Don't worry about what is normal. Most lab tests have a range of normal values for blood tests that is based on a "bell curve," a visual representation of the results of many patients. For instance, if 1,000 healthy patients are tested to measure, say, a particular blood enzyme, chances are that most of them would have similar results - say 50 on a scale from 1 to 100. But some results would fall between 55 and 60 and some would be slightly lower, say 40-45. On this curve you would also see some much lower results, say 35, and a few that are much higher, around 65. If you picture a bell-shaped curve, the top is where most of the test results cluster, but some would be spread out along the curving sides, with a very few even at the bottom. If your results are within that top cluster, they're normal even if they're not exactly the same as your friend's.
It's also important to understand that you can be perfectly healthy and have a lab test result that deviates from normal. Even if the lab identifies your result as "low" or "high," as long as you are healthy, you can often assume that the number is normal for you.
I think it is a good idea for everyone to keep copies of lab tests done when they're healthy in order to compare future results to make sure that they remain normal for you - thus if your original results came back "low" but you're perfectly fine, you'll know that something has changed should future results trend higher or lower.
Ideally, lab tests are used to confirm a diagnosis, not to make one. If you're sick, a good doctor will take the time to gather and review your medical history in order to narrow down the possibilities. Then, a blood test can help pinpoint the diagnosis and can also be used to monitor your treatment.
Even if you are sick, I would urge you to avoid unnecessary testing. Always ask what the doctor is looking for and then ask, "If you find it, what's the plan?" For example, there's no point in checking the cholesterol levels of a 95-year-old who has never been on cholesterol medication, since no reasonable doctor would suggest starting treatment of a patient that age. Yet these tests are routinely ordered.
Another rule to follow: if you already have a diagnosis and are asked to take more tests, ask how the results will change the course of treatment. Don't accept "We just want to know," or "Your insurance will cover it" as answers. If a test isn't going to help you, there's no point in having it.
Andrew Weil, M.D.