February 2018, Dr Chee L Khoo
You would have no doubt come across a few patients enquiring about non-invasive prenatal testing. Obviously, this come on top of the “optional” nuchal translucency (NT) with or without maternal serum analysis which has been around for some years. What does non-invasive prenatal tests (NIPT) exclude and what does it not exclude?
So, what does NIPT test for? NIPT uses cell free foetal DNA (cffDNA) that is present in maternal plasma. It is thought to be derived from the outer trophoblast cell layer of the placenta. The percentage of cffDNA derived from the trophoblast is termed the ‘fetal fraction’. There is a wide normal range of foetal fractions. The median value at 10 weeks of gestation is approximately 10%.
ccfDNA was first used by Lo et al to detect the presence of the Y chromosome and hence, the sex of the foetus. It is now used as a screening test for aneuploidy and single gene defects like cystic fibrosis and Huntington’s disease. There are different assays used in different laboratories. The assays examine the amount of cffDNA derived from specific chromosomes.
Aneuploidy causes the proportion of these cffDNA to deviate from the norm and with statistical analysis, the results is presented as to whether the deviations are significant or not. For the assays to be valid, there needs to have enough foetal material, the foetal fraction. The tests can be performed any time after 10 weeks gestation. For Trisomy 21, 18 and 13, the detection rate is 85-90% with a false positive rate of 4-5%.
NIPT is never 100% sensitive nor specific. It is a screening test which may or may not lead to more invasive testing. It does NOT cover all chromosomal abnormalities. It detects certain aneuploidy (21, 18, 13 and sex chromosome) well but some others less well. Low foetal fraction may can lead to a false negative result. Assays from twin pregnancies have been validated but abnormalities (include demise) of one twin may lead to false positive or false negative result. NIPT may also not detect mosaic chromosome or partial trisomies.
The major clinical benefit of NIPT is to increase the detection rate for the targeted abnormalities, while simultaneously reducing the number of false positive results and invasive tests. However, given its complexities and costs, the optimal use of NIPT remains a subject of debate. It is not Medicare rebatable and the costs vary between $450-650.
Spectrum Medical Imaging at Liverpool offers a comprehensive non-invasive prenatal screening which includes:
- Pre-test Discussion
- Viability / Dating Scan
- Blood Collection on site ($445 out of the $650 paid directly to Genea)
- GeneSyte NIPS tested on shore (results within 5 working days)
- 13-week Structural Scan
- Telephonic communication of results to patient
- Genetic counselling for NIPS, if required, as well as referral to the maternal fetal medicine unit for ongoing management if the result is abnormal.
For details of the service click here.
Reference
- Lo Y, Corbetta N, Chamberlain P, Rai V, Sargent IL, Redman CWG, et al. Presence of fetal DNA in maternal plasma and serum. Lancet 1997;350:485–7.
- https://www.racgp.org.au/afp/2017/october/non-invasive-prenatal-testing/
- Mackie FL, Hemming K, Allen S, Morris RK, Kilby MD. The accuracy of cell-free fetal DNA-based non-invasive prenatal testing in singleton pregnancies: a systematic review and bivariate meta-analysis. BJOG 2017;124:32–46.