Screening tests, diagnostic tests - is there a difference? Does it matter?

Types of tests

The majority of babies born in Australia will have had a number of tests before they were born, sometimes testing (of their parents) was even done before they were conceived. Very simply, tests divide into screening tests (to help us see if there is a higher chance of a complicated pregnancy) or diagnostic tests (which tell us, yes or no, if an issue has been identified). When talking to your midwife or doctor, it is important to understand the difference between the two.

Diagnostic tests include the following:

  • full blood count (diagnostic for anaemia, low platelets, screening for B12 and iron deficiencies and thalassaemia)

  • hepatitis B and C, HIV & syphilis (infections in the past or at present?)

  • rubella (immunity levels high enough to protect from infection?)

  • urine m/c/s (diagnostic for urine infection, screening for kidney disease)

  • CVS & Amniocentesis (testing for chromosomal abnormalities)

Screening tests include the following:

  • Non Invasive Prenatal Test (NIPT, screening for chromosomal abnormalities)

  • Nuchal translucency scan (NTS, mix of screening for chromosomal and genetic anomalies and diagnosis of abnormal anatomy)

  • Pap Smear or Cervical Screening Test (screening test for cancer of the cervix)

Sometimes parents-to-be are surprised when the result of a test comes back and, instead of knowing exactly what is happening, more tests are recommended. This can be distressing and create ethical and financial dilemmas. The classic example of this would be a NTS, organised at the end of the first third of pregnancy. If a NTS shows a high chance of a couple having a child with Down Syndrome, further DNA testing is recommended. This can be by NIPT (however a NIPT only checks for a few anomalies and if it shows a high chance of an anomaly, further testing is again recommended) or CVS or amniocentesis (which will give us a lot more information and which are the diagnostic tests).

The NIPT is not covered by Medicare and costs around $385 (Sept 2018). If done in a public hospital, CVS or amniocentesis is free, if done in the private system, there will usually be a gap between the cost of the test and the Medicare rebate. To make it even more complicated, there is a risk of miscarriage associated with the CVS and amniocentesis tests, so to find out for sure if baby does or does not have Down Syndrome, there is a risk that baby might die. And if baby does have Down Syndrome, the result of the CVS or amniocentesis test will not tell you how baby will be affected by the condition!

Some parents decide not to have this testing, as they simply don’t know what they would do with the result and indeed, don’t ask a question if you don’t want to know the answer. Others want to know for sure if their child might have a particular condition, in which case perhaps they should go straight to a diagnostic test, rather than having a screening test first.

Either way, it is important to discuss the tests you are having with your midwife or doctor so that you can make an informed decision. “Just order everything” is a request which fills me with dread, as even “everything” won’t identify all anomalies (for example, there are no tests for blindness, deafness or cerebral palsy) and it is important to think through the step beyond the result. What will you do with what we find?