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Thursday, 8 April, 1999, 10:53 GMT 11:53 UK
Blood: The risks of infection
12:30 09-02-99 blood ac
Blood donations are screened for disease
Any national blood service has the difficult task of providing thousands of litres of blood a day and ensuring that it is infection-free.

This has been made easier over the years with improved screening techniques and treatments to destroy contaminants, but it is possible for viruses to slip through undetected.

How much blood is donated?

The UK National Blood Service (NBS) collects 10,000 units of blood every day to meet demand.

The typical donor would contribute one unit (450ml) in a session, and would attend two to three sessions a year.

In the US, approximately 40,000 units of blood are needed each day.

In most countries all the blood used for transfusions is now tested for HIV. In those countries where the blood has been tested, infection through a blood transfusion is now extremely rare.

In the UK, all people at an increased risk of HIV infection have been asked not to donate blood since 1983.

All samples are screened for HIV, and should a sample turn out to be infected the donor is informed.

They are then offered counselling and advice.

How does screening of donated blood detect disease?

Routine screening of blood looks for antibodies, as Patricia Murchie of the NBS explains.

"After infection, the body starts to react against the virus and builds up antibodies to fight it. Our tests look for the antibodies," she says.

But as some viruses - particularly hepatitis C and to a lesser extent HIV - take longer to produce antibodies, the test will not always detect them.

"If you're only infected a week ago, it might not have shown up yet," Ms Murchie says.

She says that in reality it is very rare for this to happen.

Hepatitis C infections are the most likely to slip through, as it takes the body a relatively long time to start producing antibodies for the condition - up to 60 days.

However, there have only been two cases of HIV slipping through the screening programme since 1985, according to figures from the Public Health Laboratory Service (PHLS).

The use of components of these donations has led to HIV infection in five patients, it says.

The NBS is introducing more thorough screening techniques using genetic methods over the next year.

International studies have shown these to be effective at detecting HIV and hepatitis infections before antibodies are produced.

How does heat treatment improve safety?

Not all blood can be heat-treated.

Donated blood is usually broken down into its various components. This is more economic as not everyone will need whole blood - most of the time they just need one element.

It is divided into:

  • Red blood cells;
  • White blood cells;
  • Platelets;
  • Plasma;
  • Plasma derivatives.

Plasma derivatives are concentrates of specific plasma proteins and include anti-clotting factors that are used to treat people with haemophilia.

These are the parts of blood that are heat-treated to make them safe. The process kills certain viruses, including HIV and hepatitis B and C.

The technique has been available since March 1985, although in France, controversially, it was not introduced until August of that year.

The PHLS estimates there are about 500 individuals living in the UK with HIV infection acquired through treatment with blood-clotting products.

More than 1,300 HIV infections are attributed to treatment in the UK for disorders such as haemophilia A or B, the service says.

But it says there have been no reports of HIV infection due to clotting factor treatment for haemophilia in the UK since the introduction of heat-treated blood products.

Overall, the service says: "Blood transfusion and tissue transplantation have been responsible for less than 1% of reported diagnoses and Aids cases."

See also:

01 Dec 98 | Aids
29 Jan 99 | Health
09 Feb 99 | Europe
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