Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1226 page)

   Acute oliguria and prerenal azotemia

   Limitations

   Large diurnal variations exist in urine sodium levels. The rate of excretion during night is one fifth of the peak rate during the day.

   Levels are highly dependent on dietary intake and state of hydration.

TAY-SACHS DISEASE MOLECULAR DNA ASSAY
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   Definition

   Tay-Sachs disease (TSD; OMIM# 272800) molecular DNA testing identifies mutations in the hexosaminidase A gene but should be used concurrently with the hexosaminidase A (HEX A) enzyme activity assay to diagnose TSD. HEX A enzymatic activity is the primary method for diagnosing TSD or carrier identification. HEX A activity is determined by the ratio of HEX A to total hexosaminidase and can be measured in serum from women who are not pregnant and not using oral contraceptives, serum from male patients, or WBCs from individuals.

   
Normal values:
negative or no mutations are found.

   Use

   Confirmation of a clinical diagnosis.

   Carrier testing for Ashkenazi Jewish individuals.

   Carrier testing for at-risk family members of affected individuals.

   Confirmation that the reduced HEX A enzymatic activity is caused by a disease-causing allele rather than a pseudodeficiency allele, R247W or R249W. About 35% of non-Jewish individuals and 2–4% of Jewish individuals identified as heterozygotes by HEX A enzyme assay testing are carriers of a pseudodeficiency allele.

   Prenatal diagnosis: when both parental mutations are known.