Mean and standard deviation of human bone density?

Mean and standard deviation of human bone density?

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Are the human bones about equal in density? Or is there any published data on the mean + s.d. density for a given bone available?

Interesting question that I'd not considered before.

Bone density is commonly measured by assessing the bone mineral density (BMD) using dual-energy x-ray absorptiometry (DMA), literally the g/cm2 of minerals in the bone.

Having had a search I can only find values for a small number of bones commonly measured when testing for osteoporosis (characterised by reduced BMD). Unsurprisingly the age and sex of the individual has a large effect on BMD; after puberty males have higher BMD (1) and during ageing this then declines, sometimes dramatically (2).

These values are from a Vietnamese study (3) show that the hip and the spine are different.

Men [ mean (SD) ]

  • Femoral neck: 0.85 (0.13)
  • Total hip: 1.00 (0.13)
  • Lumbar spine: 1.05 (0.12)

Women [ mean (SD) ]

  • Femoral neck: 0.80 (0.11)
  • Total hip: 0.95 (0.12)
  • Lumbar spine: 0.96 (0.11)

There must be other reference values for other bones in different populations but I don't have time to look them up now! Good luck in your own research.

I found this website by the US National Institute of Health and this PDF by the World Health Organisation immeasurably enlightening.

  1. Bonjour JP, Rizzoli R. Bone acquisition in adolescence. In: Marcus R, Feldman D, Kelsey J, eds. Osteoporosis. San Diego, CA, Academic Press, 1996:465-476
  2. Cummings SR, Kelsey JL, Nevitt MC, O'Dowd KJ (1985) Epidemiology of osteoporosis and osteoporotic fractures. Epidemiol Rev 7:178-208
  3. Ho-Pham, et al. (2011) Reference Ranges for Bone Mineral Density and Prevalence of Osteoporosis in Vietnamese Men and Women. BMC Musculoskeletal Disorders, 12:182 doi:10.1186/1471-2474-12-182

Anyone can get osteoporosis. It’s more common among older women, but men can have it, too. Your chances increase as you age.


You should discuss with your doctor whether you need the test. They may recommend it if you meet any of the following:

  • You’re a woman 65 or older
  • You’re a postmenopausal woman 50 or older
  • You’re a woman at the age of menopause and have a high chance for breaking bones
  • You’re a woman who has already been through menopause, younger than 65, and have other things that give you a higher chance of osteoporosis
  • You’re a man 50 or older with other risk factors
  • You break a bone after 50
  • You’ve lost more than 1.5 inches of your adult height
  • Your posture has gotten more hunched
  • You’re having back pain without any cause
  • Your periods have stopped or are irregular although you’re neither pregnant nor menopausal
  • You’ve gotten an organ transplant
  • You’ve had a drop in hormone levels

Some types of prescription drugs can cause bone loss. These would include glucocorticoids, a class of drugs used to reduce inflammation. Tell your doctor if you’ve been on cortisone (Cortone Acetate), dexamethasone (Baycadron, Maxidex, Ozurdex), or prednisone (Deltasone).


Peak bone mass attained during childhood and adolescence is a major determinant of bone health in adults [1]. Children with certain conditions including osteogenesis imperfecta, immobilization, thalassemia, HIV and those who receive prolonged glucocorticoid treatment are at risk of developing osteoporosis [2], [3], [4], [5], [6]. Early detection and accurate assessment of low bone mass in these children can lead to early and appropriate intervention.

