Justification for reporting negative value counting results as an alternative to minimum detectable activity

Proper measurements of sample radioactivity require removal of background activity as a final step in generating an analytical result. When the sample response is within the standard deviation of the background activity, results often are reported as less than detectable or at a minimum detectable activity (MDA) value. Using the MDA creates a truncated distribution or censored data situation. Where a sample point has repeated measurements a bias is introduced. Further, real results can have analytical error statements. These error statements are lost when a MDA is reported. Investigation of a large number of net counting data for low activity sample points suggests that reporting negative results can provide more information than merely truncating to the MDA when results are near background activities. Four cases are examined: method performance, determination of low level activities, general background stability, and quality of results.