Another false-negative finding on placental sampling

(Potential) false-negative diagnoses in chorionic villi and a review of the literature

OBJECTIVES

To assess the incidence of (potential) false-negative findings of cytogenetic diagnosis in STC-villi and/or LTC-villi and to determine the best strategy for karyotyping chorionic villi in order to avoid false-negative results.

METHODS

2476 chorionic villus samples were received for prenatal cytogenetic investigations. Karyotyping was routinely performed on STC- and LTC-villi preparations by G-banding. Fluorescence in situ hybridization (FISH) analyses were performed in addition to standard chromosome analysis when necessary. Sometimes follow-up investigations like amniocentesis were performed before a definite prenatal cytogenetic result could be reported.

RESULTS

In 2389/2476 (96.5%) of the cases, both STC- and LTC-villi were investigated. Normal STC- with abnormal LTC-villi results and finally an abnormal fetal karyotype were detected in ten cases (10/2389; 0.42%); in 9/10 of the cases the indication was fetal ultrasound abnormalities. Normal STC- and LTC-villi and finally an abnormal fetal karyotype were detected in two cases (2/2389; 0.08%).

CONCLUSION

The most reliable technique for prenatal diagnosis after chorionic villus sampling (CVS) is the combination of the analysis of both STC- and LTC-villi to reduce the incidence of false-negative findings to a minimum. In the case of fetal ultrasound abnormalities with a small amount of villi available, the investigation of LTC-villi is recommended over that of STC-villi.

Detection of confined placental mosaicism in trisomy 18 conceptions using interphase cytogenetic analysis

Fluorescence in situ hybridization provides a rapid and accurate technique for detecting chromosomal aneuploidy. It is an excellent method for identifying mosaicism in placental tissues following prenatal diagnosis. Mosaicism, in the form of confined placental mosaicism, occurs im approximately 1%-2% of viable pregnancies studied by chorionic villus sampling at 9-11 weeks of gestation. It has been detected in pregnancies with both diploid and trisomic fetuses and appears to have an important effect on the intrauterine fetal survival. Using both standard cytogenetic analysis and fluorescence in situ hybridization, we have studied 12 placentas from pregnancies with trisomy 18 for the presence of chromosomal mosaicism. These included 2 that were spontaneously aborted, 5 that were terminated after prenatal diagnosis, and 4 that were delivered as either stillborn or liveborn. Significant levels of mosaicism, confined exclusively to cytotrophoblast, were detected in 7 pregnancies. This study demonstrates the usefulness of interphase cytogenetic analysis of uncultured tissues as an alternative method for the detection of mosaicism.

Cytogenetic diagnoses after chorionic villus sampling are less reliable in very-high-or very-low-risk pregnancies

An increasing number of cytogenetic prenatal diagnoses are performed on chorionic villus samplings. The accuracy of this method is influenced by chromosomal mosaicism, mostly confined to direct preparation methods. Especially those investigators who have experienced false-negative and false-positive findings propagate the combined use of direct and culture methods. Yet large collaborative studies have shown that in approximately two-thirds of diagnostic cases only one procedure is applied. Moreover, the accuracy of a cytogenetic investigation depends not only on the ontogenetic origin of the tissues investigated, but also on interacting factors such as the 'a priori risk' and the 'predictive value of a cytogenetic finding'. On this basis a differentiated prenatal diagnostic procedure is discussed, including either sole short-term culture (STC), combined STC and long-term culture (LTC), primary amniocentesis (AC), or primary percutaneous umbilical blood sampling (PUBS). The predictive value of the cytogenetic diagnosis from CVS varies significantly dependent on the a priori risk of a chromosome aberration and, in the case of an abnormal karyotype, on the specific chromosome involved. A non-mosaic and 'non-lethal' trisomy detected in STC is highly representative of the embryo/fetus, but there are exceptions of limited predictive value, e.g., trisomy 18. Guided by the strategy of an optional follow-up by LTC, AC, or PUBS in 1317 successive CV samplings, we are not aware of a false-negative diagnosis, but probably had one false-positive diagnosis: 47,XXY after STC; 46,XY after LTC. When referring to the rate of fetuses with an unbalanced karyotype expected in the different indication groups, a relative increase of false-positive findings in the very-low-risk group (maternal age < or = 35 years of age) and of false-negative findings in the very-high-risk group (abnormal ultrasonographic findings) of pregnant women when only performing CVS becomes obvious. Because of this dilemma, AC or--especially in the latter group--PUBS might be primarily offered to these indication groups instead of CVS.