Several techniques for bone mineral density (BMD) measurement are currently available. One of these modalities, a dual Energy X-ray Absorptiometry (DXA) is widely used as the method of choice because of the relatively low radiation exposure, short scanning time, and its precision and accuracy [7], [8]. Variations of BMD measurement techniques undoubtedly affect values of BMD parameters. Moreover, ethnic difference also affects childhood BMD values. Failure to use appropriate BMD reference values to compare and calculate appropriate Z-score (standard deviation score) may result in an under- or over- diagnosis of osteopenia and/or osteoporosis [9]. Therefore, ethnic-specific normative BMD data using a similar BMD measurement technique should be used for accurate BMD interpretation. There have been previous reports of normative BMD data for children of Caucasian [10], [11], [12], [13] and Asian populations including Japanese, Chinese, Korean and Indian [14], [15], [16], [17]. However, the normative BMD data for Southeast Asian including Thai children and adolescents are not currently available.

Therefore, our objectives of the present study were 1) to develop normative BMD, apparent BMD of the lumbar spine (BMADLS), bone mineral content (BMC), bone area (BA) and lean body mass (LBM) reference data for Thai children and adolescents aged 5–18 years and compare these parameters with reported BMD references and 2) to evaluate the relationships between BMD vs. age, sex, puberty, weight, height, calcium intake and the age of menarche in our population.

Racial differences in mortality following fracture

Jacobsen and colleagues, using data from the Health Care Financing Administration (vide supra), calculated mortality rates of patients with first incident hip fracture between 1984 and 1987 (7). Mortality rates were higher in men than in women in both racial groups: 33.7 and 17.2 per 1000 in white men and women, respectively, and 33.5 and 22.9 per 1000 in black men and women, respectively. While mortality rates did not differ by race among men, they were higher in black than white women. These relative differences in mortality rates persisted even after stratifying by age at time of hip fracture and by number of comorbid medical conditions.


The overall characteristics of the study subjects are summarized in Table 1 and overall results are presented in Fig. 1a, b. Bone turnover markers became significantly decreased after denosumab administration mean (SD) BAP values before the treatment change were 10.9 (3.7) μg/L, while those at 4 months of denosumab were 7.8 (2.5) μg/L (P < 0.001). Mean (SD) urinary NTX values just before denosumab were 24.9 (10.2) nmol BCE/mmol Cr, and those at 4 months were 13.2 (6.8) nmol BCE/mmol Cr (P < 0.001).

a Change in BMD at lumbar spine. In both groups, lumbar BMD values increased significantly from before 2 years to after 1 year of denosumab treatment (P < 0.001). However, BMD values in the non-responsive group did not reach those in the BP-responsive group at 1 year of denosumab therapy (P = 0.080). b Change in BMD at hips. In the non-responsive group, BMD values were significantly decreased (P < 0.001) after 2 years of BP treatment. After denosmab treatment, hip BMD values increased in both groups to a comparable degree (P = 0.709). As a result, BMD values in the non-responsive group were lower than those in the responsive group at 1 year of denosumab treatment (P = 0.065).

At the lumbar spine, there were no significant differences in age, duration of BP use, serum Ca, serum BAP, urinary NTx, or BMD at baseline between the groups (Table 2). In the responsive group, BMD values were significantly increased (P < 0.001) after 2 years of BP treatment, which were significantly decreased (P < 0.001) in the non-responsive group. At this point, a significant difference was observed for lumbar BMD at the baseline (P = 0.021) between the test groups. After denosumab administration, BMD values increased in both groups. There was no significant difference in the increase of lumbar BMD values (P = 0.147), although the augmentation tended to be greater in the non-responsive group. As a result, the difference in BMD values between the groups decreased after a year of denosumab treatment to become comparable (P = 0.080). In both groups, lumbar BMD values increased significantly over the study period (P < 0.001). However, BMD readings in the non-responsive group remained lower compared with responsive group at 1 year of denosumab treatment (Fig. 1a).

