Reference range

In medicine and health-related fields, a reference range or reference interval is the range or the interval of values that is deemed normal for a physiological measurement in healthy persons (for example, the amount of creatinine in the blood, or the partial pressure of oxygen). It is a basis for comparison for a physician or other health professional to interpret a set of test results for a particular patient. Some important reference ranges in medicine are reference ranges for blood tests and reference ranges for urine tests.

The standard definition of a reference range (usually referred to if not otherwise specified) originates in what is most prevalent in a reference group taken from the general (i.e. total) population. This is the general reference range. However, there are also optimal health ranges (ranges that appear to have the optimal health impact) and ranges for particular conditions or statuses (such as pregnancy reference ranges for hormone levels).

Values within the reference range (WRR) are those within normal limits (WNL). The limits are called the upper reference limit (URL) or upper limit of normal (ULN) and the lower reference limit (LRL) or lower limit of normal (LLN). In health care–related publishing, style sheets sometimes prefer the word reference over the word normal to prevent the nontechnical senses of normal from being conflated with the statistical sense. Values outside a reference range are not necessarily pathologic, and they are not necessarily abnormal in any sense other than statistically. Nonetheless, they are indicators of probable pathosis. Sometimes the underlying cause is obvious; in other cases, challenging differential diagnosis is required to determine what is wrong and thus how to treat it.

A cutoff or threshold is a limit used for binary classification, mainly between normal versus pathological (or probably pathological). Establishment methods for cutoffs include using an upper or a lower limit of a reference range.