A reference range is an interval derived from measurements in a specified reference population under specified analytical conditions. It gives context for a laboratory result; it is not automatically a boundary separating disease from health. Different methods, ages and reference populations can lead to different intervals. [ref]
How to Understand Your Lab Results by MedlinePlus. Reference ranges: laboratory differences, healthy people outside the interval and illness within it
For an illustrative central 95% interval, about 5% of measurements from the defining population lie outside by construction. If 20 independent tests each have a 5% outside probability, the probability of at least one outside result is . Independence is an assumption, but the calculation shows why many tests increase incidental flags. A clinical decision threshold can instead be chosen for a particular diagnostic or treatment purpose.
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