The effects of HIV can take up to 12 weeks to show in the blood. So it is best to have a test three months after the point when you think you might have been at risk to infection.

The test: Dr David Owen, of the John Hunter Clinic, explains the procedures involved. First, the client is asked to fill in a registration form (he or she does not have to give his or her real name or address) and is given a confidential personal identification number.

The client then sees a trained health adviser on a one-to-one basis for about an hour - or longer if so desired. The client's sexual history is discussed and the level of risk of exposure to HIV discerned.

'We try to explain the risk attached to certain activities and discuss safe sex methods. We also test for sexually transmited diseases, says Dr Owen.

'If the client has had unsafe sex several times, then we discuss what may happen in the event of a positive result. We also talk about behavioural changes for those who consistently find themselves in non-safe situations.'

At this stage, 90 per cent of clients decide to have the test. A nurse takes a blood sample, which is analysed. If positive, the result is checked again and again. The client returns within three days to be given the result by the counsellor who initially met him or her. Further confirmatory tests can be requested, and no results are ever given over the telephone.

If the result is positive, the counsellor will ensure that the client is put into direct contact with support organisations.

Case studies: Jeanette is 40 and lives in west London. She was a senior catering manager until she was sacked when her employers learnt she was HIV positive in 1984. She now has full-blown Aids. My counsellor tried to encourage me not to have the test, and wanted to make sure I was ready for it. But I knew that I had to have it because I had slept with someone who turned out to be positive.

'In those days, it was a seven-week wait for the result. Now it's so much faster. It was a shock to learn that I, too, was HIV positive. I didn't believe it, so I asked for another test. But it was still the same.

'The testing process is so much more humane now. Back then, nobody really knew how to cope with people's anxieties. They weren't very considerate. The support people get now is great. We've all learned so much.'

Simon 32, lives in northwest London. He went for a test at a major London hospital in June 1991, where he tested positive. It was only by accident that he discovered, at another London hospital in September 1993, that he was in fact negative. He has since had further tests which have confirmed the negative result.

'The counsellor prepared me really well. I was dreadfully disappointed when I got the positive result, although I knew that I had been at risk. I was numb. They were very helpful in telling me that you can live for years with HIV, that it isn't just a death sentence. But it took a long time to come to terms with it.

'I went to another clinic two years later, just to accompany a friend who was nervous of having his first test. He didn't know about my previous test result, so I had another test as well to make him feel OK.

'When we came back for our results, we sat in the waiting room with about 20 other people. I went in and the doctor said, 'Good news, you're clear'. I was stunned. My friend and I - he was negative - then went off for a long drink. I was both happy and angry. Angry that I had spent so much time thinking the worst, although I wouldn't take it any further with the hospital concerned. They have few enough resources as it is, never mind finding the money to fight a legal battle.'

Professor Jangu Banatavala, head of virology at St Thomas's Hospital, says that such misdiagnosis is extremely rare nowadays.

'If an initial blood sample tests positive, we subject it to two different sets of tests. Should it still remain positive, a further sample from the individual is then tested in three different ways. The chance of an incorrect result after this is about one in 4 million.

Where to go: NHS clinics are all free.

The HIV Education Centre, Colindale Hospital, Colindale Ave NW9 (081-905 9779) Mon-Fri, 9-5pm. Same-day result service Mon and Wed, otherwise one week. Appointments necessary.

James Pringle House, 73-75 Charlotte St W1 (071-380 9141). Open Mon-Fri, 9-6pm. One-week result service.

Community Health and Drug Service National Temperance Hospital, 122 Hampstead Road NW2 (071-380 9762). One-week result service.

Royal Free Hospital, Pond St, Hampstead NW3 (071-431 0970). Mon/Tues/Thurs 9-10.30am. Appointment necessary.

St Bartholomew's Hospital (2nd Floor) West Smithfield EC1 (071-601 8090). Mon/Thurs/Fri 9-3pm; Tues-Wed 2-6.30pm. Walk-in clinic, 24 and 48-hour result service.

Ealing Hospital (Pasteur Suite) Uxbridge Road, Southall UB1 (081 967 5555). Mon-Fri 9.30-5pm. Appointment necessary. One-week result service.

John Hunter Clinic St Stephen's Clinic, 369 Fulham Road SW10 (081-846 6155). Mon-Fri 9.30am-4.30pm/closed Fri 12-2pm. Walk-in clinic, three-day result service.

Victoria Clinic for Sexual Health 6 Osbert Street (off Vauxhall Bridge Road) SW1 (081-746 8066). Mon 9-6.30pm, Tues-Fri 9-5.30pm (Tues 5.30-7pm men only). Mon and Tues same-day result service, otherwise 48 hours. Appointment necessary.

St Thomas's Hospital (Lydia Department, Lambeth Wing, 1st Floor) Lambeth Palace Rd SE1 (071-928 9292, ext 2328). Mon-Fri 10-6pm. Walk-in clinic. One-week result service.

Wolverton Centre Kingston Hospital, Wolverton Rd, Kingston, Surrey (081-546 7711, ext 2843). Tues 9-12 noon/Thurs 3-5.30pm. Two-day result service.

Support: Body Positive 51B Philbeach Gdns SW5 (071-835 1045). Drop-in social and information centre. Helpline 071-373 9124 Mon-Fri 7-10pm/Sat-Sun 4-10pm.

Women's group meet on Tues 6-9pm. Positive Youth meet Fri 6-9pm.

London Lighthouse 111-117 Lancaster Rd W11 (071- 792 1200). Residential care and information centre providing support to people with HIV, Aids and those close to them.

National AIDS Helpline (0800 567 123). 24-hour advice service.

Postively Women Helpline (071-490 2327). Mon-Fri 12-2pm.

The Terence Higgins Trust Helpline (071-242 1010). 12-10pm.

Black HIV/Aids Network (BHAN) (081-749 2828). Counselling and advice to black and minority communities. Appointment necessary. Drop-in advice and social centre at 111 Devonport Road W12, Mon-Fri 9.30-5.30.

(Photograph omitted)