Q:

How is a reference interval determined?

A:

Reference intervals are based upon sound clinical and statistical principles, as appropriate to the individual test. Examples of principles used to establish reference intervals include, but are not limited to: use of so called "normal" or "healthy" subjects, statistical analysis of previously analyzed specimens (e.g., use of the laboratory database), and use of published reference intervals.

NCCLS publishes guidelines as presented in document C28-A2: How to Define and Determine Reference Intervals in the Clinical Laboratory; Approved Guideline, Second Edition.

To ascertain the reference data, the following are determined for a given analyte:

Reference individual: A person selected for testing on the basis of well-defined selection and/or exclusion and partition criteria. Exclusion criteria may include recent illness or surgery, alcohol consumption, tobacco use, drug abuse, prescription drug use, genetic factors, obesity, lactation, occupation, etc. Partition criteria may include sex, age, race or ethnic background, geographic location, diet, blood group, stage of pregnancy, etc.

Reference population: A hypothetical group consisting of all the reference individuals.

Reference sample group: An adequate number of persons selected to represent the reference population. In a priori sampling, the exclusion and partition criteria are defined before the selection of reference individuals. In a posteriori sampling, the process of exclusion and partitioning takes place after analyte testing and is often performed on existing data. The ideal number of reference subjects is 120 per partition, but often less are used.

Reference value: A value (i.e., test result) obtained by the observation or measurement of a particular type of quantity on a reference individual. Reference values are obtained from assays performed on samples from the reference sample group.

Reference distribution: The distribution of the reference values. The parametric method assumes that the observed values follow a known distribution curve, for example, a Gaussian (i.e., "normal" or "bell") probability curve. The nonparametric method of estimation makes no specific assumption about the shape of the probability distribution. The method employed depends on the analyte.

Reference limit: A value derived from the reference distribution and used for descriptive purposes. For most analytes, the lower and upper reference limits are assumed to demarcate at the 2.5^th and 97.5^th percentiles of the reference values. Thus 5 percent of the values are classified as outside the limit, (2.5 percent at each the upper and lower end), and the reference limit assumes to enclose 95 percent of the values for the population.

Reference interval: The interval between, and including, the lower reference limit and the upper reference limit. In some cases, only the upper reference limit (x) is important, and the corresponding reference interval would be 0 to x.

Observed value (patient laboratory test result): The value of a particular type of quantity, obtained by observation or measurement of a test subject (i.e., patient), to be compared with reference values, reference distributions, reference limits, or, most frequently, reference intervals.