The International Federation of Clinical Chemistry (IFCC) published between 1987--91 a series of six recommendations on reference values in laboratory medicine. This paper reviews the history and scope of the fifth part of these recommendations. This fifth recommendation deals with statistical methods used for analysis of reference values and estimation of reference intervals. The RefVal program, which implements the recommended method, is also described.
Background: The aim of this study was to develop new methods for partitioning biochemical reference data, covering in particular nongaussian distributions. Methods: We recently proposed partitioning criteria for gaussian distributions. These criteria relate to proportions of the subgroups outside each of the reference limits of the combined distribution (proportion criteria) and to distances between the subgroup distributions as correlates of these proportions (distance criteria). However, distance criteria do not seem to be ideal for nongaussian distributions because a generally valid relationship between proportions and distances cannot be established for these. Results: Proportion criteria appear preferable to distance criteria for two additional reasons: (a) The prevalences of the subgroup populations may have a considerable effect on stratification, but these are hard to account for by using distance criteria. Two methods to handle prevalences are described, the root method and the multiplication method. (b) Tied reference values, another complication of the partitioning problem, could also be hard to take care of using distance criteria. Some solutions to the problems caused by tied reference values are suggested. Conclusions: Partitioning of biochemical reference data should preferably be based on proportion criteria; this is
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