If II is below what value is subject based RI preferred?

• A. II < 0.6
• B. II > 1.4
• C. II > 0.6
• D. II ≈ 1.0

Answer: (A)

The index of individuality tells you whether a population reference interval or a subject-specific baseline is more informative for interpreting a test result. It’s the ratio of within-subject biological variation to between-subject variation (II = CV_I / CV_G).

When this index is very small, meaning an individual’s results vary little over time while the population shows wide variability, a person’s own data are most informative. In practice, if II is below 0.6, subject-based reference intervals are preferred because a single population range doesn’t adequately reflect an individual’s typical values.

If II is high (above about 1.4), results vary more within an individual than between individuals, so a population reference interval can help distinguish whether a result is typical for the population rather than for that specific person.

Values in between are intermediate, where either approach can be acceptable depending on the clinical context. So for a threshold, subject-based RI is preferred when II is below 0.6.