[en] At AWE plc, workers potentially exposed to long-lived alpha emitters, such as plutonium, provide urine samples on a routine basis. Each sample is collected during working hours over a number of days. The activity concentrations in the samples are determined by radiochemical analysis. The creatinine in the samples and the ICRP reference daily excretion rate (1700 mg and 1000 mg for men and women respectively) are used to normalise the activity to a nominal twenty-four hour excretion for dose assessment purposes. This study examines the average and daily variation of creatinine excretion in healthy male and female adults. The advantages of taking samples over an extended period are assessed. There is a large variation in creatinine excretion between individuals and significant deviation from the ICRP recommended values. (author)