OST Osteoporosis Self-Assessment Tool

What the OST measures

The Osteoporosis Self-Assessment Tool (OST), developed by Koh and colleagues and published in Osteoporosis International in 2001, is the simplest validated osteoporosis screening tool. It uses only two variables, body weight and age, to estimate osteoporosis risk. The formula is elegant: OST = (Weight in kg − Age in years) × 0.2, truncated to an integer. The logic is intuitive: lighter and older people are at higher risk of osteoporosis, so the score subtracts age from weight. A low or negative score indicates higher risk.

Why weight and age?

Body weight is one of the strongest predictors of bone mineral density (BMD). Heavier people have higher BMD because mechanical loading stimulates bone formation, and because body fat produces estrogen (which protects bone). Age is the other major risk factor, bone loss accelerates after menopause in women and accelerates gradually in men after 70. The OST captures these two dominant risk factors in a single number, without requiring any lab tests or imaging.

Sex-specific thresholds

For postmenopausal women: OST >1 is low risk, −3 to 1 is intermediate, and <−3 is high risk. For older men: OST >3 is low risk, −1 to 3 is intermediate, and <−1 is high risk. The thresholds differ because men tend to have higher body weight and different bone loss trajectories. The OST has been validated in multiple populations and shows good sensitivity (approximately 80-90%) for identifying osteoporosis (T-score ≤−2.5 on DXA), though specificity is lower (approximately 40-60%).

When to use OST

The OST is a triage tool, it identifies who should get a DXA scan, not who has osteoporosis. It is particularly useful in resource-limited settings where DXA is not readily available, and in primary care for quickly assessing which patients warrant referral. The USPSTF recommends DXA screening for all women ≥65 and for younger women with risk factors; OST can help identify younger women and men who might benefit from earlier screening. Intermediate or high OST scores should prompt DXA confirmation.

Limitations

The OST is intentionally simplistic, it does not capture many important osteoporosis risk factors including family history, glucocorticoid use, smoking, alcohol, rheumatoid arthritis, secondary osteoporosis (hyperthyroidism, hyperparathyroidism, malabsorption), and prior fractures. A patient with normal weight and age but multiple other risk factors may have a low-risk OST score but still warrant DXA. The OST should be used as a starting point, not a definitive assessment. For comprehensive fracture risk assessment, the FRAX tool (which incorporates more risk factors) is preferred once DXA results are available.

Osteoporosis prevention, what actually works

Regardless of OST score, all adults should follow bone-health basics: 1,000-1,200 mg/day calcium from food (dairy, leafy greens, fortified foods), 600-800 IU/day vitamin D (or enough to maintain 25-OH-D ≥30 ng/mL), weight-bearing exercise (walking, running, resistance training) for 30+ minutes most days, smoking cessation, and moderate alcohol (≤2 drinks/day for men, ≤1 for women). For patients with confirmed osteoporosis (T-score ≤−2.5) or prior fragility fractures, pharmacologic treatment (bisphosphonates, denosumab, teriparatide, romosozumab) is typically indicated in addition to lifestyle measures.