Sequencing-Based Tests to Determine Trisomy 21 from Maternal Plasma DNA
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National guidelines recommend that all pregnant women be offered screening for fetal chromosomal abnormalities, the majority of which are aneuploidies (an abnormal number of chromosomes). The trisomy syndromes are aneuploidies involving 3 copies of 1 chromosome. Trisomies 21, 18, and 13 are the most common forms of fetal aneuploidy that survive to birth. There are numerous limitations to standard screening for these disorders using maternal serum and fetal ultrasound. Commercial noninvasive, sequencing-based testing of maternal serum for fetal trisomy 21, 18, and 13 has recently become available and has the potential to substantially alter the current approach to screening.
Fetal chromosomal abnormalities occur in approximately 1 in 160 live births. The majority of fetal chromosomal abnormalities are aneuploidies, defined as an abnormal number of chromosomes. The trisomy syndromes are aneuploidies involving 3 copies of 1 chromosome. Trisomy 21 (Down syndrome, T21), trisomy 18 (Edwards syndrome, T18), and trisomy 13 (Patau syndrome, T13) are the most common forms of fetal aneuploidy that survive to birth. The most important risk factor for Down syndrome is maternal age, with an approximate risk of 1/1500 in young women that increases to nearly 1/10 by age 48.
Current national guidelines recommend that all pregnant women be offered screening for fetal aneuploidy (referring specifically to trisomy 21, 18, and 13) before 20 weeks of gestation, regardless of age. (1) Combinations of maternal serum markers and fetal ultrasound done at various stages of pregnancy are used, but there is not a standardized approach. The detection rate for various combinations of noninvasive testing ranges from 60% to 96% when the false-positive rate is set at 5%. When tests indicate a high risk of a trisomy syndrome, direct karyotyping of fetal tissue obtained by amniocentesis or chorionic villous sampling (CVS) is required to confirm that trisomy 21 or another trisomy is present. Both amniocentesis and CVS are invasive procedures and have an associated risk of miscarriage. A new screening strategy that reduces unnecessary amniocentesis and CVS procedures and increases detection of trisomy 21, 18, and 13 has the potential to improve outcomes.
Commercial, noninvasive, sequencing-based testing of maternal serum for fetal trisomy syndromes has recently become available and has the potential to substantially alter the current approach to screening. The test technology involves detection of fetal cell-free DNA fragments present in the plasma of pregnant women. As early as 8 to 10 weeks of gestation, these fetal DNA fragments comprise 6% to 10% or more of the total cell-free DNA in a maternal plasma sample. The tests are unable to provide a result if fetal fraction is too low, that is, below about 4%. Fetal fraction can be affected by maternal and fetal characteristics. For example, fetal fraction was found to be lower at higher maternal weights and higher with increasing fetal crown-rump length.(2)
Sequencing-based tests use 1 of 2 general approaches to analyzing cell-free DNA. The first category of tests uses quantitative or counting methods. The most widely used technique to date uses massively parallel shotgun sequencing (MPS; also known as next generation or “next-gen” sequencing). DNA fragments are amplified by polymerase chain reaction; during the sequencing process, the amplified fragments are spatially segregated and sequenced simultaneously in a massively parallel fashion. Sequenced fragments can be mapped to the reference human genome in order to obtain numbers of fragment counts per chromosome. The sequencing-derived percent of fragments from the chromosome of interest reflects the chromosomal representation of the maternal and fetal DNA fragments in the original maternal plasma sample. Another technique is direct DNA analysis, which analyzes specific cell-free DNA fragments across samples and requires approximately a tenth the number of cell-free DNA fragments as MPS. The digital analysis of selected regions (DANSR™) is an assay that uses direct DNA analysis.
The second general approach is single-nucleotide polymorphism (SNP)-based methods. These use targeted amplification and analysis of approximately 20,000 SNPs on selected chromosomes (eg, 21, 18 and 13) in a single reaction. A statistical algorithm is used to determine the number of each type of chromosome.
In order to be clinically useful, the technology must be sensitive enough to detect a slight shift in DNA fragment counts among the small fetal fragment representation of a genome with a trisomic chromosome against a large euploid maternal background. Whether sequencing-based assays require confirmation by invasive procedures and karyotyping depends on assay performance. However, discrepancies between sequencing and invasive test results that may occur for biological reasons could make confirmation by invasive testing necessary at least in some cases, regardless of sequencing test performance characteristics.
