Maternal weight correction of maternal serum PAPP‐A and free β‐hCG MoM when screening for trisomy 21 in the first trimester of pregnancy

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Abstract
Objective: To assess the suitability of either the log‐linear or reciprocal‐linear regression procedure for maternal weight correction of biochemical marker MoMs in the first trimester.Methods: Data from two prospective first‐trimester OSCAR screening programmes including 32 010 women with first‐trimester maternal serum‐free β‐hCG and PAPP‐A measured by the Kryptor analyser was analysed by regression analysis to provide parameters for the log‐linear and reciprocal‐linear MoM correction procedures. Assessment was made by goodness of fit to the data. The impact on detection rate and false‐positive rate of the different correction procedures was assessed using statistical modelling with biochemical markers alone.Results: Both log‐linear and reciprocal‐linear correction were shown to fit the data well. For free β‐hCG, the log‐linear procedure was marginally superior to the reciprocal‐linear procedure (r2 = 0.986 v 0.980), whilst for PAPP‐A the reciprocal‐linear procedure was marginally better (r2 = 0.991 v 0.985). Log‐linear correction reduced the variance for both markers more than did the reciprocal‐linear procedure. For free β‐hCG, the sd was reduced from 0.2675 to 0.2605 and for PAPP‐A, it was reduced from 0.2545 to 0.2336. Correcting for maternal weight was shown to reduce the population false‐positive rate from 7.0 to 6.5%, whilst maintaining the same detection rate at a risk cut‐off of 1 in a 100. At individual levels, a two‐fold variation in risk was demonstrated depending upon the individual's weight.Conclusions: To provide accurate individual patient‐specific risks for trisomy 21, maternal weight must be taken into account and should be a mandatory data item for screening programmes. Maternal weight correction in the first trimester using free β‐hCG and PAPP‐A can be best achieved using the log‐linear procedure. Copyright © 2003 John Wiley & Sons, Ltd.

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