Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1198 page)

   Do not use sodium azide in collection tubes.

   Hemolysis produces false-negative results.

   Hemoglobin concentrations >100 mg/dL may interfere.

   Confirmation

   Sample pretreatment required

   Gas chromatography: derivatization may be necessary

   HPLC: preferred technique for distinguishing metabolites

   Limit of quantitation: 50 μg/mL

SCREENING FOR FETAL CHROMOSOME ABNORMALITIES AND NEURAL TUBE DEFECTS

   Definition

   Noninvasive testing with goal of limiting invasive diagnostic procedures that carry risk to pregnancy

   Use

   Screening modalities have been developed for Down syndrome/trisomy 21 detection because trisomy 21 is the most common viable autosomal chromosome abnormality. However, screening also provides specific risk assessment for trisomy 18 and neural tube defects.

   In addition, with inclusion of early ultrasound examination, increased fetal nuchal translucency may indicate other chromosome abnormalities including Turner syndrome (45,X), trisomy 13, and triploidy.