None of the commercially available sequencing assays for detection of trisomy 21, 18 and 13 or other chromosomal abnormalities has been submitted to or reviewed by the U.S. Food and Drug Administration (FDA). Clinical laboratories may develop and validate tests in-house (laboratory-developed tests or LDTs; previously called “home-brew”) and market them as a laboratory service; LDTs must meet the general regulatory standards of the Clinical Laboratory Improvement Act (CLIA). Laboratories offering LDTs must be licensed by CLIA for high-complexity testing. Information on commercially available tests is as follows:
- In October 2011, Sequenom (San Diego, CA) introduced its MaterniT21™ test to test for trisomy 21, 18 and 13. The test is offered through the company’s CLIA laboratory, the Sequenom Center for Molecular Medicine. (Uses MPS; reports results as positive or negative.)
- In March 2012, Verinata Health (Redwood, CA) launched its VerifiŽ prenatal test for trisomy 21, 18, and 13. (Uses MPS and calculates a normalized chromosomal value [NPS]; reports results as 1 of 3 categories: No Aneuploidy Detected, Aneuploidy Detected, or Aneuploidy Suspected.)
- In May 2012, Ariosa Diagnostics (San Jose, CA) (formerly Aria) launched its Harmony™ test for trisomy 21 and 18, which is available from Integrated Genetics, a division of LabCorp. (Uses directed DNA analysis, results reported as risk score.)
- In March 2013, Natera (San Carlos, CA) introduced its Panorama™ prenatal test for detecting trisomy 21, 18 and 13, as well as for detecting select sex chromosome abnormalities. The test is available at ARUP Laboratories. (Uses SNP technology; results reported as risk score.)
- First Trimester Screening for Chromosomal Abnormalities using Fetal Ultrasound Assessment of Nuchal Translucency Combined with Maternal Serum Assessment (Policy #008 in the Obstetrics Section)
1. Noninvasive prenatal testing using cell free fetal DNA from the plasma of pregnant women is considered medically necessary in women with high-risk singleton pregnancies who meet at least one of the following criteria from the American College of Obstetricians and Gynecologists (ACOG):
2. Other uses of noninvasive prenatal testing using cell free fetal DNA from maternal plasma is not considered medically necessary including, but not limited to, the following:
- Maternal age 35 years or older at delivery;
- Fetal ultrasonographic findings indicating an increased risk for aneuploidy;
- History of previous pregnancy with a trisomy;
- Standard serum screening test positive for aneuploidy; or
- Parental balanced robertsonian translocation with increased risk of fetal trisomy 13 or trisomy 21.
- women with multiple gestations;
- average- or low-risk singleton pregnancies.
[INFORMATIONAL NOTE: The policy is based on a 2012 TEC Assessment and also includes a search of the MEDLINE database through October 31, 2013. The TEC Assessment focused on detection of trisomy 21/Down syndrome because a relatively large number of cases were available, and it also reviewed the available data for detection of trisomy 18 and 13. Both the TEC Assessment and the policy limit their scope to the evaluation of tests that are available in the United States.
Assessment of a diagnostic technology such as maternal plasma DNA sequencing tests typically focuses on 3 parameters: (1) analytic validity; (2) clinical validity (ie, sensitivity and specificity) in appropriate populations of patients; and (3) demonstration that the diagnostic information can be used to improve patient health outcomes (clinical utility). The evidence on these 3 questions is described below.
What is the analytic validity of the available maternal plasma DNA sequencing-based tests?
No studies were identified that provided direct evidence on analytic validity. Each of the commercially available tests uses massively parallel sequencing (MPS; also called next generation sequencing), a relatively new technology but not an entirely new concept for the clinical laboratory. Currently, there are no recognized standards for conducting clinical sequencing by MPS. On June 23, 2011, the U.S. Food and Drug Administration (FDA) held an exploratory, public meeting on the topic of MPS, in preparation for an eventual goal of developing “a transparent evidence-based regulatory pathway for evaluating medical devices/products based on next generation sequencing, NGS that would assure safety and effectiveness of devices marketed for clinical diagnostics.”(3) The discussion pointed out the differences among manufacturers’ sequencing platforms and the diversity of applications, making it difficult to generate specific regulatory phases and metrics. It was suggested that “the process may need to be judged by the accuracy and fidelity of the final result.” A consistent discussion trend was that validation be application-specific. Thus, technical performance may need to be more closed linked to intended use and population and may not be generalizable across all sequencing applications. Each of the companies currently offering a maternal plasma DNA sequencing test for fetal trisomy 21 has developed a specific procedure for its private, CLIA-licensed laboratory where all testing takes place.
