What is "normal" for lab test results?

Standard "normal" ranges for tests with numeric values are based upon use of a bell shaped curve. "Normal" is defined as those test values that fall within 2 standard deviations of the mean, which includes 95% of all results.

Thus, there is a 1 in 20 chance that an "abnormal" test may really be normal. If you perform 20 or more independent tests (which is not uncommon), then there is a greater than 50% likelihood that one or more tests will be "abnormal" just from statistical variation.