Blood tests have many different uses. They can help diagnose a condition, monitor a particular organ in the body such as the kidney or liver, and they can also be used to give measurements of bodily processes such as blood sugar or current levels of inflammation.
CRP as it is commonly known stands for C-reactive protein, which is a protein made by the liver in response to tissue injury or inflammation, and also rises in response to an infection. It is a good test to show the level of infection or inflammation, but doesn't indicate where it is coming from. If you cut your toe, had a heart attack, had a flare of rheumatoid arthritis or had the flu, your CRP levels would all rise above normal in response.
If your doctor is ordering a CRP test as part of your blood tests, it is because they want to get some information on levels of infection or inflammation. If you attended your doctor with flu symptoms, for example, a normal CRP level would be reassuring that there was not a serious amount of inflammation or infection. By itself, it is difficult to interpret, it needs to be matched with your symptoms, and it may prompt the need for more tests to find out the cause. The level of CRP can also point to how much inflammation is present. Once treatment is started - for an autoimmune condition, for example - the level can be used to guide how effective a treatment is over time, as you would expect anti-inflammatory medication to reduce these levels if it's working as it should. Hopefully your symptoms will also be improved.
The CRP increases very quickly after an infection or inflammation, the levels would change day-to-day. It therefore gives an indication of levels of infection or inflammation at a short term snapshot in time. The levels of CRP also drop very quickly when the inflammation or infection starts to get better. For this reason, doctors sometimes use it as a test to monitor how an infection is progressing, and how effective a treatment has been. This compares to ESR, another marker of infection, which rises only after a few days but continues to be elevated during a inflammation phase. It's therefore less useful or an acute infection, and better for long term inflammatory conditions.
You do not need to be fasting for this blood test, there are no special requirements. It can be taken like any normal blood test and is usually taken in a gold or yellow colour bottle. Remember to press hard for a good few minutes after the needle has been removed and keep your elbow straight, to prevent a nasty bruise!