Biochemistry Flashcards
What two functions of the kidneys are assessed using biochemical measurements in renal disease?
- Glomerular function
- Tubular function
What do…
- Glomerular function
- Tubular function
… actually mean?
- Glomerular function: the ability of the glomeruli to filter the blood
- Tubular function: the ability of the renal tubules to reabsorb the filtered blood from the glomeruli
Glomerular function can be assessed by investigating which two factors?
- Glomerular filtration rate (GFR)
- Proteinuria
What is the single best test for assessing kidney function?
GFR
Describe some properties that the ‘ideal marker’ for assessing glomerular filtration rate (GFR) would have
- Endogenous (produced by the body)
- Easy to measure (e.g., in urine)
- Appears at a constant rate
- Freely filtered at the glomerulus
- Not reabsorbed from the renal tubule
- Not secreted into the renal tubule
- Doesn’t undergo extra-renal elimination
Name the plant carbohydrate which gives an almost perfect indication of GFR
Inulin
Why is measuring inulin clearance not gold standard for assessing GFR?
Inulin is not endogenous - it needs to be injected into the body before being measured which makes it impractical
Clearance of which two substances can be used as a marker for GFR instead of inulin?
Creatinine (main one)
Urea
What is urea?
The final breakdown product of amino acids found in proteins and the main component of urine
What properties make urea a…
-Good
-Bad
… marker for assessing GFR?
Good:
- Endogenous
- Easy to measure
- Freely filtered by the glomerulus
- Not secreted into the renal tubule
Bad:
- Levels fluctuate post-prandially (not present at a constant rate)
- Reabsorbed from the renal tubules
- Extra-renal elimination by the gut so GFR will be underestimated
What is the most significant limitation for use of urea as a marker of GFR?
It’s extra-renal elimination by the gut (~25% of total urea)
What is creatinine?
A waste product of muscle metabolism
What properties make creatinine a good marker for assessing GFR?
- Endogenous
- Easy to measure
- Appears at a constant rate
- Freely filtered by the glomerulus
- Not reabsorbed from the renal tubule
- Doesn’t undergo extra-renal elimination
What is the limitation of using creatinine as a marker of GFR?
~5-10% of creatinine is secreted into the renal tubule so GFR is overestimated
What is the problem with measuring serum creatinine and what is the solution?
Problem: serum creatinine isn’t sensitive to changes in GFR until the GFR is significantly lowered (i.e., serum creatinine stays within the reference interval even when GFR has nearly halved)
Solution: urine creatinine clearance is sensitive to changes in GFR at these higher GFR levels (60-120 mL/min)
What is urinary creatinine clearance?
The volume of blood that’s cleared of creatinine per minute by glomerular filtration