Prenatal Risk Calculation (PRC) 3.0: An Extended DoE-Based First-Trimester Screening Algorithm Allowing For Early...

Both previous versions of the German PRC algorithm developed by our group for routine first-trimester screening relied on the assumption that maternal blood sampling and fetal ultrasonography are performed at the same visit of a pregnant women. In this paper we present an extension of our method allowing also for constellations where this synchronization is abandoned through preponing blood sampling to dates before 11 weeks of gestation.

In contrast to the directly measured concentrations of the serum parameters PAPP-A and free ß-hCG, the logarithmically transformed values could be shown to admit the construction of reference bands covering the whole range from 16 to 84 mm CRL [corresponding to 63 to 98 days of gestation]. Prior to determining reference limits from which the DoEs for each individual patient had to be calculated, the log concentrations of all PAPP-A and free ß-hCG values were transformed once more using the calibration approach established in 1 for the elimination of the influence of maternal weight.

Although that part of the database which was available for estimating the reference bands for blood sampling times prior to 11 weeks of gestation was comparatively sparse (898 out of 186 215 pregnancies with euploid outcome), the key statistical characteristics of the extended risk-calculation procedure turned out to be very satisfactory. Using the same cutoff value of 1:150 for the posterior risks of trisomy 21 and 13/18, the overall FPR (false positive rate) for diagnosing a T21 was found to be 3.42%. The corresponding DTR (detection rate) was obtained to be 86.8% and thus exceeded the DTR attained by PRC 2.0 for trisomy 21. For trisomies 13 and 18, the proportions of patients with calculated posterior risks exceeding the cutoff value of 1:150 were obtained to be 1.60% (=FPR) and 86.4% (=DTR).

Transforming the measured concentrations of PAPP-A and free ß-hCG to the logarithmic scale allows one to extend the DoE-based algorithm developed by the FMF Germany for diagnosing trisomies 21 and 13/18 in such a way that it can be applied to constellations where blood sampling is done before 11 weeks of gestation.