Does negative nitrite on urine dipstick reliably exclude bacteriauria?
No
Nitrites in urine are from reduction of nitrates. Only gram-ve bacteria can do this, so does not exclude infection with a gram+ve organism
What do RBC casts on urine microscopy suggest
Glomerular dysfunction