At the hips, there were no significant differences in age, duration of BP use, serum Ca, serum BAP, urinary NTx, or BMD at 2 years prior to baseline between the groups (Table 3). Similarly to the lumbar data, in the responsive group, BMD values were significantly increased, while those for non-responders were significantly decreased, after 2 years of BP treatment. However, a significant difference was not observed for hip BMD at the baseline (P = 0.053). After denosumab initiation, hip BMD values increased comparably in both groups (P = 0.709), and thus, the BMD in the non-responsive group remained lower than that in the responsive group (P = 0.065 Fig. 1b). In the responsive group, hip BMD values increased significantly over the study period (P < 0.001). However, over the study period, hip BMD in the non-responsive group did not change or significantly increase.

Simple correlation coefficients were −0.337 for the lumbar spine and −0.339 for the hips (both P = 0.001) for BMD changes over the study period (Fig. 2a, b). The less lumbar and hip BMD values were increased by initial BPs, the more significantly BMD values were increased by denosumab.

a Simple correlation coefficient for lumbar spine. Simple correlation coefficient was −0.337 for BMD at the lumbar spine (P = 0.001) for 2 years of BP treatment and 1 year of denosumab treatment. The lower was the effect on BMD by BPs, the more significant was the effect of denosumab treatment on BMD amelioration. b Simple correlation coefficient for hips. Simple correlation coefficient was −0.339 for BMD at the hip (P = 0.001) for 2 years of BP treatment and 1 year of denosumab treatment. The lower was the effect on BMD by BPs, the more significant was the effect of denosumab treatment on BMD amelioration.

Multiple linear regression analysis revealed that a decrease in BMD from 2 years before baseline to baseline (t value = −3.502, P = 0.001) was significantly associated with an increased change in BMD by denosumab treatment at the lumbar spine (adjusted R square = 0.113). At the hips, multiple linear regression analysis also showed that a decrease in BMD from 2 years before baseline to baseline (t value = −3.526 P = 0.001) was significantly associated with an increased change in BMD by denosumab therapy (adjusted R square = 0.115).

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Bone density tests are not recommended for people without risk factors for weak bones, [4] [5] which is more likely to result in unnecessary treatment rather than discovery of a true problem. [5]

Indications for testing Edit

The risk factors for low bone density and primary considerations for a bone density test include:

  • females age 65 or older. [5]
  • males age 70 or older. [5]
  • people over age 50 with:
    • previous bone fracture from minor trauma. [5]
    • rheumatoid arthritis. [5]
    • low body weight. [5]
    • a parent with a hip fracture. [5]

    Other considerations that are related to risk of low bone density and the need for a test include smoking habits, drinking habits, the long-term use of corticosteroid drugs, and a vitamin D deficiency. [5]

    Conditions found Edit

    A bone density test may detect osteoporosis or osteopenia. [5] The usual response to either of these indications is consultation with a physician. [5]

    Results of the test are often reported in three terms:

    • Measured areal density in g cm −2 . : the number of standard deviations above or below the mean for the patient's age, sex and ethnicity. : the number of standard deviations above or below the mean for a healthy 30-year-old adult of the same sex and ethnicity as the patient.

    While there are many different types of BMD tests, all are non-invasive. Most tests differ according to which bones are measured to determine the BMD result.

    DXA is currently the most widely used, but quantitative ultrasound (QUS) has been described as a more cost-effective approach to measure bone density. [7] The DXA test works by measuring a specific bone or bones, usually the spine, hip, and wrist. The density of these bones is then compared with an average index based on age, sex, and size. The resulting comparison is used to determine the risk for fractures and the stage of osteoporosis (if any) in an individual.

    Average bone mineral density = BMC / W [g/cm 2 ]

    Results are generally scored by two measures, the T-score and the Z-score. Scores indicate the amount one's bone mineral density varies from the mean. Negative scores indicate lower bone density, and positive scores indicate higher.

