What is the definition of the “Healthy Reference Interval”?

The “Healthy Reference Interval” is a statistical term referring to the most-common 95% of values seen in the healthy reference population. For parameters that have a bell-shaped normal (or Gaussian) distribution, these values fall between the 2.5th percentile and the 97.5th percentile, or within 2 standard deviations on either side of the mean, as shown in Figure 2.

Figure 2. Illustration of the boundaries of the healthy reference interval on a Gaussian shaped distribution

For parameters that have skewed distributions, the reference interval is defined either as the interval below the 95th percentile (p95) or above the 5th percentile (p5), depending on the direction of the skew. Figure 3 illustrates a positively skewed parameter:

Figure 3. Illustration of the boundaries of the healthy reference interval for a positively skewed parameter.

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Updated January 3, 2023