Body & HealthMental model
One healthy person in twenty falls outside the normal range.
A standard reference range is defined so that 95 percent of a healthy comparison group falls inside: 2.5 percent below, 2.5 percent above. A value outside it is therefore only statistically unusual at first, not pathological. The more values you measure at once, the likelier at least one lands outside.
Useful: A flagged lab value is a pointer to a question, not the answer to it.
Source: en.wikipedia.org