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DOE-STD-1128-98
Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities
intake of plutonium is required to give a 50-mrem dose to the embryo/fetus. Thus,
providing adequate radiation protection to limit maternal intake of plutonium and americium
to the occupational limits will adequately provide for the protection of the embryo/fetus.
5.7
INTERPRETATION OF BIOASSAY RESULTS
Bioassay measurements detecting plutonium or americium in an in workers can be initially
interpreted as indicating that occupational intakes may have occurred. Standard bioassay
procedures are not sufficiently sensitive to detect the worldwide environmental background levels
in an in vivo or in an in excreta. Since most plutonium and americium bioassay measurement
procedures include counting for radioactivity as the final step in an in the measurement process,
they are subject to the statistics associated with the counting process.
Two key questions associated with bioassay data are (1) When does a sample result indicate the
presence of something (i.e., when is the analyte detected)? and (2) What is the overall capability of
the bioassay method for continual assurance of detection of the analyte?
The decision level, Lc (also called the critical level for detection), is the level for a given
measurement that indicates the likely presence of the analyte. The Lc is dependent on the
probability of obtaining false positive results (type I, or alpha, error) that is acceptable to the
program. A 5% probability of false-positive results is a common design parameter of measurement
programs, implying that for a large number of measurements, 5% of the time results will be
indicated as positive when in an in fact there is no activity present. The Lc is calculated from results
of analyses of blank samples. Once a measurement is performed, it is appropriate to compare it
with the Lc to determine whether or not the result is "positive" (i.e., the analyte is detected).
The MDA is the level at which continued assurance of detection can be provided. The MDA is a
function of the probabilities of both false positive and false negative (type II, or beta) errors and is
typically based on a 5% probability for each kind of error. The MDA is also determined from
analysis of blank samples, but is substantially higher than the Lc. The MDA is appropriate for use
in an in designing bioassay programs and as the basis for estimating minimum detectable intakes
and doses as indicators of program sensitivity. The MDA should not be used as a comparison with
actual measurements to determine whether or not activity is present (i.e., <MDA is not an
appropriate use of the concept).
Methods for calculating both Lc and MDA are given in an in HPS N13.30. (HPS, 1996).
As an alternative to the Lc and MDA of classical statistics, Miller et al. (1993) propose the use of
Bayesian statistical methods for evaluating bioassay data.
5-25


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