Answer to Question #709 Submitted to "Ask the Experts"

Q

In determining bioassay results, what approach(es) do you use in establishing background values for ¹⁴C bioassays?

A

You know how they say your employees are your best assets. In this case it is absolutely true. What you need to determine a background for ¹⁴C in urine is a background population that will provide you with representative data. In this case this population is a group of your employees who do not handle radioactive materials, your administrative staff and business managers, or sales people. You should obtain approximately 20-30 urine samples (24-hour urines if available) and perform the identical analysis that you plan to perform on your occupationally exposed employees. Now you have a set of data which should be fairly representative of your employees. This is your background data. An alternative to this method would be to buy or make synthetic urine (yes they make artificial everything these days). The National Institute of Standards and Technology provides freeze-dried urine samples with toxic substances and other drug metabolites and may be able to provide standard freeze-dried urine—phone: 301-975-6776, fax: 301-948-3730, or email: srminfo@nist.gov for the latest prices. A recipe for synthetic urine is available (done by Pacific Northwest National Laboratory [PNNL]) Report 6490 (1988) from the National Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Road, Springfield, VA 22161. You should now perform the standard statistical analysis on this data to determine the average background, standard deviation of background, etc. Plug this into your favorite minimum detectable activity (MDA) or lower limit of detection (LLD) formulas and Voila. You have the LLD for your entire process that includes your individual variations, and all. If you are interested in a very rigorous analysis (that is, you are a real glutton for punishment) you can check out the following link for PNNL's Bayesian Analysis Web page to help you determine the appropriate LLD for your samples. Two references for determining the LLD/MDA can be found in:

• Currie LA. Limits for qualitative detection and quantification determination. Analytical Chemistry 40(3):587-593; 1968.  
• Altshuler B, Pasternak B. Statistical measures of the lower limit of detection of a radioactivity counter. Health Physics 34(9):293-298; 1963.

Shawn Googins, CHP