Which statement about II is true for guiding RI interpretation?

• A. Population RI is always preferred
• B. Subject-based RI is never useful
• C. Population-based RI is sufficient when II is high
• D. Both RI types are equally informative when II is high

Answer: (C)

The index of individuality tells you how much a lab value varies between different individuals relative to how much it fluctuates within the same individual over time. When this index is high, between-subject differences are large while within-subject variation is relatively small, so a population reference interval provides meaningful context for interpreting a value in an individual patient. In that scenario, relying on population-based RI is sufficient because the individual’s result can be reasonably assessed against the broad population norms, and personal baseline changes add little extra information. If the index were low, the opposite would be true: within-subject variation dominates, and a subject-based (individualized) RI would be more informative to detect meaningful changes for that patient. The other statements fail because they overstate the utility of one approach in all cases or suggest equal informativeness when II guides a clear preference.