Health ministers were under pressure last night to explain why they delayed telling the public that a blood test for the Aids virus was faulty, and that people who have tested negative could be infected with HIV.
Up to 40,000 blood samples are to be retested and hundreds of people are now facing an anxious Bank Holiday, after the Department of Health confirmed that the IMX HIV1/HIV2 antibody test, used in some NHS laboratories since September 1995, was not reliable.
Aids charities, hospitals, and helplines are being swamped with calls but can do little to reassure people at present. The Terrence Higgins Trust is advising anyone who has had an HIV test between September and March to use condoms during sex, and to go for re- testing as soon as possible.
The IMX test is also available throughout Europe, South America and Asia, posing huge problems for Abbott Laboratories, the Chicago-based manufacturer, which halted distribution of the test on 25 March.
Doctors say that only 1 per cent of the 60,000 HIV tests carried out annually are positive, and that the vast majority of people tested with the faulty kits will be reconfirmed as negative.
The Department of Health had known of problems with the IMX test, one of several used in the NHS, since last week, but said it wanted to delay the announcement until next Tuesday, when plans for retesting blood were in place, and Aids charities had been briefed.
Instead, Aids workers say they have been left to deal with thousands of worried people, unable to get medical advice because most clinics are closed for Easter.
Harriet Harman, Labour's health spokeswoman, said that the Easter Bank Holiday had exacerbated the crisis for many. "It is unfortunate that the Department of Health did not notify people as soon as they got the information, rather than wait for a public holiday."
Susie Parsons, executive director of London Lighthouse, Europe's biggest HIV centre, where extra staff were brought in yesterday to man the switchboards, said lack of formal guidance from the Government had hindered its work.
"We understand that the manufacturers of this test suspended it on 25 March, which is quite some time ago, and we had to read about it in the press this morning. We have been trying to get through to the Department of Health all morning to see if it can give us some information which will enable us to help the people who are calling us, but unfortunately the lines are just blocked."
Dr Graham Winyard, deputy chief medical officer, said he "deeply regretted" the anxiety caused by the premature announcement. He advised people to make use of helplines over the weekend, adding: "The samples are already being tested and I am confident that the vast majority that have tested negative will still be negative."
A letter sent on 29 March by Abbott Laboratories in Maidenhead, Berkshire, to laboratories which use the test, is believed to have been the source of the leak. A spokesman for Abbott Laboratories in Chicago said yesterday that the company had been alerted to a potential problem in late March when European laboratories reported inaccurate results with the test. A doctor in Portsmouth was among the first to query its reliability when he used it on a patient he knew to have full-blown Aids, and the result was negative.
Professor Jangu Banatvala, of the clinical virology unit at St Thomas' Hospital, London, where 5,000 samples are already being re-tested, said that only people with a very high number of HIV antibodies were at risk of testing negative instead of positive with the IMX test. Antibody status may be related to the stage of infection, with people only recently exposed to HIV and those on the verge of Aids being most likely to fit this profile.
Nick Partridge, chief executive of the Terrence Higgins Trust, said anybody who was wrongly tested as negative might be able to sue Abbott Laboratories.
A spokesman for one of the London clinics which carried out the tests, at St Mary's Hospital, Praed Streetsaid: "We expect the retesting to take about one month."