OSCAR stands for “One-Stop Clinic for Assessment of Risk”. This is a  prenatal screening test for Down Syndrome (Trisomy 21), Edwards Syndrome (Trisomy 18) and Patau Syndrome (Trisomy 13).  The screening test is performed from 11-13 (+6d) weeks of gestation. 

This screening test is safe, non-invasive and our local published data indicates that the test is able to identify 93% of babies which are subsequently diagnosed with Down syndrome (HKMJ paper) .

How is the test performed? 

  • An ultrasound scan is performed to confirm the gestational age and fetal size. The fetal Nuchal Translucency, the skin thickness at the back of the babies neck, is measured.
  • Blood is then taken from the mother for  measurement of the levels of Pregnancy Associated Plasma Protein-A (PAPP-A), and free beta-human Chorionic Gonadotropin (free beta-hCG). The result will be available after 3 hours.
  • Software is then used to determine the risk of Down syndrome. This software  is developed by the Chinese University of Hong Kong and is customised for Chinese population.

How is the result reported?

  • The test is reported as being High Risk (positive) if the Gestational Adjusted Risk is higher than 1:180 for Trisomy 21 and higher than 1:150 for Trisomies 13 & 18.
  • About 5% of all women undergoing screening will have a  high risk test result.
  • A low risk indicates that the chance of having Down fetus is very low, though it cannot be totally excluded.

The data collected may be used for both authorized internal and external audit of the laboratory service.

Followup options following a high risk result

Further testing is needed to confirm whether the fetus has Down syndrome. The options are:

  • Have a chorionic villus sampling (CVS) procedure and have the specimen assessed by standard karyotyping, BoBs or microarrays. We recommend CVS sampling as it can be performed before 14 weeks.
  • Have an amniocentesis and have the specimen assessed by standard karyotyping, BoBs or microarrays. This is usually performed at 16 weeks. 
  • Undergo a NIPT to look at the placental DNA in maternal blood and have a CVS or amniocentesis if the NIPT test is reported as high risk.

When choosing a followup test the points to consider are:

  1. That NIPT is still a screening test and can detect approximately 99% of Trisomies 21 and 18  affected pregnancies. The detection rate for other less common chromosomal abnormalities is lower or may not be detected. In a small percentage of cases no result will be reportable, usually due to insufficient DNA in the mothers blood sample. The results are usually reported  in 5 days.  Note that a negative NIPT result does not mean that the fetus does not have aother chromosomal abnormality or structural abnormality. A fetal morphology scan performed by a fetal maternal specialist is advised.
  2. That CVS and Amniocentesis will detect all cases of Trsiomy 21, 18 and 13 as well as other less common abnormalities. The results are reported within 3-5 days for BoBS and microarrays tests and 14 days for karyotype analysis. Current evidence, both local and international, indicates that the CVS and amniocentesis procedures carry a small risk. Approximately 1 to 2 of every 1000 CVS result in the loss of a normal fetus. The risk of procedure loss is now much lower than the risks quoted in the past and by NIPT providers.     

Comparing OSCAR and other screening programs

If you wish to have a screening test for Down Syndrome, the options are:

  1. A public or private 1st trimester OSCAR test. This screens 93% of Downs syndrome. Approximately 5% of tests are reported as high risk
  2. A 2nd trimester screening test (Quadruple tests). This screens approximate 80% of Down syndrome. Approximately 5% of tests are reported as high risk
  3. A private NIPT Test screens 99% of Down syndrome. Approximately 0.1% of tests are reported as high risk for Trisomy 21 with higher rates for less common abnormalities such as Trisomy 13, sex related abnormalities such as Turners syndrome and micro deletions.  

How can I book a private OSCAR consultation?

For patient enquiry and booking, you can contact us
by phone: (852) 5600 1970
by email: pdc_obg@cuhk.edu.hk

How can I order a laboratory test?

For obstetricians in private practice, our laboratory offers analysis on serum markers and provides risk reports on Down’s Syndrome Screening.

For more information,

General Information on Down Syndrome Screening

Guideline to Laboratory Request Form

Guideline to Sample Collection, Transportation and Spillage Handling

Guideline to Reporting Details and Interpretation

Lab Request Form – 1st Trimester Combined Down Syndrome Screening (Version 1.4)

Lab Request Form – 2nd Trimester Biochemical Down Syndrome Screening (Version 1.5)

Specimen Delivery Checklist (For HA centres)

For further enquiry, feedback or complaints, please contact
Obstetrics Screening Laboratory
by phone: (852) 3505 4217
by fax: (852) 2725 2638
by email : obsscreening@cuhk.edu.hk