X-ray fluorescence: issues surrounding the application of a new tool for measuring burden of lead.

Studies of lead toxicity would greatly benefit from a convenient, noninvasive measure of body lead burden. X-ray fluorescence (XRF) promises to provide such a measure by rapidly estimating lead content in bone, the repository of greater than 95% of an adult's lead stores (73% in children). Two separate XRF techniques exist, L-XRF and K-XRF. They differ in terms of calibration method and type of bone sampled. They also involve different radiation energies; however, radiation doses and concomitant risks are similarly low. Since interpretation of an XRF measurement depends to an extent on the distribution of lead in the skeleton, this topic is reviewed. Available data suggest that trabecular and cortical bone comprise two distinct compartments with regard to lead kinetics. Within each compartment, however, there also appears to be a significant degree of variability. Furthermore, there is evidence to suggest that the subperiosteal surface layer of cortical bone has a pattern of lead absorption and release that is different from the rest of cortical bone. This may have implications for (1) the choice of XRF technique, since the L-XRF technique measures lead in surface bone, whereas the K-XRF technique derives an estimate of lead from the full thickness of bone, and (2) the selection of bone sites for taking XRF measurements. More research is necessary to fully optimize the applicability of the XRF instrument.