Factors affecting in vivo measurement precision and accuracy of Cd-109 K x-ray fluorescence measurements

Cd-109 K x-ray fluorescence (XRF) measurement systems from two research cen tres were used to measure tibia lead content in a population (n = 530) of y oung adults. The group mean bone lead contents (+/-SEM) determined by McMas ter University (n = 214) and the University of Maryland (n = 316) were 2.80 +/- 0.51 and 2.33 +/- 0.50 mu g Pb/(g bone mineral) respectively. The mean difference of 0.47 +/- 0.71 mu g Pb/(g bone mineral) was not significant. There was no evidence of a systematic difference between measurements from the two systems. Measurement uncertainties for the young adults were poorer overall than unc ertainties for a population of occupationally exposed men. This was because obese subjects and women were included in the study. Regressions of precis ion against body mass index (BMI, defined as weight/height(2)) determined t hat uncertainties increased with BMI and were poorer for women than men. Me asurement uncertainties (1 sigma) were >8 mu g Pb/(g bone mineral) for wome n with a BMI > 0.004 kg cm(-2). Poor-precision data affected population estimates of bone lead content; an inverse correlation was found between precision and bone lead content. A sm all number (0.4%) of individual measurements with poor uncertainties were i naccurate to within the precision. It is suggested that obese subjects, who se BMI > 0.004 kg cm(-2), should be excluded from Cd-109 K XRF studies, as the measurement provides limited information and may be inaccurate.