C-reactive protein test (CRP)

C-reactive protein is a protein your liver makes when there's inflammation in the body. Inflammation is the body's way of protecting its tissues after an injury or an infection. The C-reactive protein test is a blood test that measures how much of that protein is circulating.

Here's what happens inside. When inflammation or an infection starts, the main signal that alerts the liver is a messenger molecule called interleukin 6. That signal switches on the C-reactive protein gene, the set of instructions for making it, and the liver starts producing the protein. That's why it's called an acute-phase reactant: one of the substances that respond as soon as the trouble begins. Its level rises quickly, and once whatever set it off dies down, it soon drops again.

And this is where the test hits its limit. Many conditions raise C-reactive protein, from bacterial or viral infections to autoimmune diseases like lupus, in which the body's defenses attack its own tissues. It's like seeing smoke in the distance: you know something is burning, and whether there's a lot of smoke or just a little, but not what's burning or which house it is. The result works the same way: it tells you how much inflammation there is, but not what's causing it or where it is.