Tumor markers

Tumor markers are substances that can tip doctors off to cancer. They're made by cancer cells, the cells that make up a tumor, or by normal cells in the body reacting to that cancer. Labs look for them in blood, in urine, or in cells taken from the tumor itself during a biopsy, which means removing a small piece of tissue so it can be studied.

Here's why they work. Some cancer cells make certain proteins, molecules that cells produce, in larger amounts than healthy cells do. Those proteins leave a kind of trail: finding them in a sample can be a sign of cancer. And when they're measured serially, meaning several times over the course of treatment, the way their amount changes can show whether the tumor is responding.

But a tumor marker is a lot like smoke. Smoke makes you think of fire, yet an old car with a worn-out engine puts out smoke too, and some fires barely smoke at all. Tumor markers work the same way: some conditions that aren't cancer can still raise their level, and not everyone with the cancer linked to a given marker has a high level of it. That's why a high level doesn't mean cancer on its own, and the result is usually combined with biopsies or imaging tests, the kind that let doctors see inside the body.