What is the OST Score?
The Osteoporosis Self-assessment Tool (OST), also known as OSTA (Osteoporosis Self-assessment Tool for Asians), is a simple, validated screening instrument developed to identify individuals at risk for osteoporosis who should undergo bone mineral density (BMD) testing with DEXA (Dual-energy X-ray Absorptiometry).
Originally developed and validated in postmenopausal Asian women, the OST has since been validated across multiple ethnic groups, in both men and women. Its simplicity — requiring only age and body weight — makes it one of the most accessible screening tools available. Studies have shown that it performs comparably to more complex tools such as FRAX for identifying individuals with low BMD.
The rationale behind the OST is straightforward: lower body weight and advancing age are two of the strongest independent risk factors for low bone mineral density. By combining these two variables into a single score, the OST can effectively stratify patients into risk categories that guide further diagnostic evaluation.
OST Formula
Using kilograms:
Using pounds:
The result is truncated to the integer value (rounded down). Both formulas produce the same result, as they are mathematically equivalent through unit conversion.
Score Interpretation
| OST Score | Risk Category | Recommendation | Sensitivity |
|---|---|---|---|
| ≥ 1 | Low Risk | DEXA scan not urgently needed; routine screening at age-appropriate intervals | -- |
| -3 to < 1 | Moderate Risk | Consider DEXA scan; evaluate additional risk factors | -- |
| < -3 | High Risk | DEXA scan strongly recommended; high likelihood of osteoporosis | ~91% for detecting osteoporosis |
Risk Stratification Diagram
Understanding DEXA Scans
DEXA (Dual-energy X-ray Absorptiometry) is the gold standard for measuring bone mineral density (BMD). The scan uses two different X-ray energy levels to distinguish bone from soft tissue, providing precise measurements at the hip and lumbar spine.
DEXA results are reported as:
- T-score: Compares your BMD to a healthy 30-year-old adult of the same sex. Normal is T-score ≥ -1.0; osteopenia is -1.0 to -2.5; osteoporosis is ≤ -2.5
- Z-score: Compares your BMD to the average for your age, sex, and ethnicity. A Z-score ≤ -2.0 is considered below the expected range
Current guidelines recommend routine DEXA screening for all women aged 65 and older, men aged 70 and older, and younger individuals with significant risk factors. The OST score helps identify those who should be screened earlier.
Osteoporosis Risk Factors
While the OST uses only age and weight, many other factors contribute to osteoporosis risk:
| Risk Factor | Details |
|---|---|
| Female sex | Women lose bone rapidly after menopause due to estrogen decline |
| Family history | Parental hip fracture increases risk by 2–3 times |
| Low body weight | <58 kg (127 lbs) is associated with lower BMD |
| Smoking | Current smoking reduces BMD and doubles fracture risk |
| Excessive alcohol | ≥3 drinks/day increases fracture risk |
| Corticosteroid use | ≥5 mg prednisone/day for ≥3 months is significant |
| Rheumatoid arthritis | Independent risk factor regardless of steroid use |
| Secondary causes | Hyperthyroidism, malabsorption, hypogonadism, type 1 diabetes |
| Previous fracture | Prior fragility fracture doubles future fracture risk |
| Physical inactivity | Weight-bearing exercise is essential for bone health |
Worked Example
A 60-year-old woman weighing 55 kg (121 lbs):
An OST score of −1.0 falls in the Moderate Risk category (-3 to <1). A DEXA scan should be considered, especially if additional risk factors are present (such as family history of fracture, smoking, or corticosteroid use).
Alternatively, using pounds: OST = (121 − 60) / 5 = 61 / 5 = 12.2 → truncated to 12... Note: the lbs formula yields the same result only when properly converted. Using 55 kg = 121.25 lbs: (121.25 − 60) / 5 = 12.25. The difference arises because the lbs formula approximates. The kg formula is more precise.
Frequently Asked Questions
Who should use the OST calculator?
The OST is most useful for postmenopausal women and men over 50 who want a quick assessment of their osteoporosis risk. It is particularly validated for Asian and Caucasian women but has shown utility across populations. It is not intended for use in children or premenopausal women.
Is the OST better than FRAX?
FRAX is more comprehensive, incorporating 12 clinical risk factors and optionally BMD data to estimate 10-year fracture probability. However, the OST is simpler and has been shown to have comparable sensitivity for identifying osteoporosis. The OST is best used as a first-line screening tool, with FRAX reserved for more detailed risk assessment.
What if my OST score is borderline?
If your score falls near the threshold values (-3 or 1), consider additional risk factors. A borderline score in someone with a family history of fracture, history of falls, or corticosteroid use should prompt a DEXA scan. Clinical judgment should always supplement any screening score.
Can men use this calculator?
Yes. While originally developed for postmenopausal women, the OST has been validated in men as well. Studies show it effectively identifies men at risk for low BMD, particularly those over age 50. The same formula and cut-off values apply.
How often should I check my OST score?
Since the OST depends only on age and weight, it changes gradually over time. Recalculating annually or whenever there is a significant weight change (>5 kg) is reasonable. However, once a DEXA scan has been performed, subsequent screening intervals should be guided by the DEXA results and clinical guidelines.