Non-invasive prenatal testing (NIPT) for foetal sex determination
We have summarised research findings on NIPT’s diagnostic accuracy for foetal sex determination, as well as discussed clinical, health economic and ethical consequences.
- Year: December 2016
- By: Norwegian Institute of Public Health
- ISBN (digital): 978-82-8082-776-0
X-linked recessive diseases are severe hereditary diseases that are manifested solely in males. If the mother is a carrier of an X-linked recessive disease, she can either have a healthy girl, a healthy girl who is a carrier like the mother, a healthy boy, or a boy that becomes ill with the X-linked disease. Current practice in Norway is that all pregnant women at increased risk of having a child with an X-linked recessive disease, are eligible for an invasive test (chorionic villus sampling or amniocentesis), without any determination of the foetal sex beforehand. Annually, 40-60 invasive tests are performed in this group of pregnant women in Norway.
In non-invasive prenatal testing (NIPT), a blood sample of the pregnant woman is used to identify foetal sex. The method is based on the analysis of cell-free foetal DNA found in maternal blood early in pregnancy. The purpose of using NIPT for foetal sex determination is to avoid unnecessary invasive testing of pregnant women who carry a female foetus.
We have summarised research findings on NIPT's diagnostic accuracy for foetal sex determination, as well as discussed clinical, health economic and ethical consequences related to NIPT used for foetal sex determination. Based on the findings:
Diagnostic accuracy of NIPT for foetal sex determination is very high.
Maternal blood samples taken in gestational week 7 or later provide more reliable results than blood samples taken before week 7.
Assuming 50 pregnant women are tested every year, 21 of these will avoid invasive testing.
Introduction of a program where NIPT is used in determination of foetal sex, will increase the annual total health care cost expenditure with 197,000 Norwegian kroner.
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The Knowledge Centre for the Health Services at the Norwegian Institute of Public Health has been commissioned by «New methods» at «Bestillerforum RHF» to conduct a health technology assessment on non-invasive prenatal test (NIPT) for foetal sex determination in pregnant women at increased risk for severe hereditary sex-linked diseases (X-linked recessive diseases).
Most sex-linked diseases are recessive X-linked diseases which are manifested solely in males since they have only one X-chromosome. The most common X-linked recessive diseases are Duchenne muscular dystrophy and haemophilia. Although the individual X-linked recessive diseases are relatively rare, altogether these diseases are estimated to occur in about 5 out of 10 000 live births. In Norway, we assume that annually about 30 children are born with an X-linked recessive disease.
Current practice in Norway is that all pregnant women who either already have given birth to a boy with an X-linked recessive disease or are shown to be carriers of such diseases, are eligible for an invasive test (chorionic villus sampling after gestational week 11 or amniocentesis after week 15), without any determination of the foetal sex beforehand. Such invasive tests carry a small risk of miscarriage (<1%).
Fragments of cell-free foetal DNA (cffDNA) are present in maternal blood during pregnancy. This is called the foetal fraction of all cell-free DNA in the maternal bloodstream, and can be detected early in pregnancy. NIPT is based on the analysis of cell-free foetal DNA found in maternal blood. If Y chromosome DNA sequences in the maternal blood sample are detected, the foetus is classified as male. If no Y chromosome DNA sequences can be detected, one assumes the foetus is female. In the Norwegian context, the use of NIPT for foetal sex determination is considered feasible around gestational week 10.
This health technology assessment summarizes research findings on diagnostic accuracy of NIPT for foetal sex determination, and discusses clinical, health economic and ethical consequences related to NIPT. The purpose of using NIPT for foetal sex determination is to reduce the number of invasive tests, as only pregnant women who, according to a NIPT result, carry a male foetus, will be referred to invasive testing.
We conducted systematic literature searches for systematic reviews. Individual search strategies were designed for selected relevant databases. Search strategies were based on a combination of subject headings and text words for NIPT. Searches were limited to the period of 2010 to February 2016. Two reviewers independently screened all identified records and critically appraised the selected publications. The following outcomes were included: sensitivity and specificity, predictive values, likelihood ratio, inconclusive results, and the number of avoided invasive tests compared with current clinical practice. By inconclusive results, we mean cases where the laboratory cannot tell whether the foetus is a girl or boy.
