Monitoring skeletal changes by radiological techniques.

The longitudinal sensitivity of a technique, i.e., its ability to monitor skeletal changes, is affected by two parameters: the long-term precision error (PElt) and the subject group-specific response rate (i.e., annual rates of change). Both need to be considered to avoid misinterpretation of measured changes. A new concept to aid clinical decision making for longitudinal measurements is proposed which is based on three types of measures: criteria for detecting changes-the "least significant change" (LSC) is the smallest change to be considered statistically significant, but for certain clinical questions a smaller margin, the "trend assessment margin" (TAM), can be sufficient for decision making; follow-up time intervals-for follow-up exams the patient should be called in at about the time interval specified by the (population specific) "monitoring time interval" (MTI) or, about one-third of the time earlier, after the "trend assessment interval" (TAI), depending on whether the decision can be based on the LSC or the TAM; and the standard precision error (stdPE)-the smaller stdPE, the more sensitive the technique to monitor skeletal changes. Together, these three measures yield a good characterization of a technique's ability to monitor skeletal changes. Compared with previous concepts, the proposed standardization by a response ratio instead of measures of spread or response rates makes the stdPE substantially less subject group dependent. It allows comparison of stdPE across different studies and could replace the misleading concept of expressing precision as a coefficient of variation. Application of this concept should facilitate the interpretation of measured skeletal changes.