    Less than 0.5% of patients who underwent DXA-scanning were found to have a T- or Z-score of more than +4.0, often the cause of an unusually high bone mass (HBM) and associated with mild skeletal dysplasia and the inability to float in water. [8]

    T-score Edit

    The T-score is the relevant measure when screening for osteoporosis. It is the bone mineral density (BMD) at the site when compared to the young normal reference mean. It is a comparison of a patient's BMD to that of a healthy 30-year-old. [9] The US standard is to use data for a 30-year-old of the same sex and ethnicity, but the WHO recommends using data for a 30-year-old white female for everyone. [10] Values for 30-year-olds are used in post-menopausal women and men over age 50 because they better predict risk of future fracture. [11] The criteria of the World Health Organization are: [12]

    • Normal is a T-score of −1.0 or higher [13] is defined as between −1.0 and −2.5 is defined as −2.5 or lower, meaning a bone density that is two and a half standard deviations below the mean of a 30-year-old man/woman.

    Z-score Edit

    The Z-score is the comparison to the age-matched normal and is usually used in cases of severe osteoporosis. This is the number of standard deviations a patient's BMD differs from the average BMD of their age, sex, and ethnicity. This value is used in premenopausal women, men under the age of 50, and in children. [11] It is most useful when the score is less than 2 standard deviations below this normal. In this setting, it is helpful to scrutinize for coexisting illnesses or treatments that may contribute to osteoporosis such as glucocorticoid therapy, hyperparathyroidism, or alcoholism.

    There is no such thing as a least significant change, it is a self-contradictory concept. The concept of least significant difference exists. That is, if a numerical difference is significant, it may represent a change, or not, depending on the circumstances. In the case of your data, it does not. Your measurements are a collection of bone density machine and patient slightly variable results obtained on the same day. Neither of those change on the same day. That is, the drift in machine calibration takes, typically, months to occur. Changes in patient bone mineral density typically occur over months to years.

    If differences, even >5 IQR beyond quartile outliers were seen on the same day, it would require an acute event occurrence, like an interval lumbar spine compression fracture between measurements, mistaken comparison between two different patients, a machine failure like slippage of an x-ray mask, or mistakenly using a different machine for the duplicate scan, to be called a change. It that case, we might ask if we can detect a change (although then correct, it would still be pointless). The concept of least significance change is an invented quantity (by BMD machine manufacturers: Lunar, Hologic) that appears in the BMD literature, and nowhere else. "Least significant change" would likely not pass muster during statistical review. For example, it hasn't lasted for six inches into text on this post, which is heavily peer reviewed.

    The same day measurements you have can indeed be used to establish machine precision (read as machine+patient precision). You likely have 30 or so same-day (or nearly same-day) repeat BMD measurements from which differences can be constructed. ANSWER: To find the precision from that data, you cannot average the standard deviations of those differences without performing small number correction of standard deviation.

    A simpler alternative is to perform root mean square of the standard deviations of those differences. That is, 1) sum the squares of the standard deviations of the pairs of repeat measurements. 2) divide that by $n$, the number of repeated measurements. 3) Take the square root to get a relatively unbiased standard deviation. That standard deviation is then a single standard deviation of same day machine error, i.e., the precision you asked for.

    Caution: 1) The precision number you get from only 30 patients is itself not very precise, it has a standard deviation of (very) approximately 25%. 2) BMD measurements are not of proportional error type. The errors are linear and should be expressed in projected density units (gm/cm$^2$), i.e., not in percentages.

    The better way to approach this problem is as in the abstract below, where thousands of patients have been used to obtain results. In that abstract, from Lunar BMD data, the difference numbers listed (as TDD) correspond to approximately $p=0.05$ significance, but only after an elapsed time of approx. 1 or 2 years.

    The statistically correct method for examining BMD differences has been presented.

    Bone turnover markers as determinants of bone density and fracture in men with distal forearm fractures: the pathogenesis examined in the Mr F study

    The pathogenesis for low-trauma wrist fractures in men is not fully understood. This study found that these men had evidence of significantly higher bone turnover compared with control subjects. Bone turnover markers were negative predictors of bone mineral density and were a predictor of fracture.


