Biochemical Measurement Flashcards
What would be the ideal features of a GFR marker?
Appear at constant rate, be freely filtered at the glomerulus, not be reabsorbed or secreted from the renal tubule, not undergo extra renal elimination
What are the benefits of inulin as a GFR marker?
Appears at constant rate, freely filtered at glomerulus, not reabsorbed or secreted from renal tubule, doesn’t undergo extra renal elimination
What are the issues of using inulin as a GFR marker?
Isn’t endogenous and difficult to measure
What are the benefits of using urea as a GFR marker?
Endogenous, easy to measure, freely filtered at glomerulus, not secreted from renal tubule
What are the issues of using urea as a GFR marker?
Doesn’t appear at a constant rate, reabsorbed from the renal tubule, undergoes extra renal elimination
What are the benefits of using creatine as a marker for GFR?
Endogenous, easy to measure, appears at constant rate, freely filtered at glomerulus, not reabsorbed from the renal tubule, doesn’t undergo extra renal elimination
What are the issues of using creatine as a marker for GFR?
Its secreted into the renal tubule
What are the confounders of creatine as a GFR marker?
Age, sex, muscle mass, diet
What will frankly elevated creatine correlate with?
Biochemically obvious kidney disease
How does the kidney process creatine?
90-95% is filtered in the kidney, 5-10% is secreted by the distal tubule
What does clearance represent?
The volume of plasma that is theoretically cleared of a substance per minute
How is urinary creatine clearance calculated?
Urine creatine concentration x urine volume/serum creatine concentration x duration of collection
Can people have the same creatine concentration as each other but a different GFR?
Yes = two people can have the same serum concentration of creatine but a huge difference in GFR
What is the concept that underpins GFR measurement?
Clearance
What is stage 1 chronic kidney disease?
Kidney damage with normal or increased GFR (>=90)
What is stage 2 chronic kidney disease?
Kidney damage with mild decrease in GFR (60-89)
What is stage 3 chronic kidney disease?
Moderate decrease in GFR (30-59)
What is stage 4 chronic kidney disease?
Severe decrease in GFR (15-29)
What is stage 5 chronic kidney disease?
Kidney failure (<=15 or dialysis)
What are the to concepts related to creatine clearance?
Cockcroft-Gault and 4-variable MDRD
What are some features of Cockcroft-Gault creatine clearance measurement?
Incorporates age, sex and weight
Widely used to calculate drug dosages
What are some features of 4-variable MDRD?
Incorporates age, sex and ethnicity
Widely used on biochemistry reports
What is GFR a measurement of?
Glomerular function
What is the validated reference standard for measuring GFR?
51Cr-EDTA
What are some markers of GFR?
Creatine = widely used but variably useful
Urinary creatine clearance = more sensitive than serum creatine but mostly historical
eGFR = better than creatine, not accurate at higher GFR
What does proteinuria provide a measure of?
Glomerular function
Is protein in the urine sometimes normal?
Yes = plasma proteins are usually retained
How can proteinuria be estimated?
By doing a 24hr collection
Protein:creatine ratio also helpful
What does protein >150 mg/day in the urine signify?
Significant glomerular damage
What is microalbuminuria?
Excretion of albumin in abnormal quantities but is still below the limit of protein detection by dipstick
What is the earliest expression of diabetic nephropathy?
Microalbuminuria
What is glomerular filtration useful for?
Efficient way to selectively rid the body of waste
What occurs in counter-current multiplication?
Energy is used to generate osmotic gradient that enables water reabsorption from the tubular fluid = produces concentrated urine
