Should You Take Calcium Supplements Before a DEXA Scan?
You should NOT take calcium supplements within at least 30 minutes (and ideally several hours) before your DEXA scan, as calcium tablets can artificially elevate bone mineral density measurements and potentially lead to misclassification of your bone health status.
Evidence for Avoiding Calcium Before DEXA
Direct Impact on BMD Measurements
Research demonstrates that calcium supplement ingestion can significantly interfere with DEXA scan accuracy:
In a controlled study of 36 subjects, 36% showed L1-L4 BMD changes exceeding the least significant change threshold after ingesting calcium tablets (ranging from 315-600 mg elemental calcium), with effects occurring within 30 minutes of ingestion 1
A separate study of 15 subjects found that 33% demonstrated clinically significant BMD changes outside the least significant change after calcium ingestion, with effects persisting at both 15 and 30 minutes post-ingestion 1
Phantom and volunteer studies showed that a single 500 mg calcium tablet overlying bone could increase measured BMD by up to 12.6% for individual vertebral bodies, particularly in patients with lower baseline BMD 2
Clinical Significance of the Artifact
The magnitude of error matters for your diagnosis and treatment:
While L1-L4 combined measurements showed ≤2.2% mean increase (which may seem small), individual vertebral body measurements can be substantially affected, potentially changing diagnostic categorization from osteopenia to normal, or from osteoporosis to osteopenia 2
The precision of monitoring BMD over time is adversely affected by tablet artifact, especially problematic if only 2-3 vertebral levels are available for analysis 2
Misclassification risk is highest in patients with lower baseline BMD and when smaller regions of interest are measured 2
Important Technical Considerations
Modern DEXA software has limitations in detecting calcium interference:
Recent DXA software versions can successfully identify tablets in the soft-tissue field as artifact and exclude those pixels, BUT tablets overlapping both bone and soft tissue require manual operator intervention 2
Visualization of calcium tablets on the scan was NOT reliably associated with BMD changes, meaning the artifact can be present even when not visually detected 1
Undetected tablets directly overlying bone pose the greatest risk for measurement error 2
Practical Recommendations
Pre-Scan Instructions
Abstain from all calcium supplements for at least 4-6 hours before your DEXA scan to ensure complete gastrointestinal transit and dissolution 1, 2
Avoid calcium-fortified foods and antacids containing calcium on the morning of your scan 1
What About Ongoing Calcium Supplementation?
The guidelines universally support calcium supplementation for bone health, but timing matters:
All patients at risk for osteoporosis should receive calcium supplementation (1000-1500 mg/day depending on age and sex) as part of comprehensive bone health management 3
Resume your calcium supplements immediately after completing the DEXA scan 3
Common Pitfalls to Avoid
Don't assume that because you can't see the tablet on the scan that it isn't affecting the measurement—artifact can occur without visual detection 1
Don't think that taking calcium the night before is safe—tablets can remain in the GI tract and affect morning scans 2
Don't rely solely on technologist review—even experienced operators may miss subtle tablet artifacts that affect individual vertebral measurements 2
Why This Matters for Your Care
Accurate DEXA measurements are critical for:
Determining whether you meet treatment thresholds (T-score ≤-2.5 for osteoporosis diagnosis) 4
Monitoring treatment response over time (typically reassessed after 1-2 years of therapy) 5, 6
Making decisions about initiating or continuing bisphosphonate therapy, which carries its own risks with long-term use 4
The bottom line: A falsely elevated BMD could delay necessary treatment or give false reassurance about treatment effectiveness, potentially increasing your fracture risk. The simple step of avoiding calcium before your scan ensures the most accurate assessment of your bone health.