Although all currently available commercially available tests use MPS, actual performance and interpretive procedures vary considerably. Clinical sequencing in general is not standardized or regulated by FDA or other regulatory agencies, and neither the routine quality control procedures used for each of these tests, nor the analytic performance metrics have been published.
What is the clinical validity of the available maternal plasma DNA sequencing-based tests for trisomy 21 compared to the gold standard of karyotype analysis?
Studies evaluating sequencing-based tests for detecting trisomy 21 in high-risk singleton pregnancies are summarized in Table 1 in the Appendix.(4-12) Sensitivity and specificity of the tests, as shown in Table 1, were uniformly high. Sensitivity ranged from 99.1% to 100%, and specificity from 99.7% to 100%.
Tests from 4 commercial sources were identified: 2 studies used the Sequenom test, 2 studies used the Verinata test, 4 studies used the Ariosa Diagnostics test, and 1 study used the Natera test. All but 2 studies were prospective and all but 2 were industry funded; in the non-industry-funded study, testing was provided by the company without charge. The enrolled study populations included women at increased risk due to older age and/or positive standard screening results or because they were already scheduled for amniocentesis or chorionic villous sampling (CVS). Studies generally included women at a wide range of gestational ages (eg, 8-36 weeks or 11-20 weeks) spanning first and second trimesters.
The approach to analysis varied. Some studies analyzed samples from all enrolled women and others analyzed samples from all women with pregnancies known to have a trisomy syndrome and selected controls (ie, nested case-control analysis within a cohort). The studies evaluated the results of maternal fetal DNA testing in comparison to the gold standards of karyotyping or, in individual cases when a sample did not allow karyotyping, fluorescence in situ hybridization (FISH) for specific trisomies. All studies included testing for trisomy 21 (T21) and some additionally tested for trisomy 18 and/or trisomy 13. There were fewer cases of T18 and T13 per study compared to T21. Four studies had 50 or more cases of T21, and 1 study, Palomaki et al,(5) had 212 cases.
Data from the available published studies consistently reported a very high sensitivity and specificity of maternal plasma DNA sequencing-based tests for detecting trisomy 21 in high-risk women with singleton pregnancies. Thus, there is sufficient evidence that the tests are accurate when used in this population.
Two studies have evaluated sequencing-based tests available in the U.S. for detecting trisomy 21 in average-risk singleton pregnancies. The studies were conducted by the same research group in the U.K. and both used the Ariosa (Harmony) test, which provides risk scores rather than a positive versus negative result. The first study, by Nicolaides et al did a preliminary analysis of the accuracy of cell-free DNA testing in a general population sample.(13) The authors evaluated archived samples from 2,049 women attending their routine first pregnancy visit at 11 to 14 weeks’ gestation. Karyotyping results were available for only a small percentage of women in the study; for the rest of the enrollees, ploidy was imputed by phenotype at birth obtained from medical records. This study was judged to have a high risk of bias due to a high number of exclusions from analysis. Twenty-eight pregnancies ending in stillbirth or miscarriage were excluded for lack of karyotype; while unavoidable, these exclusions likely affect the case detection rate. Cases were primarily verified by phenotype at birth from medical records. Results were available for 1949 of 2049 cases (95%). In the remaining 5%, either the fetal fraction was too low or the assay failed. Overall, using the risk cutoff for the Harmony test, the trisomy detection rate was 100% (ie, 10 of 10 cases identified), and there was a false-positive rate of 0.1%. The risk score was over 99% in all of the 8 cases of trisomy and both cases of trisomy 18. In the 1939 known or presumed euploid cases, risk scores for trisomy 21 and trisomy 18 were less than 0.01% in 1939 (99.9%).
