K-X-RAY-FLUORESCENCE MEASUREMENTS OF BONE LEAD CONCENTRATION - THE ANALYSIS OF LOW-LEVEL DATA

K line x-ray fluorescence (KXRF) measurements of bone lead have emerge d as a promising new biological marker of internal lead dose in epidem iological studies. Some disagreements exist, however, over the analysi s of data at low levels of bone lead concentration. In this study, we performed 30 serial measurements on each of three phantoms containing spiked amounts of lead. Chemical analysis of these phantoms using an i nductively coupled plasma mass spectrometer (ICPMS) indicated that the lead concentrations were 0.30, 5.77, and 11.57 mu g g(-1) Analysis of the data was performed using several definitions of a minimum detecta ble limit (MDL) to recode data below the MDL, and using all of the con tinuous point estimates of lead concentration in the phantom (includin g negative estimates). The results demonstrate that the use of MDLs to recode low-level observations reduces the efficiency of the analysis and the ability to distinguish between the phantoms. Retaining all poi nt estimates of KXRF-measured bone lead concentration provides less bi as and greater efficiency in comparing the mean or median levels of bo ne lead of different populations.