11/12/2017
Why cannot be ruled out after a screening test.
With a huge controversy brewing about missing the diagnosis of Down Syndrome, I thought it appropriate to put things in the right perspective.
Understanding the test and its predictability, cut-off and statistics is the most important thing for couple opting for this test.
This isn’t a blog about what Down Syndrome is. If you are reading why someone failed in predicting it, then you must be having a fair idea of what it is. In brief, it is an irreversible multi-organ abnormality syndrome due to an extra chromosome ( The 21st pair of chromosomes are 3 in number instead of 2) So a definitive diagnosis of Down Syndrome has only 2 options- Termination of Pregnancy at the earliest or if termination is against one’s Faith; to continue pregnancy, deliver the baby and care for it. However, as varied degrees of mental retardation is one of the features of Down Syndrome and hence considered ‘not normal’ ; and as caring for a child with special needs is perceived as a burden; most couple opt for termination of pregnancy once a definitive diagnosis of Down Syndrome is made.
As abortion is not an option in certain Faiths, many couple in Dubai opted not to take the test for detecting Down syndrome. I respected their decisions after ensuring they knew about the condition. So not taking the test is also an option.
Also we need to understand the difference between a screening test and a definitive test. A screening test is to detect the at-risk population and hence has to be done on all the people in the cohort. Example- if we wanted to detect diabetics in a group of 50, we would do a random blood sugar for all 50 of them. And then if 10 of them had unacceptable values, they would be further subject to definitive tests like fasting or post-prandial sugars.
Several decades back, we thought only older mothers delivered Down’s babies. But now we know and have seen younger women also are at risk. However, the risk of conceiving a Down’s baby increases with the age of mother. Example the risk is 1: 1500 at 20 years; 1:1250 at 25 years; 1:800 at 30 years,1:350 at 35 years and !;!50 at 40 years and so on. This is attributed to the aberration occurring in cell division as the eggs age. Hence, these figures are just age related risks. Adding more information to this, like the scan and blood test findings fine tunes risk assessment further.
Currently the best time to detect Down’s is between 12 to 14 weeks of pregnancy by the Combined Testing which includes an Ultrasound (for detection of Nasal Bone, thickness of fetal neck and some early features of the heart) and a blood test (free beta HCG and PAPP-A). We also add other facts of the patient such as ethnicity, weight, pre-existing diabetes, Bleeding/ medication in pregnancy etc. Now, the information from the Scan, Blood test report and patient profile is fed into a special software that reassigns Down Syndrome risk.
Interpreting the re-assigned risks by the computer explains to us the accuracy and limitations of this test. Eg. 1:800 at 35 years means if 800 women of 35 years got pregnant; one of them is at risk of having a Down’s baby. If after the screening, the reassigned risk is 1:9000, it means a reduced risk compared to what is expected for her age. So we tell the couple that their chances of having a Down’s baby is less. I stress on the point that with the available report 8999 babies are normal but we could still miss that 1 baby. And since the risk is reduced, it doesn’t merit a definitive test. Similarly, if for this patient, the combined test reassigns the risk as 1:100 it means that her chance of having a Down’s baby after screening has increased. Even here, 99 babies are normal and only 1 baby is abnormal. However, as the new risk is more than the background age-related risk, we offer further definitive testing methods for confirming the fetal abnormality before deciding what to do.
It is unfortunate that currently screening tests available only reassigns risk and even the best of definitive tests may be only 99.1% correct. But this is as far as we can go today and this is with years of research and earnest efforts. Medicine is an inexact science and always comes with risk/ odds ratios, predictability and cut-off values. We all want to see that day when we can confidently tell patients that a test result is 100 % accurate 100 % of the times. Till then we continue to miss cases, unfortunately.
Till then, it is imperative that couple who opt for this test know its limitations and it is the ’s to ensure that.
PEOPLE TREE Hospitals
People Tree Hospitals - Obstetrics & Gynaecology
