Differences in distribution of T-scores and Z-scores among bone densitometry tests in postmenopausal women (a comparative study).

Background: To determine the character of T-score and Z-score value distribution in individually selected methods of bone densitometry and to compare them using statistical analysis.

Methods: We examined 56 postmenopausal women with an age between 43 and 68 years with osteopenia or osteoporosis according to the WHO classification. The following measurements were made in each patient: T-score and Z-score for: 1) Stiffness index (S) of the left heel bone, USM (index). 2) Bone mineral density of the left heel bone (BMDh), DEXA (g of Ca hydroxyapatite per cm2). 3) Bone mineral density of trabecular bone of the L1 vertebra (BMDL1), QCT (mg of Ca hydroxyapatite per cm3). The densitometers used in the study were: Ultrasonometer to measure heel bone, Achilles plus LUNAR, USA; DEXA to measure heel bone, PIXI, LUNAR, USA; QCT to measure the L1 vertebra, CT, SOMATOM Plus, Siemens, Germany.

Statistical Analysis: Differences between measured values of T-scores (Z-scores) were evaluated by parametric or non-parametric methods of determining the 95% confidence intervals (C.I.)

Results: Differences between Z-score and T-score values for compared measurements were statistically significant; however, these differences were lower for Z-scores.

Conclusions: Largest differences in 95% C.I., characterizing individual measurements of T-score values (in comparison with Z-scores), were found for those densitometers whose age range of the reference groups of young adults differed the most, and conversely, the smallest differences in T-score values were found when the differences between the age ranges of reference groups were smallest. The higher variation in T-score values in comparison to Z-scores is also caused by a non-standard selection of the reference groups of young adults for the QCT, PIXI and Achilles Plus densitometers used in the study. Age characteristics of the reference group for T-scores should be standardized for all types of densitometers.