What is the next step after a prenatal screening indicates a risk greater than 1:300 for trisomy 21?

Next Step After Positive Down Syndrome Screening (Risk >1:300)

Offer noninvasive prenatal screening (NIPS/cell-free DNA testing) as the next step, which has a dramatically superior positive predictive value (50-95%) compared to traditional screening (2.2-3.6%), reducing unnecessary invasive procedures while maintaining high detection rates. 1

Why NIPS is the Preferred Next Step

When traditional first-trimester combined screening or second-trimester serum screening returns positive (risk >1:300), you face a critical decision point. The 2023 ACMG guidelines explicitly state that many patients now use NIPS as a secondary screen instead of proceeding directly to diagnostic testing after positive traditional screening results 1. This approach is both clinically sound and patient-centered.

The Numbers That Matter

The performance difference is striking:

• Traditional screening PPV: Only 2.2-3.6% of positive screens are truly affected
• NIPS PPV: 50-95% of positive results are truly affected
• This means traditional screening requires 28-45 diagnostic procedures to confirm one case of trisomy 21, while NIPS requires only 1.1-2 procedures 1

Clinical Algorithm

For patients with positive traditional screening (risk >1:300):

1. Counsel about NIPS option - Explain that NIPS can refine their risk before deciding on invasive testing
2. If NIPS is negative - Further testing usually not recommended, even with soft markers on ultrasound 1
3. If NIPS is positive - Proceed to diagnostic confirmation with amniocentesis or CVS

When to Proceed Directly to Diagnostic Testing

While NIPS is preferred for most patients, diagnostic testing (amniocentesis or CVS) remains the gold standard and should be offered directly when:

• Patient desires definitive diagnosis immediately
• Structural abnormalities are seen on ultrasound
• Patient declines NIPS or has contraindications

Diagnostic Testing Specifics

Recommended approach 2:

• Chromosome analysis for trisomy 21,18,13, or sex chromosome aneuploidies
• Sample type considerations:
  • CVS: Earlier timing (10-13 weeks) but risk of confined placental mosaicism
  • Amniocentesis: Later timing (15+ weeks) but more definitive for mosaicism concerns
• Confirmatory result: Nonmosaic trisomy on karyotype confirms diagnosis
• Mosaic CVS result: Cannot be considered confirmatory; requires follow-up amniocentesis 2

Critical Pitfalls to Avoid

Don't dismiss moderate-risk results: Studies show 67.65% of missed trisomy 21 cases fell into the "moderate risk" category, and 79.31% occurred in women under age 35 3. The 1:300 cutoff is a screening threshold, not an absolute dividing line.

Don't ignore isolated abnormal markers: Even single abnormal serum markers (elevated free β-hCG, low AFP) warrant attention and consideration of NIPS 3.

Beware of very high β-hCG levels: When free β-hCG exceeds 5 MoM, risk calculations are typically truncated, but actual risk continues to increase parabolically up to 10 MoM 4. These patients need careful counseling about potentially underestimated risk.

NIPS "No-Call" Results

If NIPS is performed and returns a "no-call" result (occurs in ~1% of cases):

• Offer diagnostic testing per ACOG/SMFM recommendations 1
• Most common cause is low fetal fraction
• Repeat testing succeeds 75-80% of the time
• Associated with higher rates of aneuploidy in some studies

Summary of Approach

The modern standard is sequential screening: traditional screening → NIPS for risk refinement → diagnostic testing only for NIPS-positive results. This maximizes detection while minimizing invasive procedures and their associated risks (procedure-related loss rate ~0.1-0.3%). The 2023 ACMG guidelines support this pathway as both acceptable and increasingly preferred in clinical practice 1.