The development of the polymerase chain reaction has had a major impact on medicine. The ability to take a very tiny sample of DNA, even from a single cell, and amplify it to give enough material for easy analysis has opened many doors in medicine. PCR has triggered the development of diagnostic techniques which would have been impossible just a few years ago.

Medical developments which have resulted or are in progress as a result of PCR include:

• Infection detection. Amplifying the genetic material from a single bacterium or virus can provide a speedy and accurate diagnosis for serious infections such as AIDS, viral meningitis and TB, where getting the right treatment quickly can mean the difference between life and death. What is more, PCR makes it possible to identify new pathogens at a speed which would have been unimaginable a few years ago. The rapid identification of the Sars virus (see SARS attack) is one very recent example of the way PCR can help fight infections.
   
• Genetic screening – PCR makes it easier to identify individuals who carry genes which can cause problems like cystic fibrosis and muscular dystrophy. It has also made it possible to look at the genetic material taken from a single cell of a very early embryo during in vitro fertilisation, amplify it and identify potential problems relatively easily. In future, screens may even be developed for the genetic variations which give us an increased risk of developing problems such as heart disease or high blood pressure. This would allow people to alter their lifestyle to reduce their risk. Screening may also indicate whether a particular medicine is the best one for that person - this idea is called "personalised medicines."
   
• Cancer warning – cancers develop when small changes in the DNA of a cell mean that it loses the normal control of the cell cycle and grows and divides far too rapidly. Amplifying the DNA using PCR gives doctors and scientists the opportunity to pick up these genetic changes in cancerous cells early in the development of the disease. The earlier cancers are detected the greater the likelihood that they can be successfully treated.
   
• Diagnosis of bowel cancer often involves an investigation of the colon and tissue samples being taken from any areas which look suspicious. Using PCR, bowel cancer can now be detected from the DNA of cells shed in the faeces. This is an easy, quick and non-intrusive way (pleasanter for all concerned!) of making a diagnosis which gives treatment a much better chance of success.
   
• Tissue matching – in organ transplants, a close tissue match between the donor and the recipient reduces the chances that the new organ will be rejected. In the past this matching has been based on blood groupings and a few other major tissue markers. PCR will lead to increasingly sophisticated levels of tissue matching at the DNA level, as a DNA bank of all the people needing transplants can be built up and PCR enables analysis of a potential donor’s tissue to be carried out quickly and effectively. This in turn should lead to more successful transplants.