Gill et al prospectively studied 1005 pregnant women.(14) They evaluated a testing strategy that included analysis of serum markers (ie, pregnancy-associated plasma protein-A [PAPP-A] and free beta-human chorionic gonadotropin) and cell-free DNA at 10 weeks and ultrasound markers (ie, nuchal translucency and presence or absence of fetal nasal bone) at 12 weeks. Parents were counseled primarily on the finding of the Harmony test if it indicated either a high or low risk of trisomy. If no results were available on the Harmony tests, parents were counseled based on combined first-trimester serum marker and ultrasound findings. Risk scores from cell-free DNA testing were available for 984 cases (98%); 27 of these required a second round of sampling. Risk scores were greater than 99% for trisomy 21 in 11 cases and for trisomy 18 in 5 cases. In 1 case, the risk score for trisomy 13 was 34%. Sixteen of the 17 women with a high risk score for aneuploidy underwent CVS and the suspected abnormality was confirmed in 15 of the 16 cases. There was 1 case with a high risk score for trisomy 21 and a negative CVS; at the time the article was written, the woman was still pregnant so the presence or absence of trisomy 21 could not be confirmed.
There are fewer data on the diagnostic accuracy of cell-free DNA testing of women with average-risk singleton pregnancies. Two studies have been published—both are from the same research group in the U.K. and use the same sequencing-based test. The studies identified a small number of trisomies and did not confirm negative or positive findings in all cases. Thus, the evidence on accuracy of sequencing-based tests is less definitive for women with average-risk pregnancies as it is for women with high-risk pregnancies.
Twin and multiple pregnancies
Detection of trisomy 21 in twin pregnancies was systematically evaluated in only 1 study, published in 2012 by Canick et al; the study used the Sequenom test.(15) All 7 cases of twin pregnancies with Down syndrome were correctly classified. Five of these were discordant, where 1 twin had T21 aneuploidy and the other did not; 2 were concordant where both twins had T21 aneuploidy.
For women with multiple pregnancies, there is insufficient evidence to draw conclusions about the diagnostic accuracy of these tests for detecting trisomy 21.
What is the clinical utility of the available maternal plasma DNA sequencing-based tests for aneuploidy?
No comparative studies were evaluated that compared health outcomes in patients managed using the maternal plasma DNA tests compared to standard screening tests.
As part of the 2012 TEC Assessment, a decision model was constructed to model health outcomes of sequencing-based testing for trisomy 21 compared to standard testing. The primary health outcomes of interest included the number of cases of aneuploidy correctly identified, the number of cases missed, the number of invasive procedures potentially avoided (ie, with a more sensitive test), and the number of miscarriages potentially avoided as a result of fewer invasive procedures. The results were calculated for a high-risk population of women age 35 years or older (estimated antenatal prevalence of T21, 0.95%), and an average risk population including women of all ages electing an initial screen (estimated antenatal prevalence of T21, 0.25%). For women testing positive on initial screen and offered an invasive, confirmatory procedure, it was assumed that 60% would accept amniocentesis or CVS. Sensitivities and specificities for both standard and sequencing-based screening tests were varied to represent the range of possible values; estimates were taken from published studies whenever possible.
According to the model results, sequencing-based testing improved outcomes for both high-risk and average risk women. As an example, assuming there are 4.25 million births in the U.S. per year(16) and two-thirds of the population of average risk pregnant women (2.8 million) accepted screening, the following outcomes would occur for the 3 screening strategies under consideration:
At least two decision models have been presented in industry-funded publications, each using a different commercially available test and published estimates of sensitivity and specificity. Findings of both these models are similar to the TEC Assessment model in that detection of T21 is increased and miscarriage rates are decreased using sequencing-based testing compared to standard screening. Both of the studies specifically model use of sequencing-based tests offered to women who have had a positive standard screening test.
- Standard screening. Of the 2.8 million screened with the stepwise sequential screen, 87,780 would have an invasive procedure (assuming 60% uptake after a positive screening test and a recommendation for confirmation), 448 would have a miscarriage, and 3976 of 4200 (94.7%) trisomy 21/Down syndrome cases would be detected.
- Sequencing as an alternative to standard screening. If sequencing-based testing were used instead of standard screening, the number of invasive procedures would be reduced to 7504 and the number of miscarriages reduced to 28, while the cases of Down syndrome detected would increase to 4144 of 4200 (97.6% of total), using conservative estimates.
