Electrolyte and Renal Function Test Flashcards
(51 cards)
➢ The volume of plasma that would theoretically have to be “cleared” of the substance to account for the amount of the substance excreted in the urine during a given period.
➢ Relates the rate of urinary excretion of material to the plasma concentration of that material
RENAL CLEARANCE
➢ That amount of substances that is concentrated in the urine
➢ The plasma concentration of the substance to determine the volume of plasma needed to account for the material excreted
Cx=(UxV) / Px mL/minute
Note: Cx – clearance of a substance x
Ux – concentration of the substance in urine
Px – concentration of the substance in plasma
V – volume of urine per unit time
Clearance in mL/24 hours can be converted to clearance in mL/minute
➢ Dividing the value by
1440 (because 24 hours equals 1440 minutes)
Best overall indicator of the level of kidney function
Declines with age
➢ After age 20 to 30 years
➢ Decrease by ~1.0mL/min/1.73 m2 per year
GLOMERULAR FILTRATION RATE
Important! Molecular marker to determine GFR must be _____________________________ and __________________
minimally reabsorbed and minimally secreted by the renal tubules
MEASUREMENT OF GFR WITH EXOGENOUS SUBSTANCES
Inulin clearance
Alternative measures
➢ Urinary clearance of exogenous radioactive markers
➢ Plasma disappearance of exogenous substances
Nonradiolabeledbiothalamate in blood and urine
MEASUREMENT OF GFR WITH ENDOGENOUS SUBSTANCES
➢ Urea
➢ Creatinine
➢ Cystatin C
➢ B trace protein [BTP]
➢ B-2 microglobulin
➢ Tryptophan glycoconjugate
MEASUREMENT OF GFR WITH ENDOGENOUS SUBSTANCES that is most widely used*
*urea & creatinine – widely used; readily available
➢ Endogenous substance
➢ MW: 113 Da
Sources: ➢ Produced by the muscle from creatine and creatine phosphate through a nonenzymatic dehydration process
Rate of production of is proportionate to the creatine - creatine phosphate pool, which, in turn, is proportionate to the muscle mass
➢ Ingested meat or dietary supplements
CREATININE AS MEASURE OF RENAL FUNCTION
What are the sources of creatinine?
Sources:
➢ Produced by the muscle from creatine and creatine phosphate through a nonenzymatic dehydration process
Rate of production of creatinine is proportionate to the creatine - creatine phosphate pool, which, in turn, is proportionate to the muscle mass
➢ Ingested meat or dietary supplements
o The rate of in vitro conversion of creatine to creatinine in meat is dependent on ________ and _______.
temperature and acidity
NOTE :o High temperature and low pH increase conversion
Why is Creatinine widely used as a marker of GFR?
➢ It is an endogenous substance with a fairly constant rate of production
➢ It is not bound to plasma protein; therefore it is filtered freely by the glomerulus
➢ It is not reabsorbed by the renal tubules, and only a small amount is secreted by the tubules
Creatinine as a Measure of Renal Function Drawbacks:
➢ Although the rate of production is fairly constant, it has substantial individual variation, depending mainly on the muscle mass
o Presence of severe muscle wasting, production of creatinine could be reduced to less than 25% to the amount predicted from the body weight
➢ Quantity of meat ingestion can substantially influence total daily production
➢ A number of chromogens, both endogenous and exogenous, interfere with its measurement by this technique
➢ Most widely used method of creatinine measurement
Alkaline picrate method
➢ reaction of creatinine with trinitrophenol, an explosive, a compound also known as picric acid, enhanced by alkaline condition
Jaffe reaction
alkaline picrate method falsely high result in the presence of:
high levels of glucosediabetic ketoacidosis
hyperglycemic coma
dialysate fluid used in peritoneal dialysis
cephalosporin antibiotics
➢ alkaline picrate method
- falsely low result in the presence of:
bilirubin haemoglobin
➢ creatinine is partially secreted by the ______________________
proximal tubules via the organic cation transport pathway
➢ tubular secretion is blocked by various drugs
cimetidine, trimethoprim, pyrimethamine, and salicylate
➢ presence of renal dysfunction
o tubular secretion could involve as much as___________ of the excreted in the urine
50%
Creatinine Measurement
➢ Alkaline picrate method
➢ Enzymatic measurement with the use of creatinine amidohydrolase or creatinine iminohydrolase
➢ Converting creatinine to creatineaminohydrolase
➢ Isotope dilution – mass spectrometry (IDMS)
FORMULAS TO ESTIMATE CREATININE CLEARANCE OF AN ESTIMATE OF GFR
Cockroft-Gault formula (mL/minute) (cockroft, 1976):
([140-age]•[IBW])/(72xSCr); multiply by 0.85 if female
NOTE :Where IBW is ideal body weight and SCris serum creatinine concentration
IBW is calculated by the following formula:
Males: IBW = 50kg + 2.3kg for each inch over 5 feet
Females: IBW = 45.5kg + 2.3kg for each inch over 5 feet
Schwartz Formula:
GFR = 0.55 x height (cm)/serum creatinine (mg/dL)
GFR = 48 x height (cm)/serum creatinine (umol/L)
Modified Schwartz Formula:
GFR = (mL/min/1.73 m2 ) = 39.1[height (m)/Cr (mg/dL)]0.516 x [1.8/cystatin C (mg/L)]0.294 [30/BUN (mg/dL)]0.169 [1.099]0.210 [height (m)/1.4]
