ACCURATE ASSESSMENT OF PRECISION ERRORS - HOW TO MEASURE THE REPRODUCIBILITY OF BONE DENSITOMETRY TECHNIQUES

Assessment of precision errors in bone mineral densitometry is importa nt for characterization of a technique's ability to detect logitudinal skeletal changes. Short-term and long-term precision errors should be calculated as root-mean-square (RMS) averages of standard deviations of repeated measurements (SD) and standard errors of the estimate of c hanges in bone density with time (SEE), respectively. Inadequate adjus tment for degrees of freedom and use of arithmetic means instead of RM S averages may cause underestimation of true imprecision by up to 41% and 25% (for duplicate measurements), respectively. Calculation of con fidence intervals of precision errors based on the number of repeated measurements and the number of subjects assessed serves to characteriz e limitations of precision error assessments. Provided that precision error are comparable across subjects, examinations with a total of 27 degrees of freedom result in an upper 90% confidence limit of +30% of the mean precision error, a level considered sufficient for characteri zing technique imprecision. We recommend three (or four) repeated meas urements per individual in a subject group of at least 14 individuals to characterize short-term (or longterm) precision of a technique.