1 Departments of Obstetrics and Gynecology, Molecular Medicine and Genetics, and Pathology, Hutzel Hospital/Wayne State University, 4707 St Antoine Blvd, Detroit, MI 48201, USA, 2 Reproductive Genetics Center, Denver, CO, 3 Prenatal Diagnostic Center, Lexington, MA, USA, 4 Karolinska Institute, Stockholm, Sweden, 5 Kings College, London, UK, 6 Jefferson Medical Center, Philadelphia, PA, USA, 7 Departments of Medical Genetics and Ob/Gyn, University of Basel, Basel, Switzerland and 8 Quest-Nichols Institute, San Juan Capistrano, CA, USA
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Abstract |
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Key words: cytogenetics/FISH/karyotype/molecular cytogenetics/prenatal diagnosis
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Introduction |
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Conflicting reports have emerged as to the true sensitivity and specificity of FISH diagnoses with ranges of diagnostic accuracy reported between 8098% (Henry and Miller, 1992; Christensen et al., 1993
; Clark et al., 1993
; Ward et al., 1993
). There have also been considerable arguments about abandoning proven `gold' standards for newer techniques. In the era of `cost, not quality' there will clearly be debates as to whether or not to eliminate expensive cytogenetic culturing and karyotyping in favour of quicker and faster FISH techniques. The purpose of this study was to compare the theoretical detection of abnormalities using the five generally available FISH probes (13, 18, 21, X, Y) to the cytogenetic analysis of prenatally determined karyotypes performed in the last 5 years from eight large prenatal diagnostic centres worldwide on 146 000 karyotypes.
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Materials and methods |
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Numerical abnormalities of chromosomes 13, 18, 21, X, and Y including trisomies, monosomies, and triploidies were considered detectable by FISH. Inversions, deletions, duplications, rings, isochromosomes, and numerical or structural aberrations of other chromosomes were considered non-detectable. FISH accuracy of 100% was assumed for the percentages calculated. Inconsequential findings such as inv (9) were not counted as abnormalities. Potential detection frequencies were compared with actual karyotypes to determine those cases which would have been detectable from those which would not have been.
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Results |
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There were considerable differences in the patterns of abnormalities seen among the centres. The differences were mainly in the proportion of cases that were either trisomy 18 or 13. Two centres, Münster and London, which have a high proportion of their cases as referrals for abnormal ultrasounds, had the highest percentage of these trisomies. We therefore defined a new parameter, i.e. the ratio of trisomy 13 plus trisomy 18 divided by trisomy 21, to reflect this issue. We created this ratio to give a quick way of separating the highly varied nature of patient recruitment. Those centres with high ratio would imply a large proportion of ultrasound among referrals (e.g. London). Those with a low ratio would suggest a high proportion of advanced maternal age or other non-ultrasound referrals.
The centres varied from a low of 0.38 (Stockholm) to a high of 0.85 (London) (Table I). The incidence of sex chromosome abnormalities, inversions, duplications, markers, and others also varied, but did not show any specific pattern of variation. Breakdown of the undetectable cases shows considerable variation in the incidence of inversions, translocations, and other aberrations among the centres, but again with no specific pattern (Table I
). A number of the undetectable cases, e.g. many of the mosaics, markers, inversions, and translocations, might not have obvious phenotypic abnormalities, but could alter the prognosis later or for future pregnancies.
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These are, of course, rough estimates and, in practice, the differences could be greater or potentially considerably less. Depending upon developments in the costs of these laboratory services, potential changes in the cost of cell culturing, facilities and equipment, as well as FISH probes are all likely. Thus, all cost estimates must be interpreted with caution.
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Discussion |
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It can be argued that a considerable percentage of the `missed' cases would be clinically insignificant. There were, for example, 285 balanced translocations, of which 124 were de novo and 161 inherited. We do not know how many of the inherited ones were known beforehand, and how many were detected serendipitously because of the proband. Likewise, there were 24 trisomy 20, nine trisomy 22 and 14 trisomy 16. As a rough approximation, about half of the missed cases might have immediate consequences. Others would have implications for genetic counselling for the individual and their relatives. A detailed economic analysis is beyond the scope of this paper, but our rough analysis suggests that the economic cost of the missed cases would seem to far outweigh the saving. A thorough analysis would also have to address considerable medicolegal exposure for undetected cases that eventually caused harm to the current pregnancy, a later one, or another relative.
As experience and available probes increase, it may be possible significantly to increase the yield of potentially detectable abnormalities in which case the equation may change. For now, however, the limitations of FISH must be weighed in the balance of cost and speed. There are also considerable public health issues which need to be considered. Similar to the arguments surrounding biochemical screening that are keyed to Down's syndrome detection per se and not other aneuploid conditions, there exists a very real possibility that in the rush to lower short term medical costs, non-physician administrators might perceive that a reputed 8090% sensitivity rate of detection of trisomy 21 obviates the need for tissue culture and karyotyping. Such potential imposition of new standards should be viewed with extreme caution. At what point do decreased short term costs constitute a mandate to lower the capabilities of complete detection? This will be a social question beyond the scope of this paper. Furthermore, our data suggest that the percentage of anomalies missed would be substantially higher than numbers often quoted (Ward et al., 1993), which further changes the balance from a public health perspective, particularly since the patient has already assumed the risks of an invasive procedure.
In summary, our data suggest that (i) at its theoretical best, FISH would detect about 70% of anomalies actually found on karyotyping by eight large prenatal diagnosis laboratories in four countries; (ii) the cost of the missed cases far outweighs the saving; (iii) we believe that it is clinically reasonable to rely upon a FISH result, when that result is consistent with an ultrasound anomaly; (iv) FISH is a good methodology that will continue to improve; and (v) with such improvements, the balance of the equation may change. Finally, the development of new technologies such as FISH, while intrinsically exciting, must be viewed in the overall context of their sensitivity, specificity, costs, and social impact (Evans et al., 1998). Much more data and reflection are needed.
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Notes |
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References |
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Submitted on October 7, 1998; accepted on January 14, 1999.