Here are a few test results I've been asked about. Again I add the disclaimer that I am not a physician, nor am I qualified to make diagnoses, or even say exactly why a test was ordered or what it means. I do however, have a very good background in the science of the immune system, and can therefore offer an overview of what various tests usually mean.
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 produce 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.
High Platelet Count:
It is more common for a lupus patient to have low platelets than high platelets. It is known, however, that some patients do have high platelet counts for reasons that are not entirely understood. One theory is that in these patients the spleen, which works to remove old or damaged cells from the blood, is damaged in lupus patients with high platelet counts. This is probably only true in a minority of lupus patients, but when it is true it means that the doctors really need to monitor signs of damage to the spleen.
It’s also worth noting that some healthy people also have high platelet counts. In these people the only real concern is that if the count is too high then the person is at risk of forming blood clots, since platelets exist in the blood to stop bleeding. If there are too many platelets around they can stop blood flow even in blood vessels, which leads to different organs’ not being able to get the blood they need to survive.
As with platelets, low C4 levels are more common than high levels in lupus patients. However, C4 can serve as a general inflammatory marker. It doesn’t tell the doctor specifically what’s wrong, just that there is inflammation occurring somewhere in the body.
As I previously wrote, complement proteins (including C4) are important in the immune response. It isn’t surprising that when the immune system is active it tends to make more complement proteins. In most lupus patients these proteins are then used up as the immune system tries to kill the body’s own cells. This isn’t true in all patients, though, and the fact that C4 is elevated can serve to tell the doctors that the immune system is active and causing inflammation somewhere.