Almost 4,000 women who received potentially contaminated anti-D in the 1990s remain untested for hepatitis C.
The Blood Transfusion Service Board is unable to say when it will contact directly the women who received anti-D made from the plasma of a woman known as patient Y.
Positive Action, which represents women infected with anti-D, has said it is unacceptable that these women have not been notified of information the BTSB holds on them.
It was particularly important, said chairwoman, Ms Jane O'Brien, because of the effectiveness of early treatment for women infected in the 1990s.
The BTSB revealed that 17,545 women received doses of anti-D containing this woman's plasma in the early 1990s. This compares with 4,062 possibly infectious doses made from patient X's plasma in the 1970s.
The BTSB medical director, Dr Willy Murphy, said late last week that it had written to more than 30 per cent of the women involved, but about 3,850 remained uncontacted.
The 1990s infection was a "different sub-type of the virus" and infected women appeared not to get as sick as quickly as women infected in the 1970s, Dr Murphy said. They also responded better to treatment with Interferon, he added.
He did not know when the board could contact these women. At present, the BTSB is contacting 500 women who received potentially contaminated doses in the 1970s and have not yet been tested.
Between March 1991 and February 1994, 17,545 doses of anti-D were administered. So far, 12,000 of the 1990s women have come forward for testing.
Of those, 8,899 received PCR positive doses, which means they were infectious.
Although the remaining doses were PCR negative, Dr Murphy said they were "not entirely sure that these batches were 100 per cent not infective". These women were being treated as at "equivalent risk".
Of those 12,000 women tested, fewer than half of 1 per cent - a total of 44 women.
"What we will do when we finish this present exercise with the 1970s women is to review the situation and plan a programme to address the issues at that stage.
"However, it is likely that, as we go, on the amount of effort required to contact these people will be more difficult."
Ms O'Brien said the women who had received contaminated or potentially contaminated anti-D in the 1990s should be given equal treatment as others.
"As well as this, all the women should be assured that the proper protocols are in place independent of the BTSB to ensure they all get full information, independent counselling and referral for medical follow-through," she said.
Dr Murphy said the BTSB was installing a document-scanning facility which would speed up the process considerably.