4.4 Interpreting QCT & pQCT Results
Key Takeaways
Read the method, site and units before classifying a result. Raw attenuation is reported in HU and uses validated scanner-specific screening cutoffs.
Adult calibrated spinal trabecular QCT has an ACR approximate osteoporosis range below 80 mg/cm³. Its T-score does not use WHO DXA cutoffs.
Validated two-dimensional femoral neck and total hip QCT T-scores can support WHO classification. A validated CTXA neck areal result can be used in FRAX in the applicable setting.
A comparison over time requires comparable acquisitions, sites and measurements. Use modality- and site-specific evidence for fracture prediction or monitoring.
Classify the measurement before classifying the patient
A CT examination can yield several skeletal measurements. Check the region, units and method before making a diagnostic or monitoring decision. A raw attenuation value, calibrated spinal trabecular volumetric BMD, and a projected hip T-score answer different questions even if derived from the same scan.
| Result | Supported clinical interpretation |
|---|---|
| Raw vertebral attenuation in HU | Possible opportunistic case finding with validated scanner- and protocol-specific cutoffs and verified stability. A low value can prompt further evaluation. |
| Calibrated lumbar trabecular QCT in mg/cm³ | Use the applicable spinal QCT interpretation. The ACR practice parameter gives approximate adult density categories. The WHO DXA T-score thresholds do not apply to a spinal QCT T-score. |
| Validated two-dimensional hip projection from QCT (CTXA) | ISCD accepts equivalent femoral neck and total hip T-scores for WHO classification in the appropriate population. ACR and UK NOGG also support use of an appropriately validated femoral neck CTXA areal BMD in FRAX. |
| Peripheral QCT at the ultradistal radius | Use the device-specific risk evidence and applicable thresholds. A peripheral QCT number does not qualify as the one-third radius DXA result for WHO diagnosis. |
| QCT-based finite element estimate of vertebral or femoral strength | The estimate can support risk assessment in validated settings. ISCD supports treatment decisions with validated thresholds and clinical risk factors. A strength estimate does not supply a WHO T-score. |
Apply the lumbar QCT range to the correct result
For adult calibrated spinal trabecular QCT, the American College of Radiology (ACR) describes ranges of more than 120 mg/cm³, 80 through 120 mg/cm³, and less than 80 mg/cm³. These approximate normal, low bone mass and osteoporosis categories, respectively. The categories are specific to this use and are not consensus WHO criteria for spinal QCT. A result of exactly 80 mg/cm³ falls in the intermediate range. These thresholds do not establish a separate imminent-risk category.
For example, a 68-year-old man with extensive lumbar osteophytes has a technically valid trabecular QCT measurement of 68 mg/cm³ at L1–L2. This falls below 80 mg/cm³ and is in the ACR's approximate osteoporosis range for this measurement. The report gives the absolute density and analyzed levels. A finding of 68 HU on a routine CT would require a different interpretation, based on validated case-finding criteria for that scanner and protocol. The two measurements share a numerical value but answer different clinical questions.
Apply evidence to fracture prediction and monitoring
The 2023 ISCD positions support spinal trabecular QCT for vertebral fracture prediction in postmenopausal women. Evidence is insufficient for the same position in men or for using spine QCT to predict hip fracture in either group. Total femur trabecular QCT has evidence for hip fracture prediction in postmenopausal women and older men. Ultradistal radius pQCT has evidence for hip fracture prediction in postmenopausal women, with insufficient evidence for extrapolation to men or for predicting spine fractures in that population. Use device-specific risk thresholds when applicable. Central DXA remains preferred for treatment decisions when feasible.
Lumbar trabecular QCT can monitor changes associated with age, disease or treatment. Proximal femur QCT can monitor age- and treatment-related changes, and ultradistal radius pQCT can monitor age-related changes. Compare the same site on technically comparable exams, review the local method's reproducibility and report limitations. Diagnostic equivalence between a projected hip QCT T-score and a DXA T-score does not establish interchangeability for a serial BMD comparison. If the earlier scan was DXA and the current result is volumetric QCT, document the change of method and assess the current result on its own scale.
Document what was measured
Include the device and software, the anatomical site and ROI, the units and reference data where applicable, and the acquisition and reconstruction factors needed to interpret the result. On a follow-up study identify a comparable previous exam or explain why a quantitative comparison is unsuitable. If raw HU suggests low density on a scan obtained for another purpose, communicate the screening finding and consider appropriate follow-up in the clinical context.
ISCD Adult Official Positions · ACR QCT Practice Parameter · UK NOGG fracture risk guidance · Primary CTXA validation
A 68-year-old man with lumbar osteophytes has a technically valid L1–L2 trabecular QCT result of 68 mg/cm³. How should this result be interpreted under the ACR spine QCT practice parameter?
It is in the normal range because it is above 50 mg/cm³.
It falls in the approximate osteoporosis range, which is below 80 mg/cm³.
It is a raw HU measurement and needs no calibration.
The osteophytes automatically invalidate the result.
A postmenopausal woman has a validated projected CTXA femoral neck T-score of −2.6 and a calibrated spine QCT measurement of 100 mg/cm³. Which result can directly support WHO densitometric classification?
The hip CTXA T-score, which is in the osteoporosis range.
The spinal volumetric value, after treating mg/cm³ as g/cm².
The mean of the spine and hip results.
Both results, after treating the spinal value as HU.
An earlier examination reported PA spine DXA BMD in g/cm². The current examination reports lumbar trabecular QCT in mg/cm³. What is the appropriate comparison?
Convert milligrams to grams and report the percentage change.
Compare the T-scores directly to judge response.
Report that a quantitative serial comparison is unsuitable and interpret the current result on its own scale.
Use the same DXA least significant change after rounding both numbers.
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