Menu

Wolfson Institute of Preventive Medicine

Search - ZOO
Search - K2
Search - Categories
Search - Contacts
Search - Content
Search - Newsfeeds
Search - Weblinks

Reflex DNA Test

The Reflex DNA test involves taking two blood samples in the 11th, 12th or 13th week of pregnancy.

One sample is used straight away to carry out a Combined test while the other sample is retained and only used for a DNA test if this is indicated by the result of the Combined test. 


The Combined test involves:
 

i) performing an ultrasound scan examination to precisely determine the gestational age of the pregnancy and to measure the nuchal translucency (NT) thickness, a space at the back of the baby’s neck.

ii) taking a sample of your blood to measure the concentration of pregnancy associated plasma protein-A (PAPP-A) and free ß human chorionic gonadotrophin (free ß-hCG).

In pregnancies with Down's syndrome, the PAPP-A level tends to be low and NT measurement and the free ß-hCG level tend to be raised. In pregnancies with Edwards or Patau syndrome the NT measurement tends to be raised while the PAPP-A and free ß-hCG levels tend to be low.

If the risk derived from the Combined test is greater than or equal to 1 in 300 the additional sample taken

for DNA analysis is automatically tested without reporting a result and avoiding the need to recall you for

your DNA test. This automatic testing strategy is called reflex testing. About 1 in 20 women will have their

DNA sample tested in this way.


The DNA test involves:

i) analysing cell free DNA in your blood which contains a small amount of your baby's DNA as well as your own. In pregnancies with Down's, Edwards or Patau syndrome there will be a little bit more DNA from chromosomes 21, 18 or 13 respectively. The test depends on measuring this extra amount.

Currently 5 NHS hospitals provide reflex DNA screening to women booking for their antenatal care; the Royal London, Newham and Whipps Cross Hospitals (Barts Health), Kingston Hospital and Liverpool Women’s Hospital.

For further information about reflex DNA screening click here.

Return to top