Strands of hair may predict just how effective drug therapy is for those living with HIV, the AIDS virus. Hair samples offer a simple but accurate test as to whether anti-retrovirals are suppressing the virus.

To learn whether AIDS medications are working, doctors often rely on patients themselves for information. But Dr. Monica Gandhi of the University of California San Francisco says this method has inherent problems. 

"We often ask the patient what is their adherence to their medication? How much are they taking of their medication when we're trying to predict why they're not doing well. And self-reported adherence has its own set of limitations. There's, you know, bias in how people present their level of adherence and there's literally just memory problems in people not remembering how much they're taking," she says.

And testing blood samples to check anti-retroviral levels can give misleading results.

"The problem there was everyone's blood levels of drugs varies day by day. So depending what I ate for breakfast that morning or if I smoked that morning or if I took another medication that interfered, the levels are sort of going to be up and down every day," she says.

Dr. Gandhi led a team of US researchers that found levels of anti-retrovirals in hair samples strongly correlated with HIV levels. The hair samples can show which drugs are being used well, which are not, and even whether the patient may not be taking enough of a certain medication. Since hair grows at a certain rate, about a centimeter per month, tests can give an average reading on how well the drugs are working.

"We collect a small sample of hair from the back of their head and by small I mean 10 to 15 strands. So we collect after about a month of therapy a small thatch of hair from the back of your head and then grind it up and measure the anti-retroviral in that hair. And that gives us an idea after you start a new regimen whether you have enough in your system," she says.

Too little anti-retrovirals in a patient's system and HIV can become resistant to the medications. Too much and the patient can become ill from the side effects.

Dr. Gandhi says some hairs are good for testing, others are not.

"We have tested it on all types of hair, African hair. We actually have some projects in Uganda with African hair. It works on fine hair. It works on gray hair. One thing is that people ask can we use pubic hair for these measurements. And we don't think that those are going to be useful because hair in those areas grow to a certain length and then they stop, which is great for anyone who has this hair. But you really want to measure hair that's sort of growing continuously and that's really scalp hair," she says. 

The test is described as easy, painless, bloodless and bio-hazard free. However, while the hair clipping can be done in remote, rural areas in developing countries, an expensive machine at a central lab is currently needed to analyze the samples. That's one of the problems that must be overcome to help health care workers in developing countries better assess the effectiveness of AIDS drugs.