Optimal risk cut-offs for Down syndrome contingent maternal serum screening.

OBJECTIVES: The objective of this study is to identify the optimal cut-off points of contingent serum screening excluding nuchal translucency (NT) measurement, to categorize the risk level in the first trimester.

METHODS: A prospective database of women undergoing contingent serum screening, without NT measurement, was reviewed. In conventional categorization, the results of first-trimester screening were categorized into high risk (>1:30) (invasive diagnosis was offered); intermediate risk (1:30-1:1500) (second-trimester screening was needed); and low risk (<1:1500) (no further test). We recategorized the risk levels using various upper and lower cut-offs and compared detection rates, false-positive rates, and rates of intermediate risk.

RESULTS: Among 24,874 women, the prevalence of Down syndrome was 1:691. The previously agreed cut-offs had a detection rate of 88.9% and a false-positive rate of 8.5% with high rate of intermediate risk (38.2%). Re-categorization provided the optimal lower and upper cut-offs 1/900 and 1/50, respectively, giving a detection rate of 86.1%, a false-positive rate of 8.1%, and a rate of intermediate risk of 24.8%.

CONCLUSIONS: This is the first study on contingent serum screening without NT measurement which shows a high detection rate with an acceptable false-positive rate. The optimal cut-offs to categorize the risk levels of the upper and the lower cut-off was 1:30-1:50 and 1:900, respectively.