Interphase-FISH assay for rapid prenatal diagnosis of trisomies in amniotic fluid

Background: Specific DNA probes allow rapid prenatal diagnosis of numerical chromosome disorders (chromosomes 13, 18, 21, X, Y) by FISH on interphase nuclei. The diagnostic reliability is presently under evaluation. Study group: In a period of 1.5 years a total 1126 amniotic fluid samples was investigated by FISH compared to standard cytogenetic analysis. Results: The success rate was 93 percent (less than or equal to 30 nuclei) and 84% (less than or equal to 50 nuclei). An abnormal karyotype was detected by FISH in 27 of 28 successfully hybridised samples, including trisomy 21 [16], trisomy 13 [4], trisomy 18 [4], aberrations of sex chromosomes [4]. Two cases with clinically relevant cytogenetic abnormalities were in principle not detectable by FISH. One false-negative finding was observed, possibly arising from maternal cell contamination of the sample. 6 % of all samples, respectively 23% of the bloody samples were contaminated by maternal cells (more than 10 %). Conclusion: Maternal contamination represents the most important limitation of the diagnostic reliability in routine practice.