Health economic evaluation
We performed a cost analysis where two alternative scenarios were considered: current practice for foetal sex determination by means of invasive tests, and a program where NIPT is used for foetal sex determination. We calculated total costs of introducing NIPT, incremental costs per pregnant woman who is tested, and incremental costs per avoided invasive test.
We identified four systematic reviews that met our inclusion criteria. We chose to report results of a systematic review of high methodological quality that was published in 2016. The systematic review was based on 60 studies, including altogether results from 11,179 NIPT samples. Another systematic review from 2011 was used to report diagnostic accuracy of NIPT related to gestational age and sample type. Due to inadequate reporting of inconclusive results across studies, we chose to refer to the results from all four systematic reviews when reporting this outcome.
Diagnostic accuracy of NIPT for foetal sex determination is very high. The sensitivity of the test is 98.9% and the specificity 99.6%. The positive and negative predictive values are correspondingly high, 99.6% and 98.8% respectively.
Diagnostic accuracy of NIPT increases with gestational age. While sensitivity before week 7 is 74.5%, it is 94.8% in the gestational weeks 7-12 and 99.0% after week 20. Diagnostic accuracy using plasma and serum are equally high. The sensitivity of NIPT is 95.6% using plasma and 96.6% using serum, and the specificity 98.8% using plasma and 98.2% using serum.
Based on the test's sensitivity and specificity, in the Norwegian population with 48.6% female births, 490 of 1,000 invasive tests will be avoided if NIPT is used to determine foetal sex prior to a referral to invasive testing. However, this calculation does not take into account inconclusive results.
Some NIPT tests will result in inconclusive results. Inconclusive results were poorly reported across studies. Based on the identified systematic reviews, we estimate that inconclusive results will occur in approximately 10-20% of NIPT samples.
Health economic evaluation
In our cost analysis, we assume that approximately 50 pregnant women in Norway are eligible for foetal sex determination each year. The proportion of inconclusive NIPT results is assumed to be 15%. Based on these assumptions, we estimate that 21 pregnant women will avoid invasive testing. The total annual costs of such a program for the Norwegian health care system is calculated to be 197,000 Norwegian kroner. The incremental costs per tested pregnant woman is approximately 3,900 Norwegian kroner compared with current practice. This means an incremental cost of approximately 9,400 Norwegian kroner per avoided invasive test. However, due to uncertainty related to NIPT prices, these results should be interpreted with caution. The costs will not differ substantially from the above estimates when the test is delivered abroad to a commercial laboratory for analysis.
Foetal sex determination may be carried out for two purposes. Firstly, determination of foetal sex may be used to identify foetuses at increased risk for severe hereditary diseases. Secondly, one may seek this information to select the gender of a child. Our report deals solely with the first alternative, where NIPT presumably will not bring along any new ethical challenges compared to current practice.
The diagnostic accuracy of NIPT for foetal sex determination is very high. In contrast to invasive tests, NIPT can be carried out early in the pregnancy and involves no risk for miscarriage. Genetic counseling and thorough discussions with the pregnant women and their partners are recommended prior to NIPT.
It is recommended to perform an ultrasound examination before blood sampling, among others in order to rule out multiple pregnancies and to estimate an accurate gestational age. It is essential that the test is carried out at a time when there is a sufficient amount of cell-free foetal DNA in the mother's bloodstream. Proper methods and criteria for the validation of negative results are needed. An extra ultrasound examination in pregnant women who have received a negative test result, can help identify false negative cases.
Compared with the current practice, implementation of NIPT will potentially reduce the number of invasive tests by 42%, as only pregnant women carrying male foetuses will be referred to invasive testing.
NIPT is a test that can be used to identify foetal sex in pregnancies at increased risk of X-linked recessive diseases. Diagnostic accuracy of NIPT for foetal sex determination is very high. Testing in 7th gestational week or later provides more reliable results than testing before week 7. Implementation of NIPT will potentially reduce 42% of invasive tests carried out in this group of pregnant women. With the assumption of 50 pregnant women eligible for testing, the total costs of introducing NIPT for foetal sex determination in the health care system is 197,000 Norwegian kroner per year.