Practical 5: Use of Serum Tests to Detect Renal Dysfunction
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What are the two most common tests ordered to detect renal dysfunction?
Serum creatinine Serum urea
What is urea the product of
AN end product of protein metabolism
When might high urea be seen? (4)
High protein diet Excessive tissue breakdown In the presence of GI bleeding Dehydration
What is the role of the kidneys in relation to urea (3)
Kidneys regulate urea livers Urea is filtered in the glomeruli and reabsorbed in the tubules Excretion of urea decreases when GFR drops
Why does a drop in GFR result in a decrease in urea excretion
The longer the tubular fluid remains in the kidney the greater the reabsorption of urea into the blood
What is creatinine
A product of creatine metabolism in muscle
What do blood values of creatinine depend on?
Blood concentration of creatinine depends closely on GFR
What is normal creatinine level proportional too
Proportional to muscle mass
What does it mean if creatinine value has doubled
GFR and renal function has probably fallen to half
Is serum urea or serum creatinine a more specific indicator of GFR
Serum creatinine is more specific -> serum urea concentrations may vary independently of the GFR but a rise in serum creatinine almost always represents a reduction in GFR
When might you see low urea laevel
Not eating properly or not able to keep food down
When might you see high urea levels (4)
Unusually high protein intake GI bleed Infection Inadequate dialysis
What may indicate an inadequate dialysis
Lack of appetite for protein-containing foods or nausea when eating them
What might cause an elevation of urea
Increased metabolism -> can be the result of fever Some drugs that affect metabolism (creatinine might also be affected)
What might cause a decrease in urea/creatinine
Muscle loss (could be from not eating)
What cause uraemia? (3)
The accumulation of waste products in the blood Urea (blood urea nitrogen) and creatinine Uremia affects other organs in the body not just the kidneys
What is the principle behind measuring creatinine (2)
Creatinine in alkaline solution reacts with picric acid to form a coloured complex The amount of complex formed is directly proportional to the creatinine concentration
What is your blank for the creatinine assay
Air
What absorbance do you read your creatinine assay at
492nm
What is the method used to measure urea concentration?
Sigma or Randox kit
What is the principle behind the sigma/Randox kit (3)
Urease hydrolyses urea in the sample to liberate ammonia Ammonia reacts with phenol in the presence of hypochlorite to produce a blue coloured compount The reaction is catalysed by Nitroprusside (Berthelot’s reaction)
For how long do you incubate the urea assay
10 minutes at 37 degrees after adding 1st reagent (reagent 2) 15 minutes at 37 degrees after adding reagents 3 and 4
At what absorbance do you read the urea assay
546nm
What is MDRD
Modification of Diet in Renal Disease Study equation