Analytical quality specifications for common reference intervals

Interpretation of laboratory test results requires comparison to some type of reference value or reference interval. These comparisons can be crosssectional (populationbased reference interval and cutoff values) or longitudinal (reference change value). Quality specifications for crosssectional comparison have been established by determining the influence of analytical bias and imprecision on the percentage of the healthy population falling outside the reference limits, when sharing populationbased reference intervals in a Gaussian distribution of results. Quality specifications for longitudinal comparisons are equally important and are often overlooked, since less work has been done in this area. Some criteria suggest that a difference between consecutive results designates a true change in a patient health status when the difference is higher than the withinsubject biological variation plus the withinlaboratory analytical variation. In this chapter we discuss the clinical considerations and laboratoryrelated factors that must be considered when quality specifications are applied to sharing reference comparisons. Real life experience shows that different analytical methods can produce comparable results when common quality goals are established, and quality can be achieved through a willingness to work together. Within the existing organization, the current specifications for analytical quality and a dedication to quality health care makes it possible to achieve transferability between laboratories within a geographic area.