Researchers in the US have published a paper in Obstetrics and Gynecology questioning the value of carrying out indirect antiglobulin screening on rhesus negative women at 28 weeks of gestation (Abbey and Dunsmoor-Su 2014).
In most Western countries, all women are offered blood type and antibody screening at one of their first midwife appointments. Where a woman’s blood is found to be rhesus negative, it will be screened for pre-existing rhesus antibodies. The vast majority of women don’t have these antibodies, but a few do, and their presence indicates that the woman has previously been exposed to rhesus positive blood (for instance from a mismatched blood transfusion or a previous pregnancy with a rhesus positive baby) and has become isoimmunised. A woman for whom this is the case will be offered additional care.
The current research isn’t arguing against the value of offering this screening test as soon as possible in pregnancy. What they are questioning is whether – as is currently the case, the same test really needs to be offered again at 28 weeks, in order to identify any women who might have become isoimmunised since their first appointment.
This is an uncommon scenario, although it does happen now and again. Research carried out by Bowman et al in 1978 suggested that this happened in about 1 in 500 women (2/1086). In the current research, Abbey and Dunsmoor-Su (2014) calculated that the risk was even lower, at closer to 1 in 1000 women (2/2029). They note a number of reasons which may explain why their figure was different, including changes in practice around anti-D since 1987, but also point out that, because of the small numbers of women who became isoimmunised in these two studies, these figures aren’t actually statistically different. Sometimes, we have to accept that, especially when it comes to rare events, there are limitations to what we can know.
Abbey and Dunsmoor-Su (2014) write that the two rationales for this 28-week test are (1) to identify women who have become isoimmunised since their booking blood test and (2) to prevent unecessary administration of anti-D. They argue that these figures and the related costings do not support the use of this test for either or both of these reasons. This position, they argue, is strengthened by the fact that the fetus in the index pregnancy (in other words, the baby inside the pregnant woman at the time) is at very low risk, as isoimmunisation affects subsequent pregnancies rather than the index pregnancy.
This does seem like a logical conclusion from a cost perspective, although the removal of a screening test which has previously been offered always has the potential to be contentious, because some people may feel aggrieved that something valuable may be being taken away from them. I absolutely feel that women should have a say in what tests are available to them, but if I was advising a new planet on constructing an approach to rhesus negativity in human pregnancy given our current Earth knowledge on this, then it might be hard to justify recommending the 28 week test on these numbers…
Abbey R and Dunsmoor-Su R (2014). Cost–Benefit Analysis of Indirect Antiglobulin Screening in Rh(D)-Negative Women at 28 Weeks of Gestation. Obstetrics & Gynecology: 123(5): 938-945.
Bowman JM, Chown B, Lewis M, Pollock JM. Rh isoimmunization during pregnancy: antenatal prophylaxis. Can Med Assoc J 1978;118:623–7