There are two main misconceptions about test results and reference ranges:
Myth: "An abnormal test result is a sign of a real problem."
Truth: A test result outside the reference range may or may not indicate a problem—the only sure signal it sends is that your doctor should investigate it further. You can have an abnormal value and have nothing wrong—but your doctor should try to determine the cause.
It’s possible that you fall in that 5% of healthy people who fall outside the reference range. In addition, there are many things that can give a misleading result without indicating a major problem. A high blood sugar could be diet-related rather than caused by diabetes. A lipid result could be high because you didn’t fast before the test. If your doctor is unsure about the test result they may wish to repeat it. Some abnormal results may disappear on their own, especially if they are on the border of the reference range.
Myth: "If all my test results are normal, I have nothing to worry about."
Truth: It is certainly a good sign, but it’s only one set of tests, not a guarantee. There is a large overlap amongst the results from healthy people and those from people with diseases, so there is still a chance of an undetected problem. Just as some healthy people’s results fall outside the reference range, lab test results in some people with disease fall within the reference range. It may also be the case that the disease may not cause any abnormality in the particular tests that you had. For example early cervical cancer won’t cause any abnormality in common blood tests but may be obvious on a Pap smear.
If you’re trying to follow a healthy lifestyle, take it as a good sign, and keep it up. But if you’re engaging in high-risk behaviour, such as drug and alcohol abuse or a poor diet, it only means "so far so good," and the potential consequences haven’t caught up with you yet. A good test result is not a licence for an unhealthy lifestyle.
If you previously had abnormal results, normal results certainly provide good news. But your doctor may want to conduct follow-up tests some months later to make sure you’re still on track and to follow any trends.