- Sequencing following standard screening. Another testing strategy would be to add sequencing-based testing only after a positive standard screen. In this scenario, invasive procedures would be further decreased to 4116, miscarriages would remain at 28, but fewer Down syndrome cases would be detected (3948 of 4200, 94.0% of total).. Thus, while this strategy has the lowest rate of miscarriages and invasive procedures, it detects fewer cases than sequencing-based testing alone.
Garfield and Armstrong published a study modeling use of the Verinata test.(17) In the model, women were eligible for screening following a positive first-trimester or second-trimester screening test or following a second-trimester ultrasound. The model assumed that 71% of women at average risk and 80% of women at high risk would choose the test. In a theoretical population of 100,000 pregnancies, the detection rate of T21 increased from 148 with standard testing to 170 with VerifiŽ testing. In addition, the number of miscarriages associated with invasive testing (assumed to be 0.5% for amniocentesis and 1% with CVS) was reduced from 60 to 20.
Palomaki et al modeled use of the Sequenom sequencing-based test offered to women after a positive screening test, with invasive testing offered only in the case of a positive sequencing-based test.(4) As in the TEC Assessment, they assumed 4.25 million births in the U.S. per year, with two-thirds of these receiving standard screening. The model assumed a 99% detection rate, 0.5% false-positive rate, and 0.9% failure rate for sequencing-based testing. Compared to the highest performing standard screening test, the addition of sequencing-based screening would increase the Down syndrome detection rate from 4450 to 4702 and decrease the number of miscarriages associated with invasive testing from 350 to 34.
It is important to note that all of the above models include confirmatory invasive testing for positive screening tests. Sequencing-based testing without confirmatory testing carries the risk of misidentifying normal pregnancies as positive for trisomy. Due to the small but finite false-positive rate, together with the low baseline prevalence of trisomy in all populations, a substantial percent of positive results on sequencing tests could be false-positive results.
In 2013, Ohno and Caughey published a decision model comparing use of sequencing-based tests in high-risk women with confirmatory testing (ie, as a screening test) and without confirmatory testing (ie, as a diagnostic test).(18) Results of the model concluded that using sequencing-based tests with a confirmatory test results in fewer losses of normal pregnancies compared to sequencing-based tests used without a confirmatory test. The model made their estimates using the total population of 520,000 high-risk women presenting for first-trimester care each year in the U.S. Sequencing-based tests used with confirmatory testing resulted in 1441 elective terminations (all with Down syndrome). Without confirmatory testing, sequencing-based tests resulted in 3873 elective terminations, 1449 with Down syndrome and 2424 without Down syndrome. There were 29 procedure-related pregnancies losses when confirmatory tests were used.
There is no published direct evidence that managing patients using sequencing-based testing improves health outcomes compared to standard screening. Modeling studies using published estimates of diagnostic accuracy and other parameters predict that sequencing-based testing as an alternative to standard screening will lead to an increase in the number of Down syndrome cases detected and a large decrease in the number of invasive tests and associated miscarriages.
Ongoing Clinical Trials
Prenatal Non-invasive Aneuploidy Test Utilizing SNPs [single nucleotide polymorphism] Trial (PreNATUS) (NCT01545674)(19): This is a prospective, blinded study evaluating the diagnostic accuracy of the Natera test for diagnosing aneuploidies (chromosomes 13, 18, 21) and sex aneuploidy (X and Y). It includes women with singleton pregnancies at high or moderate risk for trisomy who were planning on undergoing invasive testing. Gestational age of the fetus is between 8 weeks 0 days and 23 weeks 6 days. The estimated enrollment is 1,000 participants and the expected final date of data collection is December 2013.
Non-invasive Chromosomal Examination of Trisomy study (NEXT) (NCT01511458)(20): This is a prospective blinded case-control study comparing the Aria test for trisomy 21 with standard first-trimester prenatal screening (maternal serum testing and nuchal translucency). Cases will consist of patients with trisomy 21 pregnancies confirmed by genetic testing, and controls will consist of patients without trisomy 21 pregnancies, as confirmed by genetic testing or live birth. The study is sponsored by Aria Diagnostics. The estimated enrollment is 25,000 individuals. The expected date of study completion is January 2014.
Clinical Evaluation of the SEQureDx T21 Test in Low-Risk Pregnancies (NCT01597063)(21): This is a prospective study and includes pregnant women between 10 to 22 weeks’ gestation who are at low risk for trisomy 21 aneuploidy (ie, no positive prenatal screening tests, and no personal or family history of Down syndrome). Blood samples will be collected at a scheduled prenatal care visit and analyzed with the SEQureDX T21 test; pregnancies will be followed until the birth outcome is recorded. The study is sponsored by Sequenom; estimated enrollment is 3000. The expected final date of data collection is December 2013.
