Renal Part 1 Flashcards
What is the best measure of kidney function
GFR
What is the normal GFR? and age-decline
120 mL/minute 7.2L/hour
age-related decline of around 1 mL/min per year
Clearance definition and use
volume of plasma that can be completely cleared of a marker substance per unit time
Clearance can be used to calculate GFR
3 criteria to be fulfilled for a marker to be used to measure GFR
- Marker is NOT bound to serum proteins
- Freely filtered by the glomerulus
- NOT secreted or reabsorbed by tubular cells
If these conditions are fulfilled, then clearance = GFR
at any one point:
- C = (U X V)/P
Inulin clearance
Exogenous, gold-standard GFR measure
- 5.2KD fructose polymer of neutral charge
- Freely filtered and not processed by the tubular cells I.E. the perfect marker
However, a steady state infusion is required, and measurement of inulin concentration is not simple
Thus, it is used as a research tool only
Exogenous markers of GFR: urea
- First endogenous marker of GFR; a by-product of protein metabolism
- Variable (30-60%) reabsorption by tubular cells (you DON’T want any reabsorption…)
- Dependent on nutritional state, hepatic function, GI bleeding
- Very limited clinical value
Serum creatinine
rate of generation affected by (4)
Derived from muscle cells, freely filtered and actively secreted into the urine by tubular cells
Creatinine: GFR relationship is non-linear:
- At lower GFRs, level of creatinine is less accurate at predicting precise GFRs
The rate of generation of creatinine is affected by:
- Muscularity (proportional to mass)
- Age
- Sex (higher in men)
- Ethnicity (higher in Afro-Caribbean)
Cockcroft-gault adjustment
estimate creatinine clearence
Not the GFR directly (just creatinine)
May overestimate low GFRs (<30mL/min)
Estimated GFR adjusted equation/MDRD
- Complex equation derived from cohort studies
- Requires information about age, sex, serum creatinine and ethnicity → got rid of weight so equation more practical
- May underestimate GFR if above average weight and young
CKD epidemiology collaboration (CKD-EPI)
The equation is based on the same four variables as MDRD but models the relationship between GFR and serum creatinine, age, sex and race differently
It is an improvement on MDRD, but it is still imprecise at higher GFRs
- Reduces bias at GFRs >60mL/min (but imprecise at higher GFRs)
- I.E. Accurate at LOW GFRs and less accurate at HIGH GFRs
MDRD AND CKD-EPI both tend to UNDERESTIMATE GFR (but CKD-EPI less so)
Cystatin C
- This is an alternative endogenous marker
- This is constitutively produced by all nucleated cells at a constant rate and is freely filtered
- Almost completely reabsorbed and catabolised by tubular cells
- NOTE: CKD NICE guidelines have included cystatin C, however, it is not used that frequently
Summary of endogenous markers
- Serum creatinine is an insensitive marker of GFR
- Other endogenous markers (i.e. cystatin C) are better
- Constant rate infusion GFR measurement is a research tool
- Single injection GFR measurement is reserved for specific situations
- In practice, estimated GFR/creatinine clearance is the best compromise
What is the most robust value of serum creatinine measurement?
Determining the change in kidney function within a individual over time
A spot urine measurement to quantify proteinuria can be done instead of a 24hr urinary collection
true or false?
TRUE
If a dipstick measurement is -ve for blood, it reliably excludes haematuria
true or false?
TRUE
Urine protein:creatinine ratio (PCR)
This is a quantitative assessment of the amount of proteinuria
Measurement of creatinine corrects for urinary concentration
Two methods to do PCR:
- 24hr urine collection (cumbersome and messy; highly inaccurate without patient education)
- Spot urine measurement
Urine examination (single sample vs 24 hour collection)
Urine dipstick testing
protein sensitivity
what else does it look at
pH = 4.5 - 8.0
Specific gravity = 1.003 - 1.035 (Bowman’s space 1.007 - 1.010)
Protein = sensitive to albumin, not BJPs (zero → trace → 1+ to 4+)
Blood
Leucocyte esterase (-ve result is significant - reliably excludes bacteria)
Nitrite (detects bacteria, esp. G-ves, cannot reliably exclude bacteria if -ve)
Urine dipstick testing
protein sensitivity
what else does it look at
pH = 4.5 - 8.0
Specific gravity = 1.003 - 1.035 (Bowman’s space 1.007 - 1.010)
Protein = sensitive to albumin, not BJPs (zero → trace → 1+ to 4+)
Blood
Leucocyte esterase (-ve result is significant - reliably excludes bacteria)
Nitrite (detects bacteria, esp. G-ves, cannot reliably exclude bacteria if -ve)
Urine microscopy: metro and what it examines
Case presentation:
- 50yo, alcoholic
- Presents unwell, seemingly intoxicated with AKI
- Urine microscopy = calcium oxalate crystals
Diagnosis = ethylene glycol poisoning (anti-freeze)
ethylene glycol metabolises to form calcium oxalate crystals
Renal imaging 1st line and 2nd line
benefit of 2nd line in differentiating 2 conditions
- 1st line: CT KUB
-
2nd line: Ultrasound KUB
- This can differentiate AKI and hydronephrosis
- Plain KUB films (can show ‘staghorn calculi’)
- IV urogram (done more in paediatrics to look for anatomical defects)
- MRI KUB
- Functional imaging (static and dynamic renograms)
- IV radiolabelled nuclei are injected, and kidney uptake is measured
- Any kidney not showing up on scans signifies a non-functional kidney
- Renal biopsy is often necessary for various diagnoses (ultrasound or CT guided)
Summary
- GFR is best measure of kidney function, but it is not easy to measure
- Urinalysis – dip, microscopy, protein measurements can be used in diagnosis and monitoring
- Renal imaging should be tailored to what you are trying to diagnose
