Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1154 page)

   Limitations

   Risk for trisomy 13 is
not
calculated; however, trisomy 13 pregnancies are typically associated with ultrasound anomalies detectable with second-trimester ultrasound examination. By definition, screening is not diagnostic; most screen-positive pregnancies are chromosomally normal, and some affected pregnancies will be missed.

CYTOGENETICS: FLUORESCENCE IN SITU HYBRIDIZATION (FISH) AND CHROMOSOME ANALYSIS
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   Definition and Use

   
FISH

   Analysis of fetal tissue to detect targeted numeric or structural chromosome aberrations.

   Interphase FISH performed on uncultured cells is used to provide a rapid (1 day) result for targeted chromosome enumeration. Typically, chromosomes 13, 18, 21, X, and Y are assessed. Metaphase FISH performed on cultured cells is used to assess chromosome aberrations too small to be detected by conventional chromosome analysis.

   Generally used only in cases with specific risk (specific ultrasound anomalies, family history).

   
Chromosome analysis

   Analysis of fetal tissue to detect numeric and structural chromosome aberrations. Most chromosome aberrations are numeric (e.g., trisomies 13, 18, 21 [Down syndrome], 45,X [Turner syndrome], 47,XXY [Klinefelter syndrome]).

   Principle indications:

   Increased risk determined from maternal screening

   Ultrasound anomaly

   Family history of chromosome anomaly (previous affect pregnancy, balanced rearrangement carrier parent)