Difference Between a Screening Test and a Diagnostic Test
Definition
This is the most frequently confused distinction in prenatal genetics and the one that causes the most anxiety:
| Screening test | Diagnostic test | |
|---|---|---|
| What it tells you | Risk, "is the likelihood increased?" | Definitive answer, "is it present or not?" |
| Examples | NIPT (cfDNA), double/triple screening | Amniocentesis, CVS, karyotype |
| What a positive result means | Risk is increased, it does not mean the condition is present | The condition is present |
| Procedure | Blood draw, no risk | Needle sampling, carries a low risk |
| What comes next | If positive, a confirmatory test is needed | No confirmation needed |
Why it matters: In cfDNA screening, a positive result means increased risk but cannot make a diagnosis; amniocentesis or CVS is needed for confirmation. In contrast, amniocentesis is a diagnostic test: it tells you whether your baby has a specific health problem, and the results are almost always accurate.
What the patient should know: The sentence "my screening test came back positive" does not mean "my baby has the condition." Families who are unaware of this distinction may go through an unnecessary and severe period of anxiety.