(An updated version of an older post).
High Creatinine Levels:
Creatinine is a normal byproduct of our muscles’ day-to-day functioning. In a normal person creatinine is made when muscles “eat” nutrients. This creatinine is dumped into the blood, and then the kidneys filter it out of the blood and dispose of it in the urine. This is one of the major roles played by kidneys: kidneys filter all sorts of chemicals out of the blood.
If everybody makes creatinine, why do doctors check creatinine blood levels? As I said, creatinine is normally filtered out of the blood by the kidneys. This means that creatinine levels in the blood of a healthy person are quite stable. However, if for some reason (like, say, lupus) the kidneys are not functioning well, then they will be unable to filter the blood properly. This will lead to creatinine build-up, as muscles continue to make creatinine but the kidneys are not removing it from the blood. So the creatinine test serves as a basic test to see if the kidneys are working properly or not. If you have high creatinine levels then the answer is most likely, “or not,” and in a lupus patient this means that your disease is active and is interfering with your kidneys.
As an analogy, think of creatinine as the packaging that comes with food. Everybody ends up with empty bags, bottles, etc., and most of us just dump it into the garbage can. The garbage men (kidneys) then come and take the garbage out to the dump (urine) so we don’t trash lying around our houses. If, however, the garbagemen go on strike or get sick, we’d all find that we had a lot of trash building up around our homes. So testing the amount of trash in someone’s home can tell you whether or not their garbagemen are doing their jobs.
A question that a lot of lupus patients have is why they retain water when their creatinine levels are high. As a matter of fact, it's not necessarily the creatinine itself that causes water retention. The creatinine result simply tells you that the kidney is not working well. Since the kidney is also supposed to control how much fluid we retain, if it's not working well then it can cause the body to retain too many fluids, which causes swelling. This swelling is particularly noticeable in the ankles and knees.