Yes. If you change the reagents used to assay the screening markers, you will need to establish normal median values for the new reagents. Ideally, this is done by assaying approximately 200 samples using the new kit, in parallel with the old one.
Samples should preferably be uniformly distributed between the weeks, say 50 each in weeks 15-18 for second trimester markers, or weeks 10-13 for first trimester markers. Smaller numbers are acceptable in other weeks.
Having collected the measurements for the new assays, the data should be tabulated by gestational week. For each week, record the median gestational age in days, the number of samples, and the median concentration of the screening marker.
You can use a spreadsheet or database software to help with calculating the weekly medians.
For example, a tabulation for AFP might look similar to the following:-
|Number of samples||Median AFP level (iu/mL)|
Having tabulated the data, use the Regression option, on the Statistics menu in alpha, to derive a regressed median equation for the new kit. The regression for the AFP data above is shown below. The A and B values printed below the graph are the coefficients of the regressed median equation. In order to use the new normal medians, you need to enter the coefficients, using Add coefficients on the System menu.
Regression Line Equation
Median MS-AFP = A*B**GA (days)
A = 2.972833
B = 1.020696
This represents a 15.4% increase in median MS-AFP per week