Clinical Input Received through Physician Specialty Societies and Academic Medical Centers
In response to requests, input was received through 3 physician specialty societies and 4 academic medical centers while this policy was under review in 2012. While the various physician specialty societies and academic medical centers may collaborate with and make recommendations during this process, through the provision of appropriate reviewers, input received does not represent an endorsement or position statement by the physician specialty societies or academic medical centers, unless otherwise noted.
There was consensus that sequencing-based tests to determine trisomy 21 from maternal plasma DNA may be considered medically necessary in women with high-risk singleton pregnancies undergoing screening for trisomy 21. Input was mixed on whether sequencing-based tests to determine trisomy 21 from maternal plasma DNA may be considered medically necessary in women with average-risk singleton pregnancies. An American College of Obstetricians and Gynecologists (ACOG) Genetics Committee Opinion, included as part of the specialty society’s input, does not recommend the new tests at this time for women with singleton pregnancies who are not at high risk of aneuploidy. There was consensus that sequencing-based tests to determine trisomy 21 from maternal plasma DNA are investigational for women with multiple pregnancies. In terms of an appropriate protocol for using sequencing-based testing, there was consensus that testing should not be used as a single-screening test without confirmation of results by karyotyping. There was mixed input on use of the test as a replacement for standard screening tests with karyotyping confirmation and use as a secondary screen in women with screen positive on standard screening tests with karyotyping confirmation. Among the 5 reviewers who responded to the following questions (which did not include ACOG), there was consensus that the modeling approach is sufficient to determine the clinical utility of the new tests and near-consensus there is a not a need for clinical trials comparing a screening protocol using the new tests to a screening protocol using standard serum screening prior to initiation of clinical use of the tests.
Published studies from all commercially available tests have consistently demonstrated very high sensitivity and specificity for detecting Down syndrome (trisomy 21) in singleton pregnancies. Nearly all of the studies included only women at high risk of trisomy 21. Direct evidence of clinical utility is not available. A 2012 TEC Assessment modeled comparative outcomes based on the published data on test performance, published estimates of standard screening performance, patient uptake of confirmatory testing, and miscarriage rates associated with invasive procedures. For each comparison and in each risk population, sequencing-based testing improved outcomes, ie, increased the rate of Down syndrome detection and reduced the number of invasive procedures and procedure-related miscarriages. In the modeling, the negative predictive value of testing approached 100% across the range of aneuploidy risk, while the positive predictive value varied widely according to baseline risk. The variable positive predictive value highlights the possibility of a false-positive finding and thus testing using karyotyping is necessary to confirm a positive result.
Based on the available evidence, including modeling in the TEC Assessment, as well as input from clinical vetting and recommendations from national organizations, nucleic acid sequencing-based testing for trisomy 21 is considered medically necessary in women with high-risk singleton pregnancies who meet criteria and not medically necessary in women with average-risk singleton pregnancies. Testing is considered investigational in women with twin or multiple pregnancies.
Practice Guidelines and Position Statements
National Society of Genetic Counselors (NSGC)(22): In 2013, the NSGC published a position statement regarding noninvasive prenatal testing of cell-free DNA in maternal plasma. The NSGC supports noninvasive cell-free DNA testing as option in women who want testing for aneuploidy. The document states that the test has been primarily validated in pregnancies considered to be at increased risk of aneuploidy, and the organization does not support routine first-tier screening in low-risk populations. In addition, the document states that test results should not be considered diagnostic, and abnormal findings should be confirmed through conventional diagnostic procedures, such as CVS and amniocentesis.
American College of Medical Genetics and Genomics (ACMG)(23): In 2013, the ACMG published a statement on noninvasive prenatal screening for fetal aneuploidy that addresses challenges in incorporating noninvasive testing into clinical practice. Limitations identified by the organization include that chromosomal abnormalities such as unbalanced translocations, deletions and duplications, single-gene mutations and neural tube defects cannot be detected by the new tests. Moreover, it currently takes longer to obtain test results than with maternal serum analytes. The ACMG also stated that pretest and posttest counseling should be performed by trained individuals.
