Down's Syndrome Screening in the First Trimester with Additional Serum Markers: Indian Parameters.

Objective: To derive a risk calculation algorithm suitable for use in India when screening for Down's syndrome using four first-trimester maternal serum markers either alone or with ultrasound nuchal translucency (NT).

Methods: Stored maternal serum samples (- 20 °C) from 411 singleton unaffected pregnancies were retrieved and measured for pregnancy-associated plasma protein (PAPP-A), free β-human chorionic gonadotropin (hCG), placental growth factor and α-fetoprotein. Samples were taken at 10-13 weeks' gestation. Equations were derived to express marker levels in multiples of the gestation-specific normal median, adjusted for maternal weight. Gaussian model parameters were derived and compared with six published non-Indian studies; NT parameters were derived from 27,647 women screened in India. On the basis of the maternal age distribution in 64,473 Indian women screened in 2016-2017, the model was used to predict test performance.

Results: The model predicted a detection rate for a serum-only protocol of 80% for a 5% false-positive rate. Using a 1 in 250 at term Down's syndrome risk cut-off, the predicted detection rate was 78% and the false-positive rate was 4.1%. When NT was also included, the rates were 95% for 5% and 90% for 1.4%, respectively.

Conclusion: First-trimester screening using four serum markers only can be carried out in India. Performance is expected to be similar to the second-trimester Quad test and will also facilitate early screening for preeclampsia and open spina bifida. A protocol of NT plus the four serum markers enhances the performance compared with NT, PAPP-A and free β-hCG.