An analyte's index of individuality assesses utility by comparing which quantities?

• A. It compares only the between-individual variation.
• B. It compares within-individual variation to between-individual variation, while considering analytical variation.
• C. It indicates the speed of change over time.
• D. It measures instrument precision.

Answer: (B)

The index of individuality is a way to judge how useful a population reference range is for interpreting a single person’s test result, by looking at how much a given analyte varies within the same person over time versus how much it varies between different people. It combines within-person (intra-individual) variation with between-person (inter-individual) variation, and it also accounts for analytical variation from the measurement process. When the within-person variation is small compared to the between-person variation, population references are less useful for that person and tracking changes against their own baseline becomes more informative. If within-person variation is large relative to between-person variation, population references are more applicable.

So the correct idea is comparing within-individual variation to between-individual variation, while considering analytical variation. The other options don’t capture this comparison or the role of measurement error: focusing only on one type of variation, or describing change speed, or measuring instrument precision alone.