Transcript Uric Acid and Ammonia

CLINICAL CHEMISTRY (MLT 301)
NONPROTEIN NITROGEN
(NPN)
LECTURE TWO
Dr. Essam H. Jiffri
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URIC ACID
- Uric acid is the final breakdown product of purine
metabolism.
- Most mammals have the ability to catabolize purines one
step further to allantoin, a much more water-soluble endproduct.
- Purines such as adenosine and guanine, resulting from
the breakdown of nucleic acids that are ingested or
come from the destruction of tissue cells, are converted
into uric acid, mainly in the liver.
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URIC ACID
- Uric acid is transported by the plasma from the
liver to the kidney, where it is filtered by the
glomerulus.
- The uric acid in the glomerular filtrate, 98 % to
100% is reabsorbed in the proximal tubule, small
amounts of uric acid are then secreted by the
distal tubules and ultimately appear in the urine.
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URIC ACID
- The most important disease associated with elevated
levels of uric acid in the plasma is gout.
- Gout is a disease found primarily in males and usually
first diagnosed between the ages of 3O and 50.
- Patients have pain and inflammation of the joints caused
by precipitation of sodium urates in the joint resulting
from the high levels of uric acid found in extracellular
fluids
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Analytical Methods
- Uric acid is readily oxidized to allantoin and thus
can function as a reducing agent in many
reactions.
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Analytical Methods
- The most poplular method of this type is the
Caraway method, which is based on the
oxidation of the uric acid in a protein-free filtrate,
with subsequent reduction of phosphotungstic
acid to tungsten blue.
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Specimen Requirements
and Interfering Substances
- Uric acid may be determined in serum, urine, or
heparinized plasma.
- Significant hemolysis, which releases
glutathione, also may give low values; a number
of drugs, such as thiazides and salicylates, have
been shown to cause elevated values for uric
acid.
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Specimen Requirements
and Interfering Substances
- Diet in general may affect the uric acid levels,
the patient need to be fasting.
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Reference Intervals
- Values are slightly lower in children and
premenopausal females.
Children
Adult males
Adult female
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2.0-5.5 mg/dl
3.5-7.2 mg/dl
2.6-6.0 mg/dl
(0.12-0.32 mmol/L)
(0.12-0.42 mmo/L)
(0.15-0.35 mmol/L)
AMMONIA
- The level of ammonia in the circulation is
extremely low (11-35 µmol/L).
- It arises from the deamination of amino acids,
which occurs mainly through the action of
digestive and bacterial enzymes on proteins in
the intestinal tract.
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AMMONIA
- Ammonia is also released from metabolic
reactions that occur in skeletal muscle during
exercise.
- Severe liver disease represents the most
common cause of disturbed ammonia
metabolism.
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Analytical Methods
- There are two distinct approaches that have
been used for the measurement of plasma
ammonia.
- One is a two-step approach in which ammonia is
first isolated from the sample and then assayed.
- The second involves direct measurement of
ammonia by an enzymatic method or ionselective electrode.
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Specimen Requirements and
Interfering Substances
- Ammonia levels rise rapidly in whole blood after
drawing because of the deamination of amino
acids.
- EDTA is the preferred anticoagulant.
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Specimen Requirements and
Interfering Substances
- Samples should be centrifuged at 0 to 40C within
20 minutes of collection, and the plasma
removed, and frozen plasma is reportedly stable
for several days at -200C.
- Because red cells contain 2 to 3 times as much
ammonia as plasma, hemolysis should be
avoided.
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Reference Intervals
Ammonia 14 to 49 µg/dL (11-35 µmol/L)
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