Use of a routine second screening test in Rhesus (Rh)-positive pregnant women was associated with considerable improvements in detecting alloimmunization, and reductions in adverse pregnancy outcomes in hemolytic disease of the fetus and newborn (HDFN), according to findings from a population-based cohort study conducted in Sweden and recently published in the journal Epidemiology.
The researchers of the current analysis sought to estimate the number and timing of missed alloimmunizations in Rh-positive pregnancies in which only a single screening was performed, and to compare the relative burden associated with additional screening and monitoring with the estimated reduction in adverse pregnancy outcomes.
The investigators utilized data from regions that offered two or more screening tests, in an effort to design a prediction model that could estimate the probability that an Rh-positive woman who tested negative at her initial prenatal visit might become alloimmunized later on in her pregnancy. With this approach, the number of “missed” alloimmunizations in other regions of the country could be estimated.
Read more about the prognosis of HDFN
Study methods
Data on pregnancy, maternal and screening results for a total of 682,126 pregnancies from Rh-positive women were obtained from Swedish national registries between 2003 and 2012. Information from counties in which a routine second screening was conducted was utilized to design and validate a logistic model. This model was intended to be used to predict the number of missed alloimmunizations in counties in which only a single screening test was being offered.
Based on interval-censored survival analysis, an optimal time window for administering a second screening test was identified. The validated model was utilized to predict the number of positive tests at a screening in the “low” counties and the “middle” counties.
Among 62,330 pregnancies in the “middle” counties, the model predicted 148 positives at least one week prior to delivery, which was thus higher than the rate of 121 positives that had been observed. In the “low” counties, in which 918 pregnancies tested positive at any time, there were only 184 positives detected at least one week prior to delivery, compared with 1222 positives that were predicted with use of the model.
Second screening test suggested at 28 weeks
Results of the study showed that use of the model provided an accurate estimate of positive tests obtained at a second screening. In those Swedish counties with the lowest rates of screening, it was estimated that use of a second screening test would increase the prevalence of alloimmunization by 33%, thus detecting 25% of the cases that presently are being missed.
Of note, the suggested timing for a second screening test was at 28 weeks’ gestational age.
“For pregnancies currently screened once, the estimated cost of a second test followed by maternal monitoring was approximately 10% the cost incurred by the excess adverse pregnancy outcomes,” the authors noted. “Investment in routine second screening can identify many alloimmunizations that currently go undetected or are detected late, with the potential for cost savings.”
