Osteoporosis Risk Calculator (OST / OSTA index from age & weight)

Understanding the osteoporosis risk calculator (OSTA) — concept and principle

The Osteoporosis Self-assessment Tool for Asians (OSTA) uses only age and body weight to estimate who is most likely to have osteoporosis and should be prioritised for a bone density (DXA) scan. It was developed for postmenopausal Asian women and has also been studied in men.

How it is calculated

OST index = 0.2 × (body weight in kg − age in years)

• Below −4: high risk.
• −4 to −1: moderate risk.
• Above −1: low risk.

From the formula, the index is above −1 when weight > age − 5 kg, and below −4 when weight < age − 20 kg.

The principle

Bone density falls with age, especially after menopause, and lighter people have less bone mass and less of the protective effect that body weight and fat have on bone. Koh and colleagues found that these two easy facts alone separated women with and without osteoporosis about as well as more complex questionnaires. Multiplying by 0.2 simply scales the index to small, easy numbers.

Worked example

Default inputs: age 65, weight 55 kg.

1. OST index = 0.2 × (55 − 65) = −2.
2. −2 is between −4 and −1 → moderate (−4 to −1).
3. Low-risk range starts above 65 − 5 = 60 kg; high-risk range below 65 − 20 = 45 kg.

How to read the result

A high-risk index is a strong reason to have a DXA scan; a moderate index suggests discussing a scan, especially with other risk factors such as a previous fracture, steroid use, smoking or early menopause. A low index makes osteoporosis less likely but does not exclude it. Only DXA measures bone density.

Common mistakes

• Using it in place of a bone density scan.
• Ignoring strong risk factors such as a previous low-trauma fracture.
• Using weight in pounds.

Frequently asked questions

What does a T-score mean?

It compares your bone density with that of young adults; −2.5 or lower defines osteoporosis on DXA.

Does the tool work for men?

It has been studied in Asian men with a cut-off of −1, but evidence is less extensive.

Can I lower my risk?

Weight-bearing exercise, enough calcium and vitamin D, not smoking and limiting alcohol help protect bone.

Sources: Koh LK, Sedrine WB, Torralba TP, et al., "A simple tool to identify Asian women at increased risk of osteoporosis", Osteoporos Int 2001;12(8):699–705