Wide Range of Lab Test Results Can Be "Normal"

Q:  Recently, I applied for life insurance that called for blood tests. I was surprised to see that my "GGT test," about which I previously knew nothing, was high. I have since learned that the GGT is a liver test, but I don't drink at all. How could it be high?

A:  Testing for the liver enzyme gamma-glutamyl transferase (GGT) is one way to look at the health of the liver and biliary system. Assuming that your other liver function tests were normal, there are several possible explanations for the elevated numbers.

First, let me comment on how the normal range for a blood test is determined. After testing a great many people, "normal" is arbitrarily set as the range of values that include the test results of 95% of the people taking the test. Laboratories presume that any value higher or lower than this range is abnormal.

You can see that an "abnormal" test does not necessarily mean that you are "abnormal." You may simply have a relatively high GGT but normal liver function. In the same way, an adult male who is 6 feet 5 inches tall is taller than more than 95% of adult men, but that does not mean he is abnormal.

When blood tests are done for insurance, they generally perform about 25 different tests, and each test has the same possibility for this kind of error. That means that if just one test is checked, about 5% of the time the test will be abnormal (by definition). When 25 tests are checked, about 70% of the time at least one test will be abnormal.

When your doctor orders blood tests, he or she interprets the tests based on your symptoms and your medical conditions. With insurance companies, they do not have that knowledge and therefore cannot put an abnormal test result in perspective.

GGT levels might be elevated for a number of reasons, including conditions like hepatitis, diabetes, pancreatic disease, kidney failure or the presence of a gallstone or tumor. But certain medications (and, as you noted, alcohol) also can also elevate the GGT, even in circumstances where the liver hasn't been injured. If you smoke or take alcohol or medications within 24 hours of your test, even that can temporarily elevate your results.

You should address your concerns with your doctor. In some situations he or she might want to do more tests to find out why your GGT is high. Further tests could be called for if the GGT is very high, if it is high on several different occasions, if you have symptoms that are unexplained, if your liver feels abnormal on physical examination, or if you have other abnormal lab tests.

Julie L. Mitchell, MD, MS, is an Assistant Professor of Medicine at the Medical College of Wisconsin. She practices internal medicine at the Froedtert & Medical College General Internal Medicine Clinic - East. Her column also appears in the Milwaukee Journal-Sentinel.