Average and incremental cost-effectiveness ratios were calculated to combine the costs and benefits of the alternative screening strategies.
The average cost per case detected was Aus$61,700 with serum screening (second trimester), Aus$63,300 with NTS, and Aus$80,200 with NTS plus serum screening (first trimester).
The average cost per live trisomy 21 birth avoided was Aus$117,100 with serum screening (second trimester), Aus$201,700 with NTS, and Aus$345,500 with NTS plus serum screening (first trimester).
The incremental cost per extra case detected was Aus$61,700 with serum screening (second trimester) over no screening, Aus$66,300 with NTS over serum screening (second trimester), Aus$218,600 with NTS plus serum screening (first trimester) over NTS, and Aus$105,500 with NTS plus serum screening (first trimester) over serum screening (second trimester).
The incremental cost per extra live trisomy 21 birth avoided was Aus$104,800 with serum screening (second trimester) over no screening, Aus$301,400 with NTS over serum screening (second trimester), Aus$476,200 with NTS plus serum screening (first trimester) over NTS, and Aus$374,800 with NTS plus serum screening (first trimester) over serum screening (second trimester).
The sensitivity analysis showed that the results were sensitive only to the extent of substitution for dating ultrasound and the assumed detection rate for NTS.
With 100% substitution, the incremental cost per extra case detected and trisomy 21 birth averted for the combination of NTS and serum screening in the first trimester, compared with serum screening in the second trimester, was less than 50% of the primary results.
The probabilistic sensitivity analysis showed that variation in the detection rates led to an incremental cost per extra case detected for the NTS and serum screening combination, compared with second trimester serum screening alone, of Aus$121,554 (95% CI: 104,109 - 141,664).