αlpha can identify cases where a single marker has a very large influence on the risk estimate raising the possibility that the risk estimate may be incorrect (1). In these cases, αlpha will notify the user of the anomalous marker measurement and give them the opportunity of removing it from the risk estimate in the screening report.
αlpha does this by calculating the risk of a Down's syndrome pregnancy using all the markers, and then recalculated for screen-negative women with each marker omitted in turn. Screen-positive women do not have their risk recalculated, thereby avoiding the possibility of Down's syndrome pregnancies being reclassified as screen negative. If the risk changes by a factor of more than three hundred, the screening result is flagged as anomalous. If the risk changes substantially when only one marker is omitted, appropriate corrective action can be taken, such as re-assaying the serum sample or reviewing the hard copy of the NT image. If no errors are found, then the risk estimate could be recalculated without the marker value concerned, and this is reported with an appropriate comment.