Mayo Test Catalog, (Sorted By Test Name) - Mayo Medical ...

OPTU
83190
metabolism of arginine. The determination of orotic acid can be useful to distinguish between various
causes of elevated ammonia (hyperammonemia). Hyperammonemia is characteristic of all urea cycle
disorders, but orotic acid is elevated in only some. Orotic acid is also elevated in the transport defects of
dibasic amino acids (lysinuric protein intolerance, and hyperornithinemia, hyperammonemia,
homocitrullinuria [HHH] syndrome), and greatly elevated in patients with hereditary orotic aciduria
(uridine monophosphate synthase [UMPS] deficiency). Ornithine transcarbamylase (OTC) deficiency is
an X-linked urea cycle disorder that affects both males and females due to random X-inactivation. In
OTC deficiency, carbamoyl phosphate accumulates and is alternatively metabolized to orotic acid.
Allopurinol inhibits orotidine monophosphate decarboxylase and, when given to OTC carriers (who
may have normal orotic acid excretion), can cause increased excretion of orotic acid. A
carefully-monitored allopurinol challenge followed by several determinations of a patient's orotic acid
excretion can be useful to identify OTC carriers, as approximately 20% of OTC mutations are not
detectable by current molecular genetic testing methods.
Useful For: Evaluation of the differential diagnosis of hyperammonemia and hereditary orotic aciduria
When orotic acid is measured after a protein load or administration of allopurinol, excretion of orotic acid
is a very sensitive indicator of ornithine transcarbamylase (OTC) activity. An allopurinol challenge may
be helpful in determining whether a female patient may be a carrier of an OTC mutation if molecular
genetic testing was not informative.
Interpretation: The value for the orotic acid concentration is reported. The interpretation of the result
must be correlated with clinical and other laboratory findings.
Reference Values:
or =11 years: 0.4-1.2 mmol/mol creatinine
Clinical References: 1. Singh RH, Rhead WJ, Smith W, et al: Nutritional management of urea cycle
disorders. Crit Care Clin 2005 Oct;21(4 Suppl):S27-35 2. Lee B, Singh RH, Rhead WJ, et al:
Considerations in the difficult-to-manage urea cycle disorder patient. Crit Care Clin 2005 Oct;21(4
Suppl):S19-25 Review 3. Brusilow SW, Horwich AL: Urea cycle enzymes. In The Metabolic and
Molecular Bases of Inherited Disease. Eighth edition. Edited by CR Scriver, AL Beaudet, WS Sly, et al.
McGraw-Hill Book Company, 2001, pp 1909-1964 4. Webster DR, Becroft DMO, van Gennip AH, van
Kuilenberg ABP: Hereditary orotic aciduria and other disorders of pyrimidine metabolism. In The
Metabolic and Molecular Bases of Inherited Disease. Eighth edition. Edited by CR Scriver, AL Beaudet,
WS Sly, et al. McGraw-Hill Book Company, 2001, pp 2663-2702 5. Harris ML, Oberholzer VG:
Conditions affecting the colorimetry of orotic acid and orotidine in urine. Clin Chem 1980;26(3):473-479
Orthostatic Protein, Urine
Clinical Information: Orthostatic proteinuria refers to the development of increased proteinuria that
develops only when the person is upright and resolves when recumbent or supine. This condition is
usually seen in children, adolescents, or young adults, and accounts for the majority of cases of
proteinuria in childhood. Orthostatic proteinuria usually does not indicate significant underlying renal
pathology, and is usually not associated with other urine abnormalities such as hypoalbuminemia,
hematuria, red blood cell casts, fatty casts, etc. Orthostatic proteinuria typically resolves over time. This
test characterizes this condition by obtaining 2 urine collections within a 24-hour time frame, 1 collection
obtained while the person is recumbent or supine, the other when upright.
Useful For: Diagnosis of orthostatic proteinuria As a second-order test for further characterization of
proteinuria 93 mg for females) is considered consistent with orthostatic
proteinuria.
Reference Values:
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