Interpretation of Bioassay Results

DOE-STD-1128-98

5.7

INTERPRETATION OF BIOASSAY RESULTS

Bioassay measurements detecting plutonium or americium 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 excreta. Since most plutonium and americium bioassay

measurement procedures include counting for radioactivity as the final step 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 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 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 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.

General follow-up actions to abnormal bioassay measurements should include data checks,

timely verification measurements, work history reviews, and performance of special in vivo

measurements or excreta sample analyses for intake and dose assessments.

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