In plain words
Positive predictive value, or PPV, is the chance that a positive result is a true positive. A screening test can be accurate and still give many false alarms when the condition is rare. That is why the same test can mean different things for different people. In pregnancy, PPV comes up with prenatal screening for chromosome conditions, such as cell-free DNA testing. The PPV of a screen is higher for an older mother, because the starting chance of a chromosome difference is higher at older maternal age. A positive screen is not a diagnosis. Mothers are usually offered a diagnostic test, such as or , to confirm it. A mother can ask her provider what the chance of a true positive is for her age and screening test.
The deeper version
PPV equals true positives divided by all positives, so it is a function of sensitivity, specificity, and pretest probability through Bayes' theorem. For a fixed test, PPV rises with prevalence, which is why maternal age changes the meaning of a positive cell-free DNA result. ACOG Practice Bulletin 226 (2020) recommends that all pregnant people be offered screening and diagnostic testing regardless of age. Reported cell-free DNA PPVs for trisomy 21 vary widely with the population studied, and screening PPVs in low-risk groups are often well below the sensitivity figures quoted by test makers. Negative predictive value is usually high for the common trisomies because prevalence is low. The main controversy is how to present PPV to patients, since it shifts so sharply with the population being counseled.
Where it comes up
- Prenatal tests and ultrasounds: what each one isPregnancy: Your Body
- Pregnancy at 35 and olderPregnancy: Your Body
- Termination for medical reasonsLoss and Complications
What mothers here say about positive predictive value
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Sources
- American College of Obstetricians and Gynecologists — Practice Bulletin 226: Screening for Fetal Chromosomal Abnormalities (2020)
- Bayes T — An essay towards solving a problem in the doctrine of chances. Philosophical Transactions of the Royal Society (1763)