clinical diagnostic biochemistry - 12 dr. maha al-sedik 2015 cls 334
TRANSCRIPT
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Clinical diagnostic biochemistry - 12
Dr. Maha Al-Sedik2015
CLS 334
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Renal function -2
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LABORATORY TESTS AIDING
IN THE EVALUATION OF
KIDNEY FUNCTION
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Glomerular functionGlomerular filtration rate
( clearance tests )
Non protein nitrogen ( NPN )
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U X V C = P
Clearance is defined as the quantity of blood or plasma
completely cleared of a substance per unit of time.
Clearance tests
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Definition of glomerular filtration rate:
The amount of plasma pass through glomeruli
per unit time.
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Some substances are filtered, enter tubules, but
are completely reabsorbed, so they did not reach
the final urine, so its clearance is zero (e.g.
Glucose).
Some substances when filtered enter the tubules
are not reabsorbed and so 100% in urine, So its
clearance = GFR (inulin clearance = gold standard
for GFR).
VIP
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Creatinine clearance also used for determination
of GFR .(but small amount of creatinine is
reabsorbed, so its cclearance <GFR.
Some substances are filtered, enter tubules, and
more of the substance is secreted enters the
tubules by excretion. Clearance>GFR
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volume / 24 hoursHow to measure V per minute ? : 24 X 60
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Definition of glomerular filtration rate:
The amount of plasma pass through glomeruli
per unit time.
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The most frequently used clearance test is based on
the measurement of creatinine.
Small quantity of creatinine is reabsorbed by the
tubules, So creatinine clearance is approximately 7%
greater than inulin clearance.
An estimate of the GFR can be calculated from the
creatinine content of a 24-hour urine collection, and
the plasma concentration within this period.
Creatinine clearance and clinical utility
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The volume of urine is measured, urine flow rate is
calculated (ml/min) and the assay for creatinine is
performed on plasma and urine to obtain the
concentration in mg per dl or per ml.
Creatinine clearance in adults is normally about of 120
ml/min,
The accurate measurement of creatinine clearance is
difficult, especially in outpatients, since it is necessary
to obtain a complete and accurately timed sample of
urine.
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• Urine creatinine: 100 mg / dl
• Serum creatinine: 1.4 mg / dl
• Urine volume: 1000 ml
• Urine collection time : 24 hours
Calculate Creatinine clearance:
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• Urine creatinine: 110 mg / dl
• Serum creatinine: 2 mg / dl
• Urine volume: 1500 ml
• Urine collection time : 24 hours
Calculate Creatinine clearance:
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• Urine creatinine: 110 mg / dl
• Serum creatinine: 1 mg / dl
• Urine volume: 1500 ml
• Urine collection time : 24 hours
Calculate Creatinine clearance:
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• Urine creatinine: 120 mg / dl
• Serum creatinine: 1.5 mg / dl
• Urine volume: 2000 ml
• Urine collection time : 24 hours
Creatinine Clearance = [(140 - age) x BW ] / (Serum cr x 72)
Calculate Creatinine clearance:
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Non protein nitrogen
( NPN )
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Non protein nitrogen
Creatinine
Urea
Uric acid
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Creatine is formed in the liver from arginine , glycine
and methionine.
It is transported to the muscles and converted to
phosphocreatine to be used as source of energy .
Creatine phosphate loses phosphoric acid to form
creatine.
Creatine loses H2O and form creatinine.
Normal range:
o Male 0.6-1.2 mg/dL.
o Female 0.5-1.0 mg/dL.
Creatinine
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Clinical significance:
☺Creatinine is produced endogenously and released
into body fluids at a constant rate and its plasma
concentration is maintained within narrow limits
predominantly by glomerular filtration.
☺Consequently, creatinine renal clearance ("creatinine
clearance") have been used as markers of the
glomerular filtration rate (GFR).
☺Creatinine level can be affected by:
• Muscle mass.
• Renal functions.
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Analytical Methodology:
Plasma creatinine is commonly measured using either
chemical or enzymatic methods.
I. Chemical methodst: Jaffe Reaction
Most chemical methods for measuring creatinine are
based on its reaction with alkaline picrate. As first
described by Jaffe in 1886, creatinine reacts with
picrate ion in an alkaline medium to yield an orange-
red complex.
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Disadvantage of Jaffe Reaction:
Lack of specificity of the test for creatinine.
Many compounds produce Jaffee like reaction such as
:
(1)ascorbic acid,
(2)pyruvate.,
(3) cephalosporins,
(4) ketone bodies
(5) guanidine,
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II. Enzymatic Methods:
Creatininase and Creatinase:
An alternative approach has been the use of
creatininase that yields sarcosine and urea, the
former being measured with further enzyme
mediated steps using sarcosine oxidase.
This produces (1) glycine, (2) formaldehyde, and (3)
hydrogen
peroxide with the latter being detected and
measured with a variety of methods.
