Eliminating bias in routine bioassay when there is an unknown time of intake

Routine bioassay programmes sometimes find evidence of an unsuspected intake. If there were no workplace indicators of exposure or intake, it is necessary to assume a value for the time of intake. Under these circumstances, the International Commission on Radiological Protection (ICRP) continues to recommend using the midpoint of the interval between routine bioassay measurements (ICRP Publication 78, paragraph 106). The assumption of T/2 as the time of intake, where T is the interval between bioassay measurements, represents the expectation value of the time of intake, (t), assuming uniform probability of an intake at any given time. This assumption results in a modest bias, of the expectation value of the intake, (I), that would have been received by a population of workers who had uniform probability over time of intake. This underestimation leads to a negative or positive bias in dose estimates derived in this fashion. The bias is characterised for realistic, routine urinalysis programs for Pu, U and 3H, as well as for in vivo measurements of 125I, 131I and 137Cs. Simple numerical methods are presented for correcting the bias. The bias is greatest for radionuclides whose half-lives are short with respect to the interval between bioassay measurements. Since the primary concern is estimating intake rather than time, the assumed time of intake should be chosen as t(I) rather than T/2. The ICRP should consider revising some of the tables in its Publication 78 to reflect this.