It is not known why so many women who got potentially infectious anti-D product tested negative when they came forward as part of the State-wide screening programme which began in 1994. Medical experts agree the risk of infection being discovered now in these women is exceptionally low, but most appear to concur that the women should be told of their exposure to the hepatitis C virus and offered retesting.
To date, 65,000 women have come forward for testing, and some 64,000 have tested negative. Of this group about 19,000 women may have received potentially infectious anti-D and subsequently tested negative.
All of the anti-D these women received was made from plasma taken from the two women, known as patient X and patient Y, who were infected with the virus. Patient X's anti-D was administered in hospitals in May 1977 to the end of 1978, and patient Y's from March 1991 to February 1994.
From patient X's plasma, 4,062 vials of anti-D were issued to women from that period: 15,833 women came forward for testing. Based on the Blood Transfusion Service Board's examination of women's batch cards, around 1,200 of these women, who received suspect anti-D, tested negative; 795 women from this period tested positive.
Of the vials issued, some women got more than one dose, which would decrease the overall figure. It is likely the remaining 2,000 women, currently unidentified because of missing batch cards, who would have received suspect product, have already been tested under the screening programme.
There were thousands more batches issued using patient Y's plasma in the 1990s but these were far less infectious that the earlier batches. From patient Y's plasma, 15,337 vials of anti-D were issued; 19,000 women have come forward for screening from this period. Around 15,000 who received the suspect product tested negative, although this figure may change as the BTSB is still sorting through the batch cards from this period. Some are incompletely filled in and some missing. Seventy-four women have tested positive and just 34 have evidence of continuing infection related to anti-D.
While the BTSB knows the anti-D came from these two sources, it is not known if every vial was infected or the level of infection. This may explain why some women became infected and others did not. In some batches, the virus may have died.
One medical expert, who did not wish to be named, said a number of factors must be considered. "One might have been sent to Portlaoise and put in a fridge and kept nice and cold, and the other could have been sent on a longer journey to Cork, not kept so cold, and the virus could have died," the expert said.
Some women did not become infected; others did but their immune systems spontaneously cleared it. It is not medically understood how this happens. These women are of particular interest to doctors because they want to find out why they responded as they did. "It could be something very simple. The response of their immune system is very important. It could help us with a vaccine."
If a woman with the virus is successfully treated with interferon, the antibody level would begin going down, but would take many years to disappear. The Y batches of anti-D are less infectious and women infected during these years appear to respond better to interferon treatment.
The women who came forward for testing under the national screening programme were tested using an Elisa (enzyme-linked immuno-sorbent assay) test. Two different Elisa tests are usually carried out. This testing method is very sensitive, often picking up false positives. In the case of these women they tested negative and it went no further until now.
But the practice if someone tests positive on the Elisa test is to do a subsequent Riba (recombinant immunoblot assay) test. This picks up four kinds of antibodies, and the level of antibody present, measuring from 0 to 4. There are 20 combinations.
According to one expert, these women who tested negative, but are known to have received suspect anti-D, may be likely to have a weak positive on one Elisa test, a level of one, out of a possible maximum level of 16, on the Riba test.
A confirmatory PCR - polymerase chain-reaction - is carried out. This tries to detect a part of the viral genetic machinery known as RNA, which shows there is virus in the blood. If this is positive, a liver biopsy is carried out to ascertain damage to the organ.
It is not known what test the BTSB would carry out if it is decided to recall the 19,000 women.