Defining laboratory reference values and decision limits: populations, intervals, and interpretations

Abstract

This article provides a brief overview of various approaches that may be utilized for the analysis of human semen test results. Reference intervals are the most widely used tool for the interpretation of clinical laboratory results. Reference interval development has classically relied on concepts elaborated by the International Federation of Clinical Chemistry Expert Panel on Reference Values during the 1980s. These guidelines involve obtaining and classifying samples from a healthy population of at least 120 individuals and then identifying the outermost 5% of observations to use in defining limits for two-sided or one-sided reference intervals. More recently, decision limits based on epidemiological outcome analysis have also been introduced to aid in test interpretation. The reference population must be carefully defined on the basis of the intended clinical use of the underlying test. To determine appropriate reference intervals for use in male fertility assessment, a reference population of men with documented time to pregnancy of < 12 months would be most suitable. However, for epidemiological assessment of semen testing results, a reference population made up of unselected healthy men would be preferred. Although reference and decision limits derived for individual semen analysis test results will undoubtedly be the interpretational tools of choice in the near future, in the long term, multivariate methods for the interpretation of semen analysis alone or in combination with information from the female partner seem to represent better means for assessing the likelihood of achieving a successful pregnancy in a subfertile couple.

Figure 1

The 95% reference intervals are derived by identifying the most outlying 5% of observed values in a reference population. Most often, these outlying observations are split evenly between the ends of the test result distribution in the reference population, 2.5% at each end of the distribution, resulting in a two-sided reference interval (A). For some tests (e.g., sperm concentration), only low values are of clinical concern, and therefore the 5% of outlying observations for the reference interval are identified and excluded only from the low end of the distribution, resulting in a one-sided reference interval (B).

Figure 2

Distributions of test results for sperm concentration are plotted for the hypothetical population subgroups of fertile and infertile men. As these are hypothetical distributions, no specific sperm concentrations are indicated on this graph. Infertile men as a group are displayed as having lower average sperm concentrations than fertile men, and more infertile than fertile men have sperm concentrations below the lower reference limit (shaded area). The relative heights of the distribution curve for each group of men are displayed for several vertical line segments, each of which represents a different sperm concentration. The ratios of distribution heights for the infertile men/fertile men represent the likelihood ratios for infertility at each sperm concentration. Thus, for the line segment at the farthest left, corresponding to a low sperm concentration, the ratio of line heights is 6/1, yielding a likelihood ratio for infertility of 6. Conversely, at the line segment to the farthest right, corresponding to a higher sperm concentration, the ratio of line heights is 4/21, which gives a likelihood ratio for infertility approximately equal to 0.2.