    Men with distal forearm fractures have reduced bone density, an increased risk of osteoporosis and of further fractures. The aim of this study was to investigate whether or not men with distal forearm fractures had evidence of altered bone turnover activity.


    Fifty eight men with low-trauma distal forearm fracture and 58 age-matched healthy control subjects were recruited. All subjects underwent a DXA scan of the forearm, both hips, and lumbar spine, biochemical investigations, and health questionnaires. Measurements of beta crosslaps (βCTX), procollagen type I N-terminal propeptide (PINP), sclerostin, Dickkopf-1 (Dkk1), and fibroblast growth factor 23 (FGF 23) were made.


    Men with fracture had significantly higher PINP than controls at 39.2 ng/ml (SD 19.5) versus 33.4 ng/ml (SD13.1) (p<0.001). They also had significantly higher βCTX at 0.45 ng/ml (SD 0.21) versus 0.37 ng/ml (SD 0.17) (p= 0.037). Fracture subjects had significantly lower aBMD and PINP was a negative predictor of aBMD at the total hip and βCTX a negative predictor of forearm aBMD. Sclerostin was a positive predictor of aBMD at the lumbar spine and hip sites. Sex hormone binding globulin (SHBG) at 37nmol/L (SD 15.0) was lower in fracture cohort compared to 47.9 nmol/L (SD 19.2) (p=0.001) in control. Multiple regression revealed that the best model for prediction of fracture included SHBG, P1NP, and ultra-distal forearm aBMD. The likelihood of distal forearm fracture was decreased by 5.1% for each nmol/L increase in SHBH and by 1.4% for every mg/cm 2 increase in ultra-distal forearm aBMD, but increased by 6.1 % for every ng/ml increase in P1NP. Men in the highest quartile of PINP had a significantly greater likelihood of distal forearm fracture than those in the lowest quartile.


    The fracture group had significantly higher PINP and βCTX compared with the control group, and these markers were negative predictors of aBMD at the total hip and forearm sites, respectively. Sclerostin was a positive predictor of the variance of spinal and hip aBMD. Likelihood of forearm fracture was best predicted by a combination of SHBG, PINP, and ultra-distal forearm aBMD. Findings of such cross-sectional data should be treated with caution, as longitudinal studies would be required to confirm or refute them.

    Bone Mineral Density Tests

    Osteoporosis (or porous bone) is a bone disease in which bones become weak and are more likely to break.

    Without prevention or treatment, osteoporosis can progress without pain or symptoms until a bone breaks (fractures).

    • Fractures commonly occur in the hip, spine, and wrist. is often the underlying cause of bone fractures.

    Osteoporosis is not just an "old woman's disease." Although it is more common in white or Asian women older than 50 years of age, osteoporosis can occur in almost any person at any age. In fact, more than 2 million American men have osteoporosis, and in women, bone loss can begin as early as 25 years of age.

    Building strong bones and reaching peak bone density (maximum strength and solidness), especially before the age of 30, can be the best defense against developing osteoporosis. Also, a healthy lifestyle can keep bones strong, especially for people older than 30 years of age.

    Osteoporosis is more or less preventable for most people. Prevention is very important because, while treatments for osteoporosis are in place, currently no cure exists. Prevention of osteoporosis involves several aspects, including nutrition, exercise, lifestyle, and, most importantly, early screening with bone density tests.

    The Importance of Screening for Osteoporosis

    Early detection of low bone mass (osteopenia) or osteoporosis is the most important step for prevention and treatment. If osteopenia or osteoporosis has occurred, a person can take action to stop the progression of bone loss.

    Remember, effective treatment or prevention cannot take place if a person does not know he or she has, or is at risk for, osteoporosis.

    How Is Osteoporosis Diagnosed?