Introduction of early amniocentesis to routine prenatal diagnosis

With growing awareness of the problems associated with prenatal cytogenetic diagnoses after CVS, attempts have been made to provide early amniocentesis as an alternative to CVS. Since 1990, at our clinic the gestational age limit for routine diagnostic amniocentesis has been successively lowered, first to 14 and then to 13 weeks of gestation. Thus, 811 prenatal diagnoses were performed after early amniocentesis at 13 weeks (n = 217) and at 14 weeks of gestation (n = 594). No problems were encountered. Culture failure was never observed in the early samples. Using the criteria 'number of colonies' and 'culture duration until harvest', early samples taken at 14 weeks did not differ significantly from the controls after standard amniocentesis performed at 15 and 16 weeks, respectively, whereas a minority of samples taken at 13 weeks experienced some delay in culturing. However, in each group at least 85 per cent of samples led to a diagnosis fulfilling our standard criteria of a safe diagnosis (at least 20 metaphases of at least five colonies from at least one primary culture after trypsinization) within 15 days. Some differences between the different groups can be recognized: culture duration of less than 11 days tends to be increasing after standard amniocentesis, whereas long culture duration (more than 20 days) is more often associated with early amniocentesis. However, this trend is only minimal and did not result in an increased risk of missing a diagnosis.

Cytogenetic results from the U.S. Collaborative Study on CVS

Cytogenetic data are presented for 11,473 chorionic villus sampling (CVS) procedures from nine centres in the U.S. NICHD collaborative study. A successful cytogenetic diagnosis was obtained in 99.7 per cent of cases, with data obtained from the direct method only (26 per cent), culture method only (42 per cent), or a combination of both (32 per cent). A total of 1.1 per cent of patients had a second CVS or amniocentesis procedure for reasons related to the cytogenetic diagnostic procedure, including laboratory failures (27 cases), maternal cell contamination (4 cases), or mosaic or ambiguous cytogenetic results (98 cases). There were no diagnostic errors involving trisomies for chromosomes 21, 18, and 13. For sex chromosome aneuploidies, one patient terminated her pregnancy on the basis of non-mosaic 47,XXX in the direct method prior to the availability of results from cultured cells. Subsequent analysis of the CVS cultures and fetal tissues showed only normal female cells. Other false-positive predictions involving non-mosaic aneuploidies (n = 13) were observed in the direct or culture method, but these cases involved rare aneuploidies: four cases of tetraploidy, two cases of trisomy 7, and one case each of trisomies 3, 8, 11, 15, 16, 20, and 22. This indicates that rare aneuploidies observed in the direct or culture method should be subjected to follow-up by amniocentesis. Two cases of unbalanced structural abnormalities detected in the direct method were not confirmed in cultured CVS or amniotic fluid. In addition, one structural rearrangement was misinterpreted as unbalanced from the direct method, leading to pregnancy termination prior to results from cultured cells showing a balanced, inherited translocation. False-negative results (n = 8) were observed only in the direct method, including one non-mosaic fetal abnormality (trisomy 18) detected by the culture method and seven cases of fetal mosaicism (all detected by the culture method). Mosaicism was observed in 0.8 per cent of all cases, while pseudomosaicism (including single trisomic cells) was observed in 1.6 per cent of cases. Mosaicism was observed with equal frequency in the direct and culture methods, but was confirmed as fetal mosaicism more often in cases from the culture method (24 per cent) than in cases from the direct method (10 per cent). The overall rate of maternal cell contamination was 1.8 per cent for the culture method, but there was only one case of incorrect sex prediction due to complete maternal cell contamination which resulted in the birth of a normal male.(ABSTRACT TRUNCATED AT 400 WORDS)

Canadian multicenter randomized clinical trial of chorion villus sampling and amniocentesis. chromosome mosaicism in CVS and amniocentesis samples

