What is PSA, and how is it linked to prostate cancer?

Kat Arney answered this question...

PSA is a molecule produced by prostate cells. There is a test: You can measure people's blood and see the level of PSA in it. This is called the PSA test. In some ways, it's a screening test for prostate cancer, but it's not part of the National Screening Programme for a number of reasons.

The big problem is that if your prostate starts growing, if it's got a cancer in it, then in many cases, it will produce lots and lots of PSA. So if a man has this test, and it shows that you have a very high PSA level, it's suggested that you might have a prostate cancer. But the problem is that at the moment - and this is what the problem with breast cancer screening is turning out to be - we can't tell which are the dangerous cancers; these dangerous prostate cancers that a man should definitely have treated. There are side effects of the treatment; it can leave men impotent, it can leave them incontinent. And we also can't tell which cancers will grow very slowly and cause no problems to a man in his lifetime. It's important to remember that most men in their 80s have some kind of prostate cancer, but it's not going to be the thing that kills them.

The other thing with the PSA test is that actually, some types of prostate cancer don't produce a lot of PSA. So, you'll miss men that do have prostate cancer because it won't pick them up.

So you could miss some dangerous cancers, and you could over-diagnose men that don't really have a prostate cancer that's going to kill them but could lead to them having unnecessary treatment, which is why it's a very difficult test to explain. There's a lot of information that doctors can give to men, deciding whether they want to have a PSA test and inform them about the options.