Clinical Utility of Testing

Diagnosis of OTC deficiency by molecular testing is preferable to the more invasive liver biopsy that is necessary for the enzymatic test. Molecular screening for OTC mutations identifies approximately 80% of mutations, while the remaining undetected mutations are expected to affect promoter function or splicing.7 The OTC gene has an approximately 50:1 male-to-female mutation ratio, and 80% of male probands inherit the mutation from their mothers, while only 23% of manifesting females inherit the mutation. Therefore, any woman who has a son with OTC deficiency has a 20% a priori risk of having another affected son in her next pregnancy. Due to the inheritabil-ity and the severity of the disease with the limited treatment available, molecular screening of at-risk couples for the purpose of prenatal testing may be beneficial. Additionally, mutation identification may be of prognostic value in OTC deficiency (see "Interpretation of Test Results," below).

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