Should the lowest femoral neck bone mineral density (BMD) be used for the FRAX calculation?

Should the Lowest Femoral Neck BMD Be Used for FRAX?

Yes, when bilateral hip measurements are available, use the lowest femoral neck BMD value for FRAX calculation, as this provides the most conservative and clinically appropriate fracture risk assessment.

Rationale from Guidelines

The 2022 ACR Appropriateness Criteria explicitly states that FRAX predicts 10-year probability of hip fracture or major osteoporotic fracture using BMD measured at the femoral neck 1. The 2025 USPSTF guideline confirms that FRAX uses "BMD measured at the femoral neck" as the specific input site 2.

Why the Lowest BMD Value?

Research demonstrates that using the lowest femoral neck BMD value (when both hips are measured) is the standard approach:

• In a study of 140 women with distal forearm fractures, the femoral neck T-score was reported as -1.69 ± 0.87 (hip with lowest BMD value), establishing this as the reference standard 3
• The side of BMD measurement can significantly influence individual patient risk assessment, with differences between lowest and highest BMD values showing limits of agreement of 0.0 to 9.5 percentage points for major fracture risk 3
• Using the lowest value provides the most conservative estimate and avoids underestimating fracture risk

Key Technical Points

Site Specificity

• Only femoral neck BMD should be used for FRAX - not total hip, not lumbar spine 1
• Lumbar spine BMD is not incorporated into the standard FRAX algorithm, though it can be used with TBS adjustments in specialized calculations 2
• The distal radius is used primarily for hyperparathyroidism, not for FRAX input 1

When BMD Cannot Be Obtained

FRAX can be calculated without BMD, though this is less accurate:

• FRAX without BMD overestimates fracture risk by an average of 4.4 percentage points for major fractures and 3.5 percentage points for hip fractures compared to FRAX with BMD 3
• However, individual patient differences can be substantial, with 95% limits of agreement ranging from -12.3 to 21.1 percentage points 3
• Concordance between FRAX with and without BMD is only 83.8%, with discordance more common in elderly patients, those with lower BMD, and those near intervention thresholds 4

Clinical Pitfalls to Avoid

1. Don't use total hip BMD - FRAX specifically requires femoral neck BMD 1
2. Don't average bilateral measurements - use the lowest value when both hips are available 3
3. Don't substitute lumbar spine BMD - this is not validated for standard FRAX calculations 2, 1
4. Be cautious with FRAX without BMD - it may significantly misclassify individual patients, particularly those near treatment thresholds 4

The evidence consistently supports using the lowest femoral neck BMD value when available, as this approach maximizes sensitivity for identifying patients at high fracture risk who may benefit from intervention.