- Design
- Prospective multicentre diagnostic accuracy study
- Population
- 2,212 pregnant carriers of CF, SMA or haemoglobinopathy (2,403 results), 9 US sites
- Primary outcome
- Accuracy of cfDNA fetal risk against fetal or neonatal outcome
- Effect
- Sensitivity 94.4%, specificity 99.5%, NPV 99.9%, PPV 58.6%
A prospective study across nine US centres, published in Obstetrics and Gynecology on 13 August, examined reflex cell-free DNA fetal risk screening in 2,212 pregnant people already found to be carriers of cystic fibrosis, spinal muscular atrophy, or beta or alpha haemoglobinopathy. Instead of testing the partner, the maternal blood sample was used to estimate whether the fetus was affected.
Neonatal or fetal outcome was known for almost 99% of 2,403 results. A high-risk result (at least 1 in 4) was returned in 1.3%. Sensitivity was 94.4%, specificity 99.5% and negative predictive value 99.9% — but the positive predictive value was 58.6%. About four in ten high-risk results were in pregnancies that turned out unaffected.
That combination is typical of a good screen: a low-risk result is highly reassuring, and a high-risk result is a reason for diagnostic testing, not a diagnosis. The approach is attractive where partner testing is hard to arrange, but no guideline yet recommends it as a primary screen, and the authors frame their findings as supporting the possibility.
The practical change is in counselling: a high-risk cfDNA result for a recessive condition must be described as a raised chance and followed by chorionic villus sampling or amniocentesis before any irreversible decision.
- Describe a high-risk cfDNA result for a recessive condition as a raised chance — roughly three in five — not as a diagnosis.
- Offer chorionic villus sampling or amniocentesis to confirm any high-risk result before decisions about the pregnancy.
- A low-risk result was highly reassuring (negative predictive value 99.9%) and can be explained as such.
- Partner carrier testing remains the standard route; cfDNA fetal risk is an option where the partner cannot be tested.
- In India, haemoglobinopathy carriers are common; where such screens are offered, the same need for diagnostic confirmation applies.
Why it matters
A screen with 99.5% specificity still produced a false positive in roughly two of every five high-risk results.
Don't overread it
This supports cfDNA fetal risk as a screen; it has not changed carrier screening guidelines.
The statistics, in plain English
Positive predictive value is the chance that a high-risk result is a true one. At 58.6%, about 41 of every 100 high-risk results were in unaffected pregnancies. The high specificity and low prevalence together explain this: even a small false-positive rate produces many false alarms when true cases are rare.
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