- 01Density vs turnover: two different questions
A DEXA scan measures how much bone you have right now, a density snapshot. Blood markers measure how fast bone is being built and broken down, a turnover speedometer. A still photo and a speed reading answer different questions, and you often need both.
- 02T-score vs Z-score
A T-score compares your bone density to a healthy 30-year-old's peak bone mass; a Z-score compares you to people your own age and sex. T-score defines osteoporosis in postmenopausal women and older men; Z-score is used for premenopausal women, men under 50, and children.
- 03Only a scan or a fracture diagnoses osteoporosis
The diagnosis is set two ways only: a DEXA T-score of -2.5 or below, or a fragility fracture, meaning a bone that breaks from a fall at standing height or less. No blood value, however abnormal, makes or rules out the diagnosis. Calcium, vitamin D, PTH, and turnover markers only find treatable causes and track treatment.
A DEXA scan and a blood test answer two different questions about your bones, and neither can do the other's job. The scan, a low-dose X-ray of your hip and spine, measures how much bone you have and hands you a T-score; a T-score at or below -2.5 is the line for osteoporosis. Your blood work (calcium, vitamin D, PTH, and sometimes bone turnover markers) never produces a T-score. It explains why bone might be draining away and tracks whether a treatment is holding it.
The two questions every bone test answers
Bone testing splits cleanly in two. One question is how much bone you have right now: that is density, and a DEXA scan measures it. The other is how fast bone is being built up and broken down: that is turnover, and blood markers measure it. A density scan is a snapshot; turnover markers are a speedometer. The snapshot diagnoses osteoporosis; the speedometer tells you which way things are moving and how fast.
DEXA scan vs blood tests: which answers which question
This is why a blood test alone cannot tell you that you have osteoporosis. The diagnosis is set by the DEXA number or by a fragility fracture (a bone that breaks from a fall at standing height or less). The osteoporosis page covers the disease itself; here the focus is how to read each test.
What a DEXA scan (bone density test) is
DEXA stands for dual-energy X-ray absorptiometry. A scanner passes two low-energy X-ray beams through your hip and lower spine and measures how much each is absorbed, which reflects how mineral-dense the bone is. It is the reference standard for bone mineral density and is read at the lumbar spine and the proximal femur (the top of the thigh bone) 1. The radiation dose is very low, less than a chest X-ray, you stay dressed, and you lie still on a padded table for about 10 to 15 minutes. It needs no fasting, no injection, and no prep.
How to read a T-score
A T-score compares your bone density to the average peak bone mass of a healthy 30-year-old, measured in standard deviations. Zero means you match that young-adult peak; each whole point below zero is one standard deviation less bone. Because the yardstick is a young adult, most people lose some ground with age even when nothing is wrong 1.
| T-score | Category | What it means |
|---|---|---|
| -1.0 or above | Normal | Bone density is in the healthy young-adult range. |
| -1.0 to -2.5 | Low bone mass (osteopenia) | Below peak but not osteoporosis; fracture risk is judged with a tool like FRAX, not the number alone. |
| -2.5 or below | Osteoporosis | The diagnostic threshold; fracture risk is high enough that it changes the plan. |
Two people can share a T-score and not share a risk. The number sizes the gap below peak bone; a tool like FRAX turns that into your own odds of a fracture.
That reference matters. A T-score uses a young adult as the yardstick, which suits postmenopausal women and older men. For a premenopausal woman, a man under 50, or a child, guidelines use a Z-score instead, which compares your bone density to people your own age and sex 1. A low Z-score is a prompt to hunt for a specific cause rather than to label age-related loss.
The blood tests: finding why bone is being lost
A bone-health blood panel is ordered to catch treatable reasons for bone loss, not to diagnose osteoporosis. A typical workup includes calcium, phosphate, PTH, 25-OH vitamin D, alkaline phosphatase (an enzyme that can rise when bone is being built quickly), kidney and liver function, and TSH (thyroid) 2. Three of these do most of the work: calcium, vitamin D, and PTH. They are read together, not one at a time.
Calcium and PTH: read as a pair
About 99% of your body's calcium is stored in your bones and teeth 7, and only a sliver circulates in blood, held in a tight lab range of roughly 8.5 to 10.5 mg/dL. That blood level is defended by PTH (parathyroid hormone), the body's calcium thermostat. A single calcium or PTH value rarely means much on its own.
PTH means nothing on its own. Read it next to a calcium drawn at the same time, because the pair is what gives the diagnosis.
Total calcium also tracks albumin (a blood protein). As the calcium page puts it, a low total can be normal calcium hiding behind low protein, and the lab corrects for that before it should cause concern. That is why a lone calcium result gets read against albumin and, when it is genuinely high or low, against a same-day PTH.
