4. Uric acid
Uric acid is a nitrogenous compound
(C5H4N40,/2,6,8-trihydroxypurine)
It is the product of catabolism of the purine nucleic
acids.
It is the principal nitrogenous component of the
excrement of reptiles and birds.
Nearly all of the uric acid in plasma is present as
monosodium urate.
5. Uric acid
At the pH of plasma (pH 7), urate is relatively
insoluble;
At concentrations greater than 6.8 mg/dL, the
plasma is saturated. As a result, urate crystals may
form and precipitate in the tissues.
acidic urine (<pH 5.75) uric acid is the predominant
species and uric acid crystals may form
7. Physiology of uric acid
purines are degraded into uric acid in the liver.
From the liver Uric acid is transported in the plasma
to the kidney,
where it is filtered by the glomerulus.
Reabsorption of 98% to 100% of the uric acid from
the glomerular filtrate occurs in the proximal tubules.
Small amounts of uric acid are secreted by the distal
tubules into the urine.
Renal excretion accounts for about 70% of uric acid
elimination; the remainder passes into the
gastrointestinal tract and is degraded by bacterial
enzymes.
8. The daily synthesis rate of uric acid is approximately
400 mg.
Dietary sources contribute another 300mg.
In men consuming a purine-free diet, the total body
pool of exchangeable urate is estimated at 1200 mg.
In women it is estimated to be 600 mg.
Patients with gouty arthritis and tissue deposition of
urate may have urate pools as large as 18,000 to
30,000 mg.
10. Hyperuricemia
Hyperuricemia is most commonly defined as
plasma uric acid concentrations greater than 7.0
mg/dL (0.42 mmol/L) in men or greater than 6.0
mg/dL (0.36 mmol/L) in women.
11.
12. Causes of hyper uricemia
Lesch-Nyhan syndrome is an X-linked genetic
disorder (seen only in males)
caused by the complete deficiency of hypoxanthine
guanine phosphoribosyltransferase (HGPRT), an
important enzyme in the biosynthesis of purines.
Lack of this enzyme prevents the reutilization of
purine bases in the nucleotide salvage pathway.
Results in increased de novo synthesis of purine
nucleotides and high plasma and urine
concentrations of uric acid.
Neurologic symptoms, mental retardation, and self-
mutilation characterize this extremely rare disease.
13. Gout
Gout is a disease found
primarily in men and usually
is first diagnosed between
30 and 50 years of age.
In women, urate
concentration rises after
menopause.
Postmenopausal women
may develop hyperuricemia
and gout.
Affected individuals have
pain and inflammation of the
joints caused by
precipitation of sodium
urates.
The big toe (first
metatarsophalangeal) joint is
the classic site for gout.
14. Gout
In 25% to 30% of these patients, hyperuricemia is
a result of overproduction of uric acid.
Plasma uric acid concentration is usually greater
than 6.0 mg/dL.
Kidney disease associated with hyperuricemia may
take one or more of several forms:
(1) gouty nephropathy with urate deposition in renal
parenchyma,
(2) acute intratubular deposition of urate crystals, and
(3) urate nephtolithiasis.
15. Causes of hyperuricemia
Chronic renal disease causes increased uric acid
concentration because filtration and secretion are
impaired.
toxemia of pregnancy (preeclampsia) probably
caused by uteroplacental tissue breakdown and
decreased kidney perfusion. Concentrations in
excess of 6.0 mg/dL (0.36 mmol/l) at 32 weeks
gestation have been noted to be associated with a
high perinatal mortality rate.'
lactic acidosis, presumably as a result of
competition for binding sites in the renal tubules.
16. Hypouricemia
Hypouricemia is defined as a condition where urate
concentrations are less than 2.0 mg/dL (0.12
mmol/L).
Causes
Liver disease
Defective tubular reabsorption (Fanconi syndrome)
Chemotherapy with azathioprine or 6-
mercaptopurine
Overtreatment with allopurinol
19. Specimen Requirements
Uric acid may be measured in heparinized plasma,
serum, or urine.
Serum should be removed from cells as quickly as
possible to prevent dilution by intracellular contents.
Diet may affect uric acid concentration overall, but a
recent meal has no significant effect and a fasting
specimen is unnecessary.
Ethylenediaminetetraacetic acid (EDTA) or fluoride
additives should not be used for specimens that will
be tested by a uricase method.
20. Specimen Requirements
Urine specimens must be alkaline (pH 8).
Stability in serum / plasma:
3 days at 20-25°C
7 days at 4 to 8°C
6 months at -20°C
21. Chemical Method
PHOSPHOTUNGSTIC ACID METHOD
Principle: Proteins in serum are precipitated with
tungstic acid. Uric acid in the supernatant reduces the
phosphotungstic acid into tungsten blue in an alkaline
medium of sodium bicarbonate.
The colour of the tungsten blue is proportional to the
uric acid concentration and is read at 660 nm.
23. Interfering substances
Proteins can cause high background absorbance,
reducing sensitivity.
Hemoglobin and xanthine can cause negative
interference.
Bilirubin and ascorbic acid,which destroy peroxide, if
present in sufficient quantity, can interfere.
Commercial reagent preparations often include
potassium ferricyanide and ascorbate oxidase to
minimize these interferences.
24. Interfering substances
Significant hemolysis may result in low values.
Drugs such as salicylates and thiazides have been
shown to increase values for uric acid.
26. Physiological variations
The concentration of plasma uric acid increases
gradually with age.
Rising about 10% between the ages of 20 and 60
years.
There is a rise in women after menopause, reaching
concentrations similar to those in men.
During pregnancy plasma uric acid concentrations
fall during the first trimester and until about 24 weeks
of gestation.
Using an enzymatic assay, reference intervals at
32,36, and 38 weeks of gestation have been
reported as 1.9 to 5.5 mg/dL , 2.0 to 5.8 mg/dL, and
2.7 to 6.5 mg/dL respectively
27. IDMS
Detection of characteristic fragments following
ionization; quantification using isotopically labeled
compound
Proposed reference method
28. HPLC
HPLC methods using ion-exchange or reversed-
phase columns have been used to separate and
quantify uric acid.
The column effluent is monitored at 293 nm to detect
the eluting uric acid.
HPLC methods are specific and fast; mobile phases
are simple; and the retention time for uric acid is less
than 6 minutes.
