What's the difference between a screening test and a diagnostic test?
In short
- A screening test gives you a chance, like 1 in 500. It doesn't give you a yes or no.
- A diagnostic test can confirm a condition, but has its own small risks.
- A "high chance" result is not a diagnosis — it just means a further test may help.
- Your doctor will explain your own result, and your choices.
What happens
- You have a screening test (a blood test, an ultrasound measurement, or both) at the recommended time in pregnancy.
- Your result comes back as a chance (like 1 in 500), not a yes/no answer.
- If your chance is higher than the cut-off used by your lab, you're usually offered a diagnostic test to confirm or rule out the condition, if you choose to have one.
The details: numbers and where they differ
Why this distinction matters
Every screening test used in pregnancy — the double marker test, the quad marker test, and NIPT — works the same way: it estimates a chance, using markers in your blood, an ultrasound measurement, or both. None of them examines your baby's own cells directly, which is why none of them can confirm a diagnosis on its own. Only a diagnostic test, such as amniocentesis or chorionic villus sampling (CVS), examines cells from the pregnancy directly and can confirm or rule out a specific condition — and because it's more invasive, it carries its own small risks that a screening blood test or ultrasound doesn't.
In the source's own words
NIPT is a screening test, not a diagnostic one, and it can produce both false positive results (indicating increased risk when the fetus is unaffected) and false negative results (indicating decreased risk when the fetus is actually affected). A positive (high-risk) NIPT result should be followed by diagnostic testing to confirm the finding before a genetic condition is considered established.
MedlinePlus Genetics (NIH/National Library of Medicine), Noninvasive prenatal testing (NIPT). Quoted verbatim.
When to call your doctor
- If you don't understand whether your result is a screen or a diagnosis — ask directly, in those words.
- If you're offered a diagnostic test and want to talk through the risks and benefits before deciding.
Discuss your results with your doctor: this page reports what these sources say, not advice for your situation.
Sources
- MedlinePlus Genetics (NIH/National Library of Medicine), Noninvasive prenatal testing (NIPT). Quoted verbatim. US federal government work, public domain.
- American College of Obstetricians and Gynecologists (ACOG), Prenatal Genetic Screening Tests (Patient FAQ). Named as the guideline this section describes; its text and tables are not reproduced here (All rights reserved (cite only)). Says a positive (higher-chance) cell-free DNA screening result should be followed by a diagnostic test, amniocentesis or CVS, before a condition is considered confirmed.
Sources checked 2026-09-26. How we source and check every page.
Educational summary of named sources, not medical advice.