International Society for Prenatal Diagnosis (ISPD)(24): In 2013, the ISPD published a position statement regarding prenatal diagnosis of chromosomal abnormalities. The statement included the following discussion of maternal cell-free DNA screening:
Although rapid progress has been made in the development and validation of this technology, demonstration that in actual clinical practice, the testing is sufficiently accurate, has low failure rates, and can be provided in a timely fashion, has not been provided. Therefore, at the present time, the following caveats need to be considered….
Reliable noninvasive maternal cfDNA (cell-free) aneuploidy screening methods have only been reported for trisomies 21 and 18….
There are insufficient data available to judge whether any specific cfDNA screening method is most effective.
The tests should not be considered to be fully diagnostic and therefore are not a replacement for amniocentesis and CVS….
Analytic validity trials have been mostly focused on patients who are at high risk on the basis of maternal age or other screening tests. Efficacy in low-risk populations has not yet been fully demonstrated….
American College of Obstetricians and Gynecologists (ACOG) and Society for Maternal-Fetal Medicine(25): In November 2012, ACOG released a committee opinion on noninvasive testing for fetal aneuploidy The Committee Opinion was issued jointly with the Society for Maternal-Fetal Medicine Publications Committee. ACOG recommended that maternal plasma DNA testing be offered to patients at increased risk of fetal aneuploidy. They did not recommend that the test be offered to women who are not at high risk or women with multiple gestations. ACOG further recommended that women be counseled prior to testing about the limitations of the test and recommended confirmation of positive findings with CVS or amniocentesis. The document noted that the content reflected emerging clinical and scientific advances and is subject to change as additional information becomes available. The Committee Opinion did not include an explicit review of the literature.
Medicare National Coverage
There is no national coverage determination (NCD). In the absence of an NCD, coverage decisions are left to the discretion of local Medicare carriers.]
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Sequencing-Based Tests to Determine Trisomy 21 from Maternal Plasma DNA
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2. Ashoor G, Syngelaki A, Poon LC et al. Fetal fraction in maternal plasma cell-free DNA at 11-13 weeks' gestation: relation to maternal and fetal characteristics. Ultrasound Obstet Gynecol 2013; 41(1):26-32.
3. Food and Drug Adminstration (FDA). Ultra High Throughput Sequencing for Clinical Diagnostic Applications - Approaches to Assess Analytical Validity, June 23, 2011. Available online at: http://www.fda.gov/MedicalDevices/NewsEvents/WorkshopsConferences/ucm255327.htm. Last accessed November, 2013.
4. Palomaki GE, Deciu C, Kloza EM et al. DNA sequencing of maternal plasma reliably identifies trisomy 18 and trisomy 13 as well as Down syndrome: an international collaborative study. Genet Med 2012; 14(3):296-305.
5. Palomaki GE, Kloza EM, Lambert-Messerlian GM et al. DNA sequencing of maternal plasma to detect Down syndrome: an international clinical validation study. Genet Med 2011; 13(11):913-20.
6. Ehrich M, Deciu C, Zwiefelhofer T et al. Noninvasive detection of fetal trisomy 21 by sequencing of DNA in maternal blood: a study in a clinical setting. Am J Obstet Gynecol 2011; 204(3):205 e1-11.
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8. Sehnert AJ, Rhees B, Comstock D et al. Optimal detection of fetal chromosomal abnormalities by massively parallel DNA sequencing of cell-free fetal DNA from maternal blood. Clin Chem 2011; 57(7):1042-9.
9. Norton ME, Brar H, Weiss J et al. Non-Invasive Chromosomal Evaluation (NICE) Study: Results of a Multicenter, Prospective, Cohort Study for Detection of Fetal Trisomy 21 and Trisomy 18. Am J Obstet Gynecol 2012.
10. Ashoor G, Syngelaki A, Wagner M et al. Chromosome-selective sequencing of maternal plasma cell-free DNA for first-trimester detection of trisomy 21 and trisomy 18. Am J Obstet Gynecol 2012; 206(4):322 e1-5.
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12. Nicolaides KH, Syngelaki A, Gil M et al. Validation of targeted sequencing of single-nucleotide polymorphisms for non-invasive prenatal detection of aneuploidy of chromosomes 13, 18, 21, X, and Y. Prenat Diagn 2013; 33(6):575-9.
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