How this instrument works
The Osteoporosis Self-Assessment Tool, or OST index, was built by Koh and colleagues in 2001 from a dataset of postmenopausal women across eight Asian countries. The formula is deliberately minimal: subtract age in years from body weight in kilograms, then multiply by 0.2. No lab work, no imaging, no questionnaire — just two numbers most people already know, chosen because lower body weight and older age are both independently associated with lower bone mineral density.
This calculator sorts the resulting score into three bands. A score above −1 falls in the low-risk band. A score from −1 down through −4, inclusive, falls in the moderate-risk band. A score below −4 falls in the high-risk band. Falling into the moderate or high band isn't a diagnosis of osteoporosis — it's a signal worth following up with an actual bone density measurement, typically a DXA (dual-energy X-ray absorptiometry) scan, which is what osteoporosis is formally diagnosed from.
One detail worth being direct about: the −1 and −4 cutoffs come from the original Asian derivation cohort and were optimized for that population. Later validation studies conducted in non-Asian populations — Caucasian, Portuguese, and African-American cohorts among them — have generally found that a higher cutoff, often around 2 rather than −1 (and sometimes higher still, depending on the study), flags an equivalent share of at-risk people more reliably in those groups, largely because non-Asian populations tend to run heavier on average, which shifts OST scores upward across the board. That doesn't make the tool useless outside its original population, but it does mean the −1/−4 bands shouldn't be treated as a universally validated standard across every population; the index is best used as a quick screening prompt, not a precise threshold.
- Enter body weight in kilograms in the Weight field.
- Enter age in years in the Age field.
- Read the OST index score — weight minus age, multiplied by 0.2.
- Check the risk category the score falls into: low, moderate, or high.
- Treat a moderate or high score as a prompt to ask a clinician about a DXA bone density scan, not as a diagnosis.
Worked example — three weight-and-age scores
A 70 kg person who is 50 years old has an OST index of (70 − 50) × 0.2 = 4, comfortably above the −1 threshold, placing them in the low-risk band where this score alone prompts no further action.
A 50 kg person who is 70 years old has an OST index of (50 − 70) × 0.2 = −4 — exactly at the boundary this calculator treats as still moderate rather than high, since the moderate band runs from −1 down through −4 inclusive.
A lighter 40 kg person who is 70 years old has an OST index of (40 − 70) × 0.2 = −6, below −4 and therefore in the high-risk band, a result worth raising with a clinician as a prompt toward DXA bone density scanning rather than a standalone diagnosis.
Questions
What does the OST index actually measure?
It doesn't measure bone density directly — it's a screening formula built only from body weight and age, standing in as a rough, easily-calculated proxy for the likelihood that a DXA scan would find low bone mineral density. Lower weight and older age both push the score down because both are independently linked to lower bone density in the population the tool was built from.
Is the −1 and −4 cutoff valid for every population?
Not with equal confidence everywhere. Those specific cutoffs were optimized on the original Asian derivation cohort from Koh et al.'s 2001 study. Validation studies in other populations, including Caucasian, Portuguese, and African-American cohorts, have generally found that a higher cutoff — often around 2 rather than −1, and sometimes higher still — performs better in those groups, since non-Asian populations tend to run heavier on average and OST scores shift upward with body weight. Treat the −1/−4 bands as a reasonable general screen rather than a cutoff validated identically for everyone.
Does a high-risk score mean I have osteoporosis?
No. A high-risk score means this quick formula suggests it's worth getting an actual bone density measurement, typically a DXA scan, which is the test osteoporosis is formally diagnosed from. The OST index itself has no imaging or lab component, so it can't confirm or rule out osteoporosis on its own.
Why does the formula use only weight and age?
That simplicity is the point. The OST index was designed as a primary-care screening step that needs nothing beyond two numbers a patient already knows, so it can flag who might benefit from further bone density testing before any imaging or lab work happens — not to replace that testing once it's warranted.
What should I do if my score lands in the moderate or high band?
Bring it up with a clinician and ask about DXA bone density testing, which can give an actual measurement rather than a weight-and-age estimate. A moderate or high OST score is a prompt for that conversation, not a result to act on by itself.
Why does higher body weight lower osteoporosis risk in this score?
Higher body weight is generally associated with greater mechanical loading on bone and, in postmenopausal women, more peripheral estrogen production from fat tissue — both linked to somewhat higher bone mineral density in population studies. That relationship is why the OST formula subtracts age from weight rather than treating them as independent factors.
References
- Koh LK, et al. — A simple tool to identify Asian women at increased risk of osteoporosis, Osteoporos Int, 2001
- Subramaniam S, et al. — Performance of the Osteoporosis Self-Assessment Tool (OST) in Predicting Osteoporosis, 2018
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.