Abstract
The reference change value, defined as RCV=1.96×2 1/2×(s I2+s A2) 1/2, where s I is within-subject biological variation and s A is analytical variation, has been used for many years to take clinical decisions in patient monitoring. Furthermore, the index of individuality was defined as II=(s I2+s A2) 1/2/s G, where s G is the between-subject biological variation. This index has been simplified by later authors to s I/s G and has been used in monitoring situations to determine the utility of population-based reference intervals. Harris stated that when the index of individuality is lower than 0.6, the specific reference interval of the individual – when available – is better than the population-based reference interval. However, if a change within a patient is equivalent to the RCV applied for the significant difference between two measurements, the probability of detecting this change is only 50% (the same probability of missing it). Therefore, to obtain a higher probability of detecting a change by the RCV (e.g., 90%) the interpretation of the index of individuality has to be reconsidered. This contribution compares the power of the RCV to the use of cut-off points and population-based reference intervals. The benefits of the RCV compared to the distance to cut-off point or reference limit are also described in relation to the index of individuality.
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