To the Editor: The introduction of non-invasive prenatal testing will revolutionise the practice of prenatal diagnosis. One of the many potential applications of non-invasive prenatal diagnosis (NIPD) is to determine fetal RHD status in pregnant Rhesus (Rh) D-negative women, and the feasibility of such testing was aptly demonstrated by Hyland and colleagues1 and commented on by Cole and Savoia.2 It is clear that once the technical problems of indeterminate results and gene variants are resolved, NIPD for RHD status will have improved clinical utility over current invasive testing, thus reducing the need for invasive procedures and prophylactic treatment of all RhD-negative women.

The assay developed by Hyland et al can also be used to determine fetal sex, indicating a potential additional use in women at risk of carrying a fetus with a sex-linked disorder, so as to halve the number of chorionic villus sampling (CVS) tests being performed. However, it is worth noting that the number of CVS tests currently performed for this indication is extremely small. In Victoria, the complete population-based dataset on prenatal diagnosis for 2007 shows that there were only eight invasive tests for sex-linked disorders.3

The most significant impact of NIPD technology will be for pregnant women who request prenatal testing for Down syndrome. If NIPD testing for Down syndrome becomes available, it is anticipated that current testing based on chromosome analysis (karyotyping) of a sample obtained by CVS or amniocentesis will become redundant. However, replacing karyotyping — a genome-wide test — with a targeted NIPD test for Down syndrome will mean that many of the other chromosome abnormalities currently detected by CVS or amniocentesis will no longer be detected.4 We do not know whether this is important to women or not.

Before implementing any NIPD test, we suggest that each application needs a separate investigation, including a careful comparison of clinical utility between current tests and the proposed replacement technology.