How long is anti-D treatment effective?
Q - How long does prophylactic anti-D treatment continue to be effective?
A - A Rhesus-negative woman may develop antibodies during her first pregnancy when her fetus is Rh-positive. Antibodies develop most frequently after the 28th week of gestation. Although mortality from RhD sensitisation has declined significantly since the introduction of routine postnatal antenatal prophylaxis using anti-D immunoglobulin treatment (anti-D Ig) in 1969, some 1.5 per cent of RhD negative women still develop anti-D antibodies.
This is due to small fetomaternal haemorrhages occurring during pregnancy which often go undetected, and this in turn leads each year to an estimated 50 infant deaths during the first week following delivery due to haemolytic disease.
To act as a safeguard, NICE recommended in May 2002 that routine antenatal anti-D prophylaxis should be offered to all non-sensitised pregnant RhD negative women.
Assuming anti-D Ig has a circulating half-life of about 21 days then giving 500 iu intramuscularly at 28 weeks' gestation and a further 500 iu at 34 weeks increases the total anti-D Ig in the mother to 625 iu at 34 weeks.
By 40 weeks' gestation this amount has decreased to about 155 iu. As a result, at least 125 iu of anti-D Ig will still be available just when the majority of bleeds occur in the last trimester.
Because of this strong evidence, giving at least 500 iu anti-D Ig at 28 weeks' gestation followed by 500 iu at 34 weeks has been endorsed as the optimum dosage regimen by the Royal College of Obstetricians and Gynaecologists as 'green top' guidelines and classified as a 'grade A' recommendation to fully protect against sensitising events.
Barry Hill is a chief biomedical scientist in the Blood Transfusion Department, Wigan Royal Infirmary