Adding ductus venosus blood flow as a categorical variable to the Combined and Integrated tests in Down's syndrome...

Ductus venosus blood flow, expressed as a pulsatility index (DVPI) has been shown to improve the performance of the Combined and Integrated antenatal screening tests for Down's syndrome using previously published data. The use of ductus venosus blood flow as a categorical marker (reversed or absent end diastolic blood flow indicating a positive result) is less discriminatory but simpler, so is sometimes preferred over DVPI. For example, with the Integrated test the false-positive rate for a 90% detection rate was 1.5% as a categorical marker compared with 1.1% expressed as DVPI.  We here provide the necessary algorithm and parameters for using ductus venosus blood flow as a categorical marker with the Combined and Integrated tests.