Re: Statistics Question (Bioassay)

["Michael Soldano"<michael.soldano@fernald.gov>]

     Tom Labone (from Savannah River) has done some work in this area.  He 
     actually drafted a paper called the "The Estimation of Confidence 
     Bands for Bioassay Data" (I don't know if he ever published it).
     
     Basically, he establishes confidence bands around the bioassay model's 
     excretion prediction, using the model and the actual bioassay data.  
     He gives a simple tritium example in his paper, but I am sure it is 
     applicable to other radionuclides.  Additional bioassay data which 
     falls outside the 95% confidence bands may be indicative of a "new" 
     intake.  However, you should also consider the error associated with 
     the bioassay sample analysis, itself, and whether the model is fitting 
     the first intake adequately.  Other statistical test (i.e. runs, 
     residuals) can help in determining this.  This method assumes that the 
     bioassay model is "correct" for the bioassay samples.  Most internal 
     dosimetrist realize that adjustment of the models parameters may 
     provide a better fit and may negate any second intake from 
     consideration. 
     
     I have found that stripping one intake from another is not always 
     justify.  You may end up with some net negative bioassay results.  
     Again, you are assuming the model is "correct".  In some cases, it is 
     better to use all the data.  In most cases, this will provide a 
     conservative estimate of intake and dose over the stripping method.
     
     I don't think there is a quick and simple method to apply to all 
     cases.  You may have to evaluate the case by several methods, in order 
     to provide a conservative and defensible dose estimate.
     
     
     Thanks
     
     Mike Soldano, CHP
     Fluor Daniel Fernald


______________________________ Reply Separator _________________________________
Subject: Statistics Question (Bioassay) 
Author:  <radsafe@romulus.ehs.uiuc.edu> at FE-INTERNET
Date:    9/25/1998 11:53 AM


<excerpt>Several people are having a debate regarding statistics relative 
to bioassay (urine) samples.  The basic question is:  How do you 
determine when a person has had a new intake of radioactive material if 
they had an intake in the past and they are still excreting activity from 
that intake?  
     
     
It seems that there are several ways to look at the issue.  Do you assume 
you basically have one "lump" of radioactive material that you are 
measuring and then simply subtract the expected activity from the 
measured result (assuming you have decided that there is a measurable 
amount of activity in the sample) and say that if the difference is 
greater than 0, it's due to a new intake?  Or, do you assume that there 
are 2 separate parts and that there is a greater than 0 background at the 
time of the second result, due to the previous intake, and calculate a 
new detection level on which to base your decision? 
     
     
Is there some other way to approach it? We are looking for a quick, 
simple, defensible way to do this. All of the samples are historical, so 
there is limited information available, and no chance of getting new 
samples.  Some thoughts that we've had regarding disregarding subsequent 
bioassay datum are when: -the adjusted bioassay activity is less than 10 
or 20 or 100% of the detection limit and the later data are reasonably 
accounted for. -the adjusted activity is less than one or two times the 
compounded error for that value. -assuming an additional intake would be 
less than 0.02 or 0.1 ALI 
     
     
As stated in HPS N13.30, we desire "a bias on the high side ... when 
uncertainties of measurement or interpretations are present in order to 
be able to state with a high degree of confidence that ... cumulative 
exposures and risks are below a certain level." 
     
     
We look forward to your thoughts on this.  Thanks in advance. 
     
     
</excerpt>
     
******************************
     
  Elizabeth M. Brackett, CHP 
     
     Sr. Health Physicist     
     
     MJW Corporation, Inc.       
     
       (330) 644-3757        
     
  mailto:brackett@bright.net 
     
******************************
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