Potential of a Second Screening Test for Alloimmunization in Pregnancies of Rhesus-positive Women: A Swedish...

There is lack of consensus regarding whether a second screening in rhesus-positive pregnant women is worthwhile, with different guidelines, recommendations, and practices. We aimed to estimate the number and timing of missed alloimmunizations in rhesus-positive pregnancies screened once and weigh the relative burden of additional screening and monitoring versus the estimated reduction in adverse pregnancy outcomes.

We extracted information on maternal, pregnancy, and screening results for 682,126 pregnancies for 2003-2012 from Swedish national registers. We used data from counties with a routine second screening to develop and validate a logistic model for a positive second test after an earlier negative. We used this model to predict the number of missed alloimmunizations in counties offering only one screening. Interval-censored survival analysis identified an optimal time window for a second test. We compared the burden of additional screening with estimated adverse pregnancy outcomes avoided.

The model provided an accurate estimate of positive tests at the second screening. For counties with the lowest screening rates, we estimated that a second screening would increase the alloimmunization prevalence by 33% (from 0.19% to 0.25%), detecting the 25% (304/1222) of cases that are currently missed. The suggested timing of a second screen was gestational week 28. For pregnancies currently screened once, the estimated cost of a second test followed by maternal monitoring was approximately 10% of the cost incurred by the excess adverse pregnancy outcomes.

Investment in routine second screening can identify many alloimmunizations that currently go undetected or are detected late, with the potential for cost savings.