Data on 1040 chorionic villus and 969 amniotic fluid samples were collected from women studied in the Canadian Multicentre Randomized Clinical Trial of Chorion Villus Sampling and Amniocentesis. Cytogenetic results were obtained from 98.0 per cent of chorionic villus samples and from 99.9 per cent of amniotic fluid samples. Level I mosaicism (a single cell with an abnormal karyotype) occurred frequently in both chorionic villus and amniotic fluid samples and appeared to have no clinical significance. Level II mosaicism occurred in 0.9 per cent of CVS mesenchyme and 1.5 per cent of amniotic fluid cultures and in general was not perceived to be of sufficient concern to warrant cytogenetic follow-up studies. Level III mosaicism was reported in 18 CVS cases (15 cytotrophoblast, 1 mesenchyme, and 2 with both cell methods) and in one amniotic fluid case. In all cases but one (fetus with trisomy 18), level III mosaicism was confined to the placenta. Maternal cell contamination occurring with a frequency of 6.4 per cent in the mesenchyme analyses was a concern. This study supports the final report of the Canadian Multicentre Randomized Clinical Trial of Chorion Villus Sampling and Amniocentesis. Cytogenetic analysis of chorionic villus samples appears to be an acceptable alternative to the analysis of amniotic fluid samples. However, because of mosaicism and maternal cell contamination concerns, the examination of both cytotrophoblast preparations and mesenchyme cultures from chorionic villus samples is recommended.

The vanishing twin: an explanation for discordance between chorionic villus karyotype and fetal phenotype

One 'erroneous' diagnosis occurred in 200 first-trimester chorionic villus samples (CVS) analysed. In direct preparations following 24 h incubation as well as in long-term cultures, a 46,XX karyotype was observed in the villi (28 and 25 cells, respectively). At 20 weeks of gestation, labour was induced because of fetal death in utero. An autopsy performed on the fetus revealed a male phenotype. Placenta and fetal tissues were not submitted for cytogenetic studies. The discordant CVS karyotype (46,XX), in view of the male fetal phenotype, prompted further cytogenetic and molecular studies. Chromosome marker studies on the parents' blood and chorionic villi confirmed both maternal and paternal inheritance of chromosomes in the CVS. DNA studies on formalin-fixed skin using a Y-specific probe, DYZ1, confirmed the presence of a Y chromosome in the fetus. The most likely cause of the discrepant CVS karyotype is the presence of an undetected degenerating dizygotic twin.

Transabdominal placental biopsy in the second and third trimester of pregnancy

Transabdominal placental biopsy under ultrasound guidance was carried out in 260 cases in the second trimester and 50 cases in the third trimester of pregnancy. Placental tissue was aspirated using an 18 or 20 gauge needle. In a total of 310 placental biopsies in the second and third trimester, 100 were performed because of suspicious ultrasonographic findings. Placental biopsy is simple in the presence of severe oligohydramnios where fetal blood sampling is usually more difficult. Oligohydramnios and polyhydramnios were the ultrasonographic findings in 50% of cases and were found to be associated with 30% of abnormal chromosomal findings. There was one (0.3%) abortion within two weeks following placental biopsy. Placental biopsy did not affect the outcome of the pregnancy.

Discordant findings in chorionic villus direct preparation and long term culture--mosaicism in the fetus

Prenatal cytogenetic study of chorionic villi showed a discrepancy between a normal female karyotype 46,XX in the direct preparation after short-term incubation, and a 45,X karyotype in the long-term culture. The subsequent amniocentesis revealed a normal karyotype in three cultures and a 45,X/46,XX mosaicism in one culture. Cytogenetic analysis of chorionic villi after termination of the pregnancy showed a normal karyotype in the direct preparation and a 45,X/46,XX mosaicism in the long-term culture. Fetal lymphocytes showed normal karyotypes, whereas fibroblast cultures revealed a 45,X/46,XX mosaicism.

Simultaneous placentacentesis and amniocentesis for prenatal karyotyping: report on 250 cases

The efficacy and risks of simultaneous transabdominal chorionic villus biopsy (placentacentesis) and amniocentesis in the second and third trimesters were evaluated in 250 singleton pregnancies. The major indications were advanced maternal age (36.0 per cent), abnormal ultrasound findings (23.2 per cent), and low maternal AFP value (17.6 per cent). Nine abnormal karyotypes were found in placental tissue (3.6 per cent). The karyotypes of placental and amniotic cells were different in three cases, including two cases of false-positive mosaicism (0.8 per cent) and one case of a false-negative result (0.4 per cent) obtained by placental karyotyping. The problem of discrepant karyotypes in embryonic and extra-embryonic tissue does not seem to be restricted to the first trimester. The post-procedure fetal loss rate was estimated as approximately 1.8 per cent. We conclude that the procedure presented here combines the advantages of rapid karyotyping (placentacentesis) and high diagnostic reliability (amniocentesis). It does not seem to be necessary to restrict its use to late presentations and suspicious ultrasound findings.

