Interpretation of Bioassay Results

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.

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