Comparative evaluation of methods to determine intra‐individual reference ranges in nutrition support team (NST)‐related tests

BACKGROUND: The intra‐individual reference range is generally narrower than the commonly used reference range. Consequently, close monitoring of changes in the laboratory test results of individuals based on the inter‐individual reference range remains challenging. METHODS: We examined the determination of individual reference ranges using four indicators of nutritional conditions: transferrin (TRF), albumin (ALB), retinol‐binding protein (RBP), and transthyretin (TTR). The subjects comprised 20 healthy individuals and blood samples were collected and tested five times at 2‐week intervals. We used the measurement results for the four indicators and examined individual reference ranges using four methods, including calculation methods based on the reference change value and Bayesian inference. RESULTS: The resulting intra‐individual reference ranges were narrower than the currently used inter‐individual reference range for all measurements using four methods. Furthermore, the intra‐individual coefficient of variation [CV (intra)] was smaller than the inter‐individual coefficient of variation [CV (inter)] for TRF, RBP, and TTR for all 20 subjects. The means CV (intra) for the four indicators were also lower than the corresponding CV (inter). CONCLUSIONS: The intra‐individual reference range can be used to validate the standard deviation and coefficient of variation for currently used indicators. Moreover, Bayesian methods are speculated to be the most versatile.