Benefits of placental biopsies for rapid karyotyping in the second and third trimesters (late chorionic villus sampling) in high-risk pregnancies

In a total of 301 placental biopsies in the second and third trimesters, 225 were performed because of suspicious ultrasonographic findings. In this group there was a high rate of aneuploidies (20%). As opposed to the alternative methods for rapid karyotyping, placental biopsies can easily be performed even in pregnancies with abnormal amounts of amniotic fluid. Oligohydramnios and polyhydramnios were key ultrasonographic findings in 38% of cases and were found to be associated with 22% of abnormal chromosomal findings. Suggestive ultrasonographic findings seem to justify the exclusive use of direct preparation.

'Late CVS' international registry compilation of data from 24 centres

Data on 2058 late CVS cases, i.e., placental biopsies after 12 completed weeks of pregnancy, were collected from 24 centres. Two major groups of indications with or without ultrasound findings suspicious of fetal chromosomal abnormalities can be differentiated. In the first group, the rates of cytogenetic anomalies (21 per cent) and fetal losses (10 per cent) are high. The respective figures for the low-risk group are 6 per cent for chromosome anomalies and 2 per cent for total fetal losses. To evaluate this rapidly spreading new method further, more data are required and will be collected by the CVS registry based in Philadelphia, U.S.A.

The predictive value of cytogenetic diagnosis after CVS: 1500 cases

The cytogenetic results of 1500 chorionic villus samples (CVS) are presented. In these 1500 samples, 23 samples (1.5 per cent) could not be provided with a diagnosis because of laboratory failure. This failure rate dropped from 3 per cent in the first 500 samples to 0.2 per cent in the last 500. In the remaining 1477 samples, 58 (3.9 per cent) chromosomal aberrations were found. Of these, 21 (36 per cent) proved not to represent the karyotype of the fetus proper. Predictive values of (different groups of) chromosomal aberrations in CVS are calculated. The impact of (differences between) the predictive value for some major chromosomal aberrations is discussed. A tissue- and chromosome-specific selection mechanism is postulated.

Down syndrome at birth not detected by first-trimester chorionic villus sampling

A case of a false-negative first-trimester diagnosis following chorionic villus sampling is reported that ended with the birth of a child with Down syndrome. Chromosome analysis of 30 metaphases from 24 h-cultured chorionic villi obtained in week 12 of gestation showed a normal chromosome constitution. However, the newborn showed manifestations of Down syndrome, and 99 of 100 metaphases analysed from cultured lymphocytes showed 47, XY, + 21. The remaining metaphase was normal (46, XY).

Abnormal pregnancy sonogram and chromosomal anomalies: four years' experience with rapid karyotyping

Over a four-year period, 140 pregnancies with different malformations detected by ultrasound were examined cytogenetically. Gestational age ranged from 13 to 36 weeks. Twenty-six fetuses (18.6 per cent) had abnormal karyotypes, including trisomies, triploidy, monosomy X, and structural anomalies. Similar malformations were found in fetuses with different chromosomal anomalies, indicating that the types of malformations are not specific for particular chromosomal anomalies. Chromosomal analysis was performed on amniotic fluid culture and by direct karyotyping of placental biopsies. Direct karyotyping is suggested to be the most rapid approach, especially if sonographic anomalies are detected close to the 24th week of gestation, shortly before delivery, and in cases of significant oligohydramnios.

Validity of cytogenetic analyses from trophoblast tissue throughout gestation

Cytogenetic findings in the Münster Chorionic Villi Sampling (CVS) program are presented after 1,184 first trimester transcervical samplings and 131 second and third trimester placentacenteses. In the first trimester series the abnormality rate is low (2.4%) in patients with only an age-dependent aneuploidy risk. In this group terminations were performed in only 1.6% because of aneuploidy. True mosaicism was found more frequently after CVS, and the risk of maternal cell contamination seems higher as compared to amniocentesis. There are no obvious differences in the overall rate of diagnostic errors after both procedures, when metaphases after direct preparation and chorionic cell cultures are analysed and doubtful findings such as mosaicism are adequately followed up by amniocentesis. The cytogenetic techniques also offer a very rapid approach to karyotyping in the second and third trimester. We found a high rate of aneuploidy (15%) when placentacentesis was performed after sonographic diagnosis of fetal abnormalities. We conclude that cytogenetic analysis from trophoblast tissue is an accurate diagnostic tool applicable from first to third trimester of pregnancy.