Per Hyltoft Petersen, Sverre Sandberg, Callum G. Fraser, Henk Goldschmidt
Influence of Index of Individuality on False Positives in Repeated Sampling from Healthy Individuals
The index of individuality is defined as the ratio of the
within-subject biological variation to the between-subject
variation, i.e., the variation between the biological
set-points. It has been disputed whether the index
of individuality has influence on the usefulness of
conventional population-based reference intervals. In
this investigation we found that, as long as only a single
sample is taken, for a certain change in an individual's
set-point, the index of individuality has no influence
on the usefulness of reference intervals. When
two or more samples are taken into account, however,
the outcome of the measurement is highly dependent
on the index of individuality. For a low index, repeat
measurement has only limited effect on the fraction of
false-positive results, as the next result will be close to
the first, but, when the index is high, the fraction of
false-positive results will be reduced considerably
through repeating the test. Moreover, the distribution
of biological set-points for which the fraction of false-positive
results originate is described and the influence
of analytical imprecision is discussed. The calculations
are performed for values of the index of
individuality from 0 to 2.0 for the traditional 95% reference
interval based on x? ± 2*stotal (stotal = total biological
variation), and also for a decision limit (cut-off
point) x? ± 3*stotal. The numbers are, of course, different,
but the effects of the index of individuality are the
same, independent of the chosen cut-off point.
This concept is related to the clinical classification
(diagnosis, prognosis, screening) and the difference
from different principles of monitoring is discussed.
Further, five examples are evaluated and aspects of index
of individuality in relation to false-positive results
are discussed.
Clinical Chemical Laboratory Medicine, Walter de Gruyter
Print ISSN: 1434-6621
Volume: 39, 04/2001
Pages: 160 - 165
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