In Vivo Count Results - doe-std-1136-20040133

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiological Protection in Uranium Facilities

procedures include counting for radioactivity as the final step in the measurement process, they are also

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 ANSI N13.30.

As an alternative to the Lc and MDA of classical statistics, there have been proposals (Miller et al.

1993) to use 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.

5.7.1 In Vivo Count Results

In vivo uranium measurements are generally relatively insensitive with regard to levels of

occupational exposure concern. This applies particularly to routine chest or lung counting, skeleton

counting, and liver counting. For that reason, any detection of uranium should be investigated. The

investigation should address the validity of the measurement by reviewing the spectrum and its associated

background subtraction. These reviews are particularly important if the result is near the Lc. Follow-up to a

positive result should include a confirming measurement. Ideally, this should be an immediate (same day)

recount of equal or higher sensitivity. The farther removed in time a verification measurement is from the

original measurement, the more important it becomes to factor in potential lung clearance in comparing the

two measurements. A follow-up measurement taken 30 days after an initial high-routine may not be

capable of providing verification if the material of concern exhibits class W behavior.

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