First trimester combined nuchal translucency and biochemical screening (OSCAR)

1. What is OSCAR?

OSCAR is a mnemonic and stands for One Stop Clinic for Assessment of Risk for Downs syndrome. In many countries, it is named ‘first trimester combined nuchal translucency (NT) and biochemical screening’;
It is a screening test performed at 11-13 6/7 weeks of gestation, mainly for Downs syndrome. It can also screen for Edwards and Patau syndrome
The following parameters are obtained to calculate the risks of having a baby affected by Downs, Edwards or Patau syndrome for each individual woman:

(i) The woman’s age;
(ii) Her serum level of pregnancy-associated plasma protein-A (PAPP-A);
(iii) Her serum level of free beta human chorionic gonadotrophin (free-beta hCG);
(iv) Thickness of the baby’s neck-fold (nuchal translucency thickness)

2. How is OSCAR performed?

Ultrasound for measuring the NT thickness of the fetus;
Taking blood from the mother for the hormonal levels;
All have to be performed at 11-13 6/7 weeks of gestation;
A computer software will calculate the risks for Downs, Edwards and Patau syndromes based on the maternal age, NT and the levels of the hormones

3. How accurate is the test?

The detection rates for Downs, Edwards and Patau syndromes are ~90%;
There is a false positive rate of ~5%, meaning that 5% of all women with normal babies will have a positive result

4. What if the OSCAR results are abnormal?

Not all women with abnormal results have abnormal babies. Approximately 5% of all women will have an abnormal result;
The reasons leading to an increase in risk can be contributed by the woman’s age, the increase in NT, abnormal hormonal profile or a combination of any of the above. The implication can be different;
It is important to discuss with a specialist in this field to determine the next appropriate option, whether it is NIPT, chorionic villus sampling or amniocentesis

5. What other abnormalities can OSCAR screen for?

NT measurement is an essential part of the OSCAR test; there is a normal range of NT for different fetal size;
Increase in NT is an important marker not just for fetal chromosomal abnormalities, but also for fetal structural defects, in particular congenital heart diseases. NT is also a good screening test for some genetic conditions

6. How is OSCAR compared with other antenatal screening tests for Downs syndrome?

OSCAR

Mid-trimester (16-20 weeks)

biochemical screening

NIPT

Detection rate

90%

60-80%

99.6%

False positive rate

0.03%