Thursday, 6 September 2007

HPV testing

As I've mentioned before, cancer of the cervix is unusual in that it is believed that a certain infection is absolutely necessary to get the cancer. This means that preventing the infection tends to prevent the cancer, and, further, that a vaccine for a cancer is theoretically possible. The infection, of course, is HPV - human papilloma virus - and the vaccine is new on the block but already up and running.

Sadly, preaching abstinence and safe sex is seldom enough. Realistically, people still get STDs (lots of people!) and so people can still get HPV... and so people can still get cervical cancer. What has made much more of an impact than the standard use-a-condom approach is screening.

Screening is particularly effective in cervical cancer because the process by which the normal cells of the cervix become cancerous (carcinogenesis) is quite slow in this case - it averages about 10 years. Therefore screening entails looking for signs that some cells might one day end up cancerous, following which the appropriate action can be performed (which might be removing the relevant piece of the cervix, for example).

Until recently, screening was limited to Pap smears. In this procedure, a scraping of the cervix is made and the cells are examined under a microscope. Cells that look precancerous are thus identified, since they usually look different enough from the normal cells. Pap smears have absolutely revolutionised cervical cancer, and almost alone account for the significant drop in the cancer's incidence since their introduction.

However, no system is foolproof. Despite their usefulness, the process still relies on human judgement. While almost every pathologist can tell completely normal from absolutely cancerous, there is naturally always going to be a grey area in between, where the significance of some slightly-odd-looking-could-be-normal-could-be-precancerous cells is uncertain. Pap smear evaluations have been devised to help alleviate this difficulty, but is there not some other way?

Well, a new screening method has recently been developed. It's called, helpfully enough, the HPV test, and it aims to identify cervical infection with the subtypes of HPV that are known to place you at high risk for cervical cancer. The nice part is that the test is mechanised, meaning that human error is largely eliminated. Essentially the test hunts for the DNA of the dangerous types of HPV, and then (almost literally) lights up when it's been located.

By itself, the test is no substitute for the Pap smear, because human interpretation is very useful in this situation, despite its downside. But no one is aiming to make you choose. Increasingly, health authorities are approving the HPV test's use in conjunction with Pap smears. Together, they are highly sensitive for identifying woman at risk for cervical cancer - one study put the figure at close to 100%.

With any test that exhibits such great sensitivity (it picks up all potential cases), there is the risk of false positives. In other words, since the test is so sure to pick up all possible cases, there will be some cases that would not have developed cancer, who are now labelled as being precancerous. The result: an increase in 'unnecessary' operations.

The recommendations work around this problem though, at least in part. If the test does come back positive, a further specialised exam is done (colposcopy) to check its significance. Furthermore, there are no doubt many women who would rather be safe than sorry, and for whom the 'false positives' problem simply isn't a problem.

In my view, therefore, the HPV test offers a potentially useful adjunct to the cervical cancer story. There is a web site on the HPV test ( which is a truly outstanding resource, offering information on HPV, diagnostic modalities and treatment options in a user-friendly yet comprehensive way. Check it out if you're interested!

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