NIPT Positive Predictive Value (PPV/NPV)

Why use it

Prevents the grave clinical error of treating a "high risk" NIPT as a diagnosis. Demonstrates that even with 99.9% specificity, a rare condition (e.g., T13 at age 25, or 22q11.2 deletion at any age) will have a surprisingly low PPV. Essential for informed pre- and post-test counseling.

About

Non-Invasive Prenatal Testing (NIPT) analyzes cell-free DNA (cfDNA) from maternal plasma to screen for fetal chromosomal abnormalities. Despite high sensitivity and specificity, a positive NIPT result does NOT confirm diagnosis — its Positive Predictive Value (PPV) depends critically on the background prevalence (prior probability) of the condition, which is primarily driven by maternal age and gestational age-specific risks for aneuploidies, and by general population rates for microdeletions (which are age-independent). This calculator uses Bayes theorem to compute PPV and NPV from any combination of prevalence, sensitivity, and specificity.

When to use

• Pre-test counseling: explain the limitations of NIPT before the patient draws blood
• Post-test counseling: calculate the true probability of aneuploidy after a "screen positive" result
• Explaining why diagnostic confirmation (CVS or amniocentesis) is mandatory after a positive NIPT
• Counseling patients who refuse invasive confirmation despite a positive NIPT
• Comparing performance of NIPT for common trisomies vs. sex chromosome aneuploidies vs. microdeletions

Formula

Pearls & pitfalls

• For T21 at age 35 (prevalence ~1/250), NIPT PPV is typically 80-95% — excellent, but still 5-20% false positive.
• For T21 at age 25 (prevalence ~1/1,000), PPV drops to ~60-80% despite same test performance.
• For microdeletions (e.g., 22q11.2, prevalence ~1/4,000), PPV is often only 10-35%, even with 99.5% specificity.
• NPV is nearly always >99.9% because these conditions are rare — a negative result is highly reliable.
• Confined placental mosaicism (CPM) and vanishing twin are major causes of false-positive NIPT.
• "High risk" NIPT = screen positive. NOT diagnostic. Invasive confirmation is the standard of care (ACOG, SMFM, ACMG).
• Sex chromosome aneuploidies (e.g., Turner 45,X) have lower sensitivity (~90%) and moderate PPV.
• Always use the maternal age- and gestational age-specific prevalence, not generic population figures.

Next steps

Always offer diagnostic testing (CVS or amniocentesis) to confirm any "high risk" NIPT result before making irreversible pregnancy decisions. A negative NIPT (NPV typically >99.9%) is highly reassuring but does not exclude all chromosomal conditions. NIPT does not screen for all birth defects — anatomy scan remains essential.

Evidence

Introduced clinically in 2011. Endorsed by ACOG (Practice Bulletin 226, 2020), SMFM, ACMG, and ISPD as a screening — not diagnostic — test. PPV calculation is required before clinical decision-making per ACOG guidelines. Large validation studies include NEXT trial (Norton et al., NEJM 2015) and CLARITY trial (Bianchi et al., NEJM 2014).

References

1. ACOG Practice Bulletin 226: Screening for Fetal Chromosomal Abnormalities. Obstet Gynecol. 2020;136(4):e48-e69. — link
2. Norton ME et al. Cell-free DNA Analysis for Noninvasive Examination of Trisomy. N Engl J Med. 2015;372:1589-1597. — link
3. Bianchi DW et al. DNA Sequencing versus Standard Prenatal Aneuploidy Screening. N Engl J Med. 2014;370:799-808. — link
4. Gregg AR et al. Noninvasive prenatal screening for fetal aneuploidy, 2016 update. Genet Med. 2016;18(10):1056-65. — link