This is a non protein nitrogenous
compound. Measurement of excretion rate of creatinine is used as diagnostic indicators
of kidney function. Creatinine is synthesized in kidney liver and pancreas. A
proportion of free creatine in muscle (1-2%/day) spontaneously and irreversible
converts to its anhydride creatinine. So creatinine production is fairly
constant and related to muscle mass.
Measurement:
Measurement:
Chemical method:
Measured by Jaffe’s reaction of
alkaline picrate method. Here creatinine reacts with picrate ion in an alkaline
medium to yield an orange red complex. Although many other compounds like
protein, glucose, ascorbic acid, ketone bodies, bilirubin (negative
interferent), and cephalosporins can produce positive interfere with the test.
True creatinine can be measured by
using hydrated aluminum silicate (Fuller’s earth, Lloyd’s reagent). Here
creatinine will only get adsorbed and precipitated. The precipitate is eluted
and the creatinine is measured by alkaline picrate method. Jaffes method
overestimates true plasma creatinine 20% due to non creatinine interferences.
Kinetic method: These are more
sensitive, specific and lack of interferences. Early studies of interferences
into the kinetic methods identified two kinds of noncreatinine chromogens,
those whose rate of adduct formation were very rapid in first 20 seconds after
mixing reagent and sample (e.g. acetoacetate) and those whose rates did not
become rapid until 80 to 100 s after mixing (e.g. protein). The window between
20 and 80 therefore was a period of signal produced by creatinine-picrate
reaction. These methods are now widely used. This is kinetic chemical method.
Enzymatic methods:
This is more popular method.
Addition of potassium ferricyanide or bilirubin oxidase is done to remove
bilirubin interference. Interference by ascorbic acid is removed by adding
ascorbate oxidase. This system is also included in POCT device using
polarographic detection.
Dry chemistry system:
Multilayer dry reagent methods are
available for creatinine measurement using enzyme reactions in POCT. A two
slide approach employs creatinine deaminase, which deaminates creatinine with
the ammonia diffusing through a semipermeable and optically opaque layer to
react with bromophenol blue to give an increase in absorbance at 600 nm. A 2nd
multilayer film lacking the enzyme was used to quantitate endogenous ammonia,
enabling blank correction. A later single-slide method used the creatininase
creatinase reaction sequence. Here the color produced in the film is quantified
by reflectance.
Reference value
Serum/plasma creatinine = 0.7 – 1.5 mg/dl in men and 0.6 – 1.1 in women.
Urine = 0.7 – 1.8 gm/day
In ESRD creatinine may exceed 11 mg/dl.
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