How are subject based RI derived?

• A. From cross-sectional population sampling
• B. From serial measurements in a single individual
• C. By pooling inter-laboratory data
• D. By random sampling across species

Answer: (B)

Subject-based reference intervals come from tracking a single individual over time. By taking serial measurements in the same patient, you capture that person’s own normal range and how much their values naturally vary (within-subject variation and the assay's analytical variation). This allows you to detect meaningful changes for that individual, even if population norms would consider the value normal or if the population range is wide. In practice, repeated measurements in one animal establish a personalized baseline and a narrow personal interval, which is why this approach is best described as derived from serial measurements in a single individual.

The other ideas describe population-level approaches or cross-lab harmonization: using cross-sectional data from many animals gives population reference intervals; pooling data from different laboratories yields broader, method-related reference ranges; and sampling across species isn’t relevant to a single-subject baseline.