Experts usually allow patients between two and four months to see if they respond to the drugs. In the meantime if the drugs do not work they may fall ill. The virus is very good at adapting itself to try and outsmart the drugs used against it. If mutations are noted, scientists then have to study how that might affect drugs' ability to fight the virus. Phenotypic testing involves growing the virus in the presence of an anti-retroviral drug.
Scientists then measure how resistant the virus is to the drug.
Anti-retroviral drugs have dramatically reduced deaths from Aids Two US studies published last week found resistance was growing. In the UK, very little research is being carried out into prevalence of drug resistance. Only by funding tests will we learn how big a headache it is likely to be. He added that if the UK had similar figures to the US that could mean quite a few people were already resistant to treatment. And he warned that the percentage affected was likely to rise as drug-resistant strains were passed on, although drug companies are producing around two new drugs a year in an effort to keep ahead of the virus.