Vitamin D: the common, treatable cause
Low vitamin D is the reason a bone panel so often gets ordered, because it is common and fixable. 25-OH vitamin D is the blood marker of vitamin D status; the Endocrine Society, in its 2011 guideline, calls below 20 ng/mL deficient and 20 to 30 ng/mL insufficient 5. When vitamin D runs low, the gut absorbs less calcium, so PTH climbs to defend the blood level, and that steady high PTH quietly draws down bone. The vitamin D page covers status and repletion in full.
Where a 25-OH vitamin D result fallsng/mL
Bone turnover markers: the speedometer
Turnover markers measure the pace of remodeling. Bone is constantly torn down and rebuilt: CTX (C-terminal telopeptide) rises when bone is being broken down (resorption), while P1NP (procollagen type 1 N-propeptide), bone-specific alkaline phosphatase, and osteocalcin rise when bone is being built (formation). The International Osteoporosis Foundation and IFCC name serum P1NP and serum CTX as the reference markers 4. They are read as a change from your own baseline: a drop in CTX a few months after starting treatment signals the treatment is working.
Here is the whole panel in one view, from the scan that diagnoses to the markers that monitor.
| Test | What it measures | How it's read | What it's for |
|---|---|---|---|
| DEXA scan | Bone mineral density (how much bone) | T-score: normal at or above -1.0, osteopenia -1.0 to -2.5, osteoporosis at or below -2.5 | Diagnoses osteoporosis; estimates fracture risk |
| Calcium (total) | Calcium circulating in blood | Roughly 8.5 to 10.5 mg/dL; read with albumin and PTH | Screens for parathyroid and other causes |
| 25-OH vitamin D | Vitamin D status | Deficient below 20, insufficient 20 to 30, sufficient 30+ ng/mL | Finds a common, treatable driver of bone loss |
| PTH | The calcium thermostat | Read next to a same-day calcium; lab-specific range | Sorts why calcium is high or low |
| Turnover markers (CTX, P1NP) | Speed of bone loss and formation | Change from your own baseline, not a fixed cutoff | Monitors treatment response |
Osteoporosis is silent, so screening is by age and risk
Bone loss has no symptoms. Osteoporosis is silent until a bone breaks, and the first sign is often a fragility fracture of the hip, wrist, or spine; spinal fractures in particular frequently pass unnoticed, showing up later as lost height or a stooped posture 2.
For most people the trigger to test is age and risk, not symptoms. The US Preventive Services Task Force recommends bone density testing for all women 65 and older, and for postmenopausal women under 65 whose fracture risk is raised by a formal risk tool 3. That tool is usually FRAX, which estimates your 10-year probability of a major fracture from risk factors like age, prior fracture, family history, smoking, and steroid use, with or without a hip density value 6.
The USPSTF recommends screening for osteoporosis with bone measurement testing to prevent osteoporotic fractures in women 65 years and older.US Preventive Services Task Force, JAMA 2018
How the tests are done and how often
- DEXA scan: a painless, low-dose X-ray, about 10 to 15 minutes, no fasting or prep. It is an imaging order, separate from any blood draw.
- Blood tests: a standard blood draw. Calcium rides along on a basic metabolic panel; PTH, 25-OH vitamin D, and turnover markers are usually added on as separate orders.
- Repeat DEXA: commonly every 2 years, sooner if you are on treatment or losing bone quickly. In the US, Medicare covers a screening DEXA every 24 months for those who qualify.
- Family history counts: a parent's hip fracture raises your own risk and is one of the FRAX inputs, so mention it when you are assessed.
Each test here has its own page. To go deeper or step back, the bone health hub gathers every marker on this panel, and the osteoporosis page covers the disease itself.
Link · US Preventive Services Task ForceScreening for osteoporosis to prevent fracturesThe USPSTF recommendation on who should get a bone density test and when.uspreventiveservicestaskforce.org
Citations
- International Society for Clinical Densitometry (ISCD), 2019 Official Positions (Adult).
- LeBoff MS, et al. The clinician's guide to prevention and treatment of osteoporosis. Osteoporosis International (BHOF), 2022.
- US Preventive Services Task Force. Screening for Osteoporosis to Prevent Fractures. JAMA. 2018;319(24):2521-2531.
- Vasikaran S, et al. Markers of bone turnover for the prediction of fracture risk and monitoring of osteoporosis treatment. Osteoporos Int. 2011;22(2):391-420.
- Holick MF, et al. Evaluation, Treatment, and Prevention of Vitamin D Deficiency: an Endocrine Society Clinical Practice Guideline. JCEM. 2011;96(7):1911-1930.
- FRAX Fracture Risk Assessment Tool, University of Sheffield / WHO Collaborating Centre.
- National Institutes of Health, Office of Dietary Supplements. Calcium: Fact Sheet for Health Professionals.
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.