DXA Calculators
T- and Z-scores, least significant change, scanner precision and body-composition indices; reference values are entered by you rather than baked in
Educational tool. It cannot classify a patient. A T-score depends entirely on which young-adult reference database is used (NHANES III for the proximal femur is the ISCD recommendation; manufacturers ship different databases for the spine). Two scanners can report different T-scores for the same bone. WHO classification applies only to specific sites, in specific populations, not to every measured region, and not to premenopausal women, men under 50, or children, for whom Z-scores are used.
Scores
T-score and Z-score
T = (BMD − young-adult mean) ÷ young-adult SD · Z = (BMD − age-matched mean) ÷ age-matched SD
Enter the mean and SD printed on your scanner’s reference page. WHO categories (T ≥ −1 normal; −2.5 < T < −1 low bone mass; T ≤ −2.5 osteoporosis) apply to postmenopausal women and men ≥ 50, at the lumbar spine, femoral neck or total hip. A Z-score ≤ −2.0 is “below the expected range for age”.
WHO classification: WHO Study Group, Technical Report Series 843 (1994); applied per ISCD 2019 Official Positions; diagnosis at lumbar spine, femoral neck or total hip, using a uniform Caucasian female reference for the femur (NHANES III).
Precision and serial change
Least significant change
LSC = 2.77 × precision (95% confidence)
2.77 = 1.96 × √2, the 95% confidence factor for the difference between two measurements. A change smaller than the LSC is not distinguishable from measurement noise. The LSC must be your own centre’s, from your own precision study.
LSC and precision: ISCD 2019 Official Positions: LSC = 2.77 × RMS-SD, precision assessment requires 30 degrees of freedom.
Precision from a repeated-measures set
RMS-SD = √(Σ SDᵢ² ÷ m) · %CV = RMS-SD ÷ mean × 100
ISCD requires 30 degrees of freedom (30 patients scanned twice, or 15 scanned three times). Five rows will compute, but it is not a valid precision assessment. Repositioning between scans is mandatory.
LSC and precision: ISCD 2019 Official Positions: LSC = 2.77 × RMS-SD, precision assessment requires 30 degrees of freedom.
Body composition
Body composition indices
BMI = wt ÷ ht² · FMI = fat mass ÷ ht² · ALMI = appendicular lean ÷ ht²
Appendicular lean mass is the sum of the four limbs. EWGSOP2 low-ALMI cut-points are 7.0 kg/m² (men) and 5.5 kg/m² (women), and low ALMI alone is not sarcopenia; low muscle strength is the primary criterion.
Sarcopenia cut-points: Cruz-Jentoft AJ et al. Sarcopenia: revised European consensus on definition and diagnosis (EWGSOP2). Age Ageing 2019;48:16–31.
Sources
WHO classification: WHO Study Group, Technical Report Series 843 (1994); applied per ISCD 2019 Official Positions; diagnosis at lumbar spine, femoral neck or total hip, using a uniform Caucasian female reference for the femur (NHANES III).
LSC and precision: ISCD 2019 Official Positions: LSC = 2.77 × RMS-SD, precision assessment requires 30 degrees of freedom.
Sarcopenia cut-points: Cruz-Jentoft AJ et al. Sarcopenia: revised European consensus on definition and diagnosis (EWGSOP2). Age Ageing 2019;48:16–31.
Educational content only. Reference means and SDs must come from your own scanner’s reference database.