Materials and methods
This systematic review was conducted following the PRISMA guidelines.
Search strategy
PubMed, Web of Science, and Scopus were the databases searched. The
keywords used were “cell-free fetal DNA”, “non-invasive prenatal
testing”, “aneuploidy”, and “trisomy”. An advanced search was
conducted, combining: keywords, synonyms, MeSH equivalents, Boolean
Operators (“AND”, “OR”), and type of study (only primary research
articles were accepted). The full search strategy for each of the
databases is available in Appendix S1 . The only filter applied
was the publication date (2017 or later, after the last best review).
The final search was conducted on 18/09/2023.
Study selection and eligibility
criteria
Firstly, duplicate articles were eliminated from the bulk retrieval
generated by the search. Next, articles were screened by title and
abstract. The remaining articles were fully reviewed and assessed
according to the eligibility criteria.
The eligibility criteria were as follows. Only primary research articles
were included. Studies including pregnant women with a singleton
pregnancy, who underwent a NIPT by cffDNA were considered eligible.
Furthermore, the cffDNA results must have been compared to confirmatory
prenatal or postnatal tests. The abnormalities included were the most
common autosomal trisomies, as described above, and SCAs.
Studies including diseases other than those previously stated were
excluded. Multiple pregnancies (including vanishing twins) were also
excluded, as they are commonly associated with inconclusive cffDNA
results.
Data extraction
Data was collected manually from the eligible studies, following the
screening process.
The methodological characteristics of the studies were extracted:
primary author, publication date, type of study, country, duration of
study, number of participants, participants characteristics (maternal
and gestational ages), cffDNA sequencing technology used and diagnostic
confirmation technique. Table S1 summarizes these
characteristics.
The statistical results comparing cffDNA to the confirmed
(gold-standard) diagnoses of each study were collected for each
aneuploidy: T21 – T18 – T13 – SCAs (45,X0 – 47,XXY – 47,XXX –
47,XYY). The positive predictive value (PPV) is presented as the key
outcome variable in this systematic review as it was the common metric
across the 14 studies, presented in Table 1 (for autosomal
trisomies) and Table 2 (for SCAs). The prevalence of a positive
NIPT test outcome was also collected for each of the studies (see Table
3 for autosomal trisomies and Table 4 for SCAs). The rate of
inconclusive results was also collected.
When the overall PPV of several aneuploidies was given, the individual
PPV for each aneuploidy was calculated when the data was available. As
for the high-risk NIPT result prevalence, it was either directly
provided or calculated as a percentage by dividing the number of
high-risk (positive) cffDNA test results by the total number of samples
tested.
Quality assessment and risk of
bias
The study selection, quality assessment and data extraction were carried
out following the PRISMA guidelines. The Critical Appraisal Skills
Programme (CASP) checklists were used to assess the risk of bias.