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Creatininase
Creatinine + H2O ------------------ Creatine
Creatinase
Creatine + H2O ------------------- Sarcosine + urea
Sarcosine oxidase
Sarcosine +O 2 + H2O --------------------- Glycine + formaldehyde + H2O2
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= BUN (blood urea nitrogen)
Catabolism of proteins and amino acids results in the
formation of ammonia then in the liver urea is
formed from ammonia, which is predominantly
cleared from the body by the kidneys.
The real urea concentration is BUN x 2.14 because
60 g of urea contains 28 g of nitrogen.
BUN is a sensitive indicator of renal disease.
Urea level depends upon :
a. diet b. liver function c. renal
function.
Plasma urea (BUN)
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Normal BUN range is 6 - 20 mg/dL.
Normal urea : 10 – 50 mg / dl
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Protein
Proteolysis, principally enzymatic
Amino acids
Transamination and oxidative deamination
Ammonia
Enzymatic synthesis in the “urea cycle”
Urea
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Urea is the major nitrogen containing metabolic
product of protein catabolism in humans.
Its elimination in the urine represents the major route
for nitrogen excretion.
More than 90% of urea is excreted through the
kidneys, with losses through the GIT and skin.
Urea is filtered freely by the glomeruli then part of it
is reabsorped through tubule.
Its clearance is less than GFR.
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Plasma concentrations also tend to be slightly
higher in males than females.
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Clinical Significance:
Measurement of blood and plasma urea has been used
for many years as an indicator of kidney function.
However, it is now generally accepted that creatinine
measurement provides better information in this
respect.
Causes of elevated urea ( related to kidney ):
Prerenal: renal hypo perfusion.
Renal: acute tubular necrosis.
Postrenal: obstruction of urinary flow.
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Analytical Methodology:
Both chemical and enzymatic methods are used to quantify
urea in body fluids.
I. Chemical Methods:
Most chemical methods for urea are based on the Fearon reaction
in which diacetyl condenses with urea to form the chromogen
diazine, which absorbs strongly at 540 nm.
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II. Enzymatic Methods:
Enzymatic methods for the measurement of urea are based on
hydrolysis of urea with urease to generate ammonia, which is
then quantified.
The most clinically used kinetic method couples the urease
reaction with L-glutamate dehydrogenase and measure the rate
of disappearance of reduced NADH at 340 nm.
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Urease
Urea + H2O ---------------------2 NH3 + CO2
2 NH4 + 2- ketoglutarate + 2 NADH --------L- Glutamate + 2 NAD + 2H2O
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In human, uric acid is the major product of the
catabolism of the purine nucleosides, adenosine and
guanosine.
Purines are derived from catabolism of dietary nucleic
acid (nucleated cells, like meat) and from degradation of
endogenous nucleic acids.
Uric acid
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Renal handling of uric acid is complex and involves four
sequential steps:
Glomerular filtration of virtually all the uric acid in capillary
plasma entering the glomerulus.
Reabsorption in the proximal convoluted tubule of about 98
to 100% of filtered uric acid.
Subsequent secretion of uric acid into the lumen of the distal
tubule.
Further reabsorption in the collecting tubule.
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• Hyperuricemia is defined by serum or plasma uric acid
concentrations higher than 7.0 mg/dl (0.42mmol/L) in men or
greater than 6.0 mg/dl (0.36mmol/L) in women.
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Analytical Methodology:
Common techniques for measuring uric acid in body fluids include:
(I) phosphotungstic acid (PTA).
(2) uricase.
(3) HPLC-based methods.
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Phosphotungstic Acid Methods:
These methods are based on the development of a blue reaction
chromogen (tungsten blue) as PTA is reduced by urate in an
alkaline medium.
The absorbance of the chromogen in the reaction mixture is
measured at wavelengths of 650 to 700 nm.
PTA methods are subject to many interferences, and efforts to
modify them have had little success in improving their specificity.
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Uricase Methods:
Uricase methods are more specific than PTA approaches.
Uricase is used either as a single step or as the initial step to
oxidize uric acid.
Uricase acts on uric acid to produce allantoin, hydrogen
peroxide, and carbon dioxide.
The decrease in absorbance as urate is converted and is
measured with a spectrophotometaetr 293 nm.
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Uric acid + O2 + 2 H2O --- Allantoin + H2O2
H2O2
4- AA + DCPS ----------- Quinoneimine + 4 H2O POD
AA : aminoantipyrine
DCPS: dichlorophenol sulphonate
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HPLC Methods:
HPLC methods using ion-exchange or reversed-
phase columns have been used to separate and
quantify uric acid.
HPLC methods are specific and fast; mobile phases
are simple; and the retention time for uric acid is
less than 6 minutes.
Because of these multiple attributes, HPLC has
been used to develop reference methods for
measuring uric acid.
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Reference: Burtis and Ashwood Saunders, Teitz fundamentals of Clinical Chemistry, 4th edition, 2000.
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