FPC: assessing renal function Flashcards
What are the main functions of kidneys?
Excretion of waste products, maintenance of fluid balance in the body, and production/secretion of enzymes and hormones.
In what instances is kidney function a factor in prognosis?
Risk of CKD complications, risk of CVD, risk of mortality.
What is the best measure of kidney function?
GFR.
What is the basic equation for GFR?
Net filtration pressure x area.
What does a declining GFR mean?
Could be due to CKD or an acute and reservable problem.
Describe GFR measurement’s role in routine care.
The measurement is complex, time consuming, and expensive, so it isn’t really practical in routine care. It’s more important to know if it is changing over time, so usually we estimate it.
What is the best method to measure GFR?
Measure urinary clearance of an ideal filtration marker from the plasma over a period of time.
What is true of ideal filtration markers?
- They have a stable plasma concentration. 2. They’re completely filtered at the glomerulus. 3. They are not reabsorbed, secreted, or metabolized by the kidneys.
What is the gold standard to measure GFR?
Inulin clearance measurement; cumbersome and expensive but works.
What is a less cumbersome way to measure GFR compared to inulin clearance?
Measuring clearance of exogenous agents that behave like inulin; often chelated to radioisotopes. Still expensive and complex.
How is GFR measured in clinical practice?
Creatinine and cystatin C.
How is BUN impacted by decreased GFR?
It decreases.
Why is BUN not used to measure GFR?
It is impacted by many other factors outside kidney function.
What is creatinine?
End product of skeletal muscle metabolism that is continuously excreted by the kidneys.
What are disadvantages of using creatinine to estimate GFR?
Its production varies among people, and it is secreted in small amounts by the renal tubules.
What are advantages to using creatinine to measure GFR?
It is endogenous, it is released into circulation constantly, freely filtered by the glomerulus and not reabsorbed or metabolized in the kidney, easy to measure and low-cost.
Why is serum creatinine limited in measuring kidney function?
The relationship between sCr and GFR is parabolic and not linear, and many factors may impact sCr.
What medications increase sCr?
Trimethoprim and cimetidine.
What medications decrease sCr?
Antibiotics.
What are disadvantages of using CrCl in estimating GFR?
It overestimates it, it is highly susceptible to error, and you have to collect urine on 2 separate occasions to improve accuracy.
What variables are used in the Cockcroft-Gault equation?
Age, lean body weight, and sCr.
What variables does the MDRD equation use?
sCr, age, sex.
What are advantages of the MDRD equation?
It is adjusted for body surface area and is more accurate than measuring creatinine clearance. It is also validated in multiple populations.
What variables does the CKD-EPI equation use?
sCr, age, sex.
Compare CKD-EPI with MDRD.
CKD-EPI tends to be more accurate and is the most widely used method.
Describe cystatin C.
Produced at a constant rate by cells and is less variable and more accurate between people than Cr. All of its removal is via the kidneys and is metabolized by tubules with only small amounts excreted in the urine.
What are the disadvantages of using cystatin C to measure GFR?
It isn’t validated in all populations and its generation can be affected by acute diseases. It is also more expensive in comparison to Cr.
When should cystatin C be used?
In cases where Cr may be inaccurate or in confirming CKD diagnosis when Cr eGFR is 45-60 and no other features of CKD are present.
What are caveats of GFR estimating formulas?
Patient must be in steady state.
What is proteinuria?
Increased protein in the urine; can indicate kidney damage.
What is glomerular proteinuria?
Increased glomerular permeability to large proteins like albumin (most common proteinuria!).
What is tubular proteinuria?
Incomplete tubular reabsorption of normally filtered proteins like B2 macroglobulin.
What is overproduction proteinuria?
Overproduction and excessive filtration of low MW proteins that exceed tubular reabsorption capacity, like immunoglobulin light chains.
What is albuminuria?
Abnormal loss of albumin in urine; marker of damage to filtration barrier.
Albuminuria is related to…
CKD progression/ESRD and CV risk.
How can we detect protein in the urine?
Urine dipstick, 24-hour urine collection, or random urine protein creatinine ratio/random urine albumin creatinine ratio.
Urine dipstick only measures…
Severe albuminuria.
What is the gold standard to measure proteinuria?
24-hour urine collection, but it can be inaccurate.
