CRP

Inflammation is the way your body clears damage: with an infection, a wound or an injury, immune cells send out signal substances, and your liver answers with a series of proteins. CRP is the fastest and the most sensitive of them. It sticks to damaged cells and to bacteria and marks them for the immune system, so it knows what needs clearing. At rest your liver makes hardly any; within six hours of the start of an inflammation the amount in your blood can be a hundred times larger. That speed is the strength of CRP. It peaks after a day or two and has a half-life of less than a day, so as soon as the cause falls away, the value is back at its old level within a few days. CRP therefore follows what is happening now, not what happened last month. A bacterial infection usually lifts the value much further than a virus, and a sizeable operation or injury does the same as an infection. The other side of CRP is quieter. Fat tissue in your belly, smoking, little exercise and poor sleep keep the liver in a light, continuous inflammatory state, and then the value stays slightly raised without you feeling anything. Your report shows CRP in mg/L. Normal is below 5 mg/L, where three of the four labs draw the line; above 10 mg/L the value is raised wherever you were tested, and for the smouldering inflammation it is mainly the area below that which counts.

CRP has two faces, and both are useful. The first is the acute one: if you have a fever, pain or are ill, CRP shows how large the inflammatory response is and whether it is heading the way of a bacterium. That is the question a GP answers with a finger prick before deciding on antibiotics, and it is also the reason that a value you measure while you have a cold says nothing about your usual level. The second face is the most interesting for anyone who wants to stay healthy. Atherosclerosis is an inflammatory process: the plaques in your vessel wall are built up by immune cells and made unstable by inflammation. Anyone with a resting CRP above 3 has over the years roughly twice the chance of a heart attack of someone below 1, independent of cholesterol. That is not a disease but a state of your body, and that state goes with exactly the things you can change: your waist, whether you smoke, how much you move, how you sleep and how you eat. Fat cells in your belly make inflammatory substances themselves, which is why CRP falls with the waist. On top of that, CRP distorts other values. Ferritin rises with inflammation and then hides an iron deficiency, serum iron and albumin fall, and glucose and triglycerides rise. A CRP from the same draw tells whether the rest of your report can be trusted. That is the reason CRP goes with almost every blood test, even when nothing is bothering you.

A low CRP is the healthy state. There is no lower limit that points to a deficiency, and a value the lab cannot even express as a number any more means that no inflammation is running right now, not even the smouldering kind. That is reassuring, especially beside a good ApoB and a normal fasting glucose. A high CRP means there is inflammation somewhere, and the height and the pace tell which. Above 10 mg/L it is almost always something acute: an infection, an injury, an operation, a flare of rheumatoid arthritis or a bowel inflammation, and then the value often runs to far above a hundred. If you know where it comes from, you need do nothing with the number; repeat it after recovery to see that it is back. If you do not know, and the rise persists or fever, pain or weight loss come with it, then a search for the source belongs with it. A value that hovers between 1 and 10 without you being ill, on two measurements at least two weeks apart, is the smouldering inflammation, and you read that beside your waist, your smoking, your sleep and your lipids. CRP belongs beside your white blood cells and neutrophils, which show whether an infection is the cause, beside ferritin, which becomes unreliable with inflammation, and beside ApoB and glucose, which together with CRP determine your vascular risk.