Assesment of renal function Flashcards
Why is it important to be able to assess renal function
Because of its vital role in maintaining homeostasis
2 clinical situations where measuring GFR would be particularly useful
- In patients with renal disease, progression of the underlying disease process results in nephron destruction and decreased nephron function.
- Many drugs are removed by the kidneys. When GFR falls, excretion falls so that [drug] in plasma may rise causing toxicity.
Total GFR = ?
Sum of all filtration by functioning nephrons
What result of what test could suggest progression of renal disease
Reduced GFR
When someone has reduced renal function what may have to be done to their drug doses and why
Doses may have to be reduced as decreased renal function 》decreased excretion 》increased [drug]
What body fluid is measured to determine GFR
Plasma
NOT URINE
What test accesses GFR
Plasma clearance test
Equation for accessing plasma clearance and what each symbol stand for
Cx(subscript) = [Ux(subscript)] Vx(subscript)/[Px (subscript)] Cx = Plasma clearance of x Ux = Urine concentration of X V = urine flow rate Px = plasma concentration of X
What does plasma clearance mean
The VOLUME of PLASMA cleared
NOT the QUANTITY of SUBSTANCE cleared
Gold standard plasma clearance test
Inulin clearance (polyfructose) Loading iv dose of inulin 》allow time to equilibrate 》sample plasma and urine simultaneously (during a timed urine sample).
Why is inulin clearance a good test for GFR
It s freely filtered at the glomerulus Not reabsorbed Not secreted Not metabolized by kidneys Does not interfere with normal renal function
Normal GFR
125 mls/min
What is GFR if
Uin = 285 mg/dl
V = 1.1 mls/min
Pin = 2.5 mg/dl
GFR = 285 (1.1/2.5)
=125.4 mls/min
What will be the clearance of substances that are filtered and reabsorbed, why
Lower clearance than inulin
[Ux] is lower because x reabsorbed into blood
[Px] will be higher because x is reabsorbed into blood
What will be the clearance of substances that are filtered and secreted, why
Higher than inulin
[Ux] will be higher as x is also secreted into urine, not just filtered at glomerulus
[Px] will be lower as x is removed from plasma after glomerulus by secretion
Women’s GFR compared to men
~10% lower
Why is eGFR measured more routinely than GFR
GFR is usually too complex and expensive to measure
Takes several hours
What is measured in eGFR and where does it come from
Creatinine clearance
Muscle breakdown of creatine
Factors affecting plasma creatinine
Muscle mass: athletes vs malnutrition
Dietary intake: creatine supplements vs vegetarians
Drugs: Some lead to increases as does KETOACIDOSIS
Clearance of glucose, urea and penicillin compared to inulin
Glucose = 0 (all reabsorbed) Urea = < inulin (some reabsorbed) Penicillin = > (filtered and secreted)
What’s RPF
Real plasma flow
What’s used to measure RPF
Para-amino-hippuric acid (PAH) clearance
Value of RPF
660 mls/min
What happens to PAH at the glomerulus
It is freely filtered
PAH remaining in the plasma is secreted into the tubule ( >90% of plasma is cleared of its PAH in one transit to the kidney)
What is PAH clearance a measure of
All the plasma flowing through the kidneys in a given time
Result when;
Filtration > excretion
Excretion > filtration
Filtration = excretion
Net reabsorption of x
Net secretion of x
No Net secretion or reabsorption of x
