Prerequisites for use of common reference intervals.

The theory of reference values was developed more than 30 years ago, but its application in most clinical laboratories is still incomplete today. This is for several reasons, the most relevant ones being the lack of standardisation of the analytical methods, resulting in method-dependent values, and the difficulty in recruiting the proper number of reference subjects for establishment of reference intervals. With the recent progress in method standardisation the first problem is reducing while the second can be addressed optimally via multicentre collaborative studies that aim to establish common reference intervals. To be effective this approach requires the following prerequisites: 1) the existence of a reference measurement system for the analyte; 2) field methods producing results traceable to the reference system; and 3) a carefully planned multicentre reference interval study. Such a procedure will produce results traceable to the reference measurement system for a large number of reference subjects, under controlled pre-analytical conditions. It will also enable a better understanding of the various sources of population variability, if there is the need for partitioning of a reference interval or if there are any limitations to adopting the established reference intervals on a national or global scale. Once reference intervals are determined, clinical laboratories can adopt a common reference interval provided: 1) the population that the laboratory services is similar to the one studied; 2) methods producing traceable results are used; and 3) analytical quality is within defined targets of precision and bias. Moreover, some validation of the interval using a small sample of reference individuals from the laboratory's population is advisable.