    • The only way to accurately test the strength and solidness of the bones is with bone mineral density (BMD) tests. Bone mineral density tests measure the solidness and mass (bone density) in the lumbar spine, hip, and/or wrist, which are the most common sites of fractures due to osteoporosis.
    • Other tests measure bone density in the heel or hand. These tests are performed like X-rays. They are painless, noninvasive, and safe. The risk of radiation is very minimal, much less than even having a chest X-ray film.

    Who Should Have a Bone Mineral Density (BMD) Test?

    Risk Factors for Osteoporosis

    Certain factors are associated with an increased risk of developing osteoporosis (see Prevention of Osteoporosis). Take a one-minute osteoporosis risk test from the International Osteoporosis Foundation.

    If a person has any of these risk factors or other signs of osteoporosis, a doctor may recommend that bone mass is measured. Risk factors for osteoporosis include the following:

    • Advancing age (age <45 years)
    • Female sex
    • Asian or white race
    • Family history of hip fracture
    • Low body weight
    • Long-term corticosteroid therapy
    • Chronic disorders associated with osteoporosis, such as anorexia nervosa or liver disease
    • Previous broken bones with minimal trauma
    • Poor diet without enough calcium and vitamin D
    • Lack of exercise

    Current Recommendations

    According to current recommendations in the United States by the National Osteoporosis Foundation, the following individuals should have a bone mineral density (BMD) test:

    • All women 65 years and older, regardless of risk factors, to screen for postmenopausal osteoporosis
    • Younger postmenopausal and premenopausal women who have one or more risk factors (other than being white, postmenopausal, and female)
    • Postmenopausal women who present with fractures (to confirm the diagnosis and determine disease severity)
    • Men aged 70 and older
    • Younger men who have broken a bone or who have one or more risk factors
    • Adults taking medication that is associated with bone loss, such as prednisone or methylprednisolone (Medrol)
    • Anyone being considered for treatment with prescription medication to strengthen bone
    • Anyone taking prescription medication to strengthen bone (to monitor treatment effect)

    Medicare and Bone Mineral Density Testing

    • Medicare covers bone mineral density (BMD) testing for the following individuals 65 years and older:
      • Women with low estrogen levels who have risk factors for osteoporosis
      • Men and women with abnormalities of the spine (vertebral abnormalities)
      • Men and women who are receiving (or are going to receive) long-term steroid (glucocorticoid) therapy
      • Individuals with primary hyperparathyroidism (excess of parathyroid hormone)
      • Men and women on drug therapy for osteoporosis who are being monitored to determine the effectiveness of the drug therapy


      What Is a Bone Mineral Density Test?

      BMD tests measure the solidness and mass (bone density) in the spine, hip, and/or wrist (the most common sites of fractures due to osteoporosis). Some bone mineral density tests measure bone in the heel or hand.

      These measurements are performed like X-ray films, and they are the only reliable way to determine loss of bone mass. They are painless, noninvasive, and safe.

      Doctors examine bone mineral density test results to do the following:

      • Determine the extent of bone loss (osteopenia or osteoporosis) before a fracture occurs
      • Confirm a diagnosis of osteoporosis if a person already has broken bones (fractures)
      • Predict the chance of a person having a fracture in the future
      • Determine the rate of bone loss and monitor the effects of treatment (follow-up tests performed to monitor treatment are usually conducted every two years)

      What Types of Bone Mineral Density Tests Are Available?

      Several tests are available to assess bone density. Densitometry means the measurement of bone density. Densitometry tests are not painful, and they are completely noninvasive, which means no surgery. Central machines measure density in the hip, lumbar spine, and total body. Peripheral machines measure density in the finger, wrist, kneecap, shinbone, and heel. The most common types of tests are listed below:

      • Dual energy X-ray absorptiometry (DXA or DEXA) scanning is a special low-radiation X-ray that can detect bone loss -- even very small amounts of bone loss. DXA scans are the most commonly used method of bone mineral density measurement. They are used to measure lumbar spine, forearm, and hip bone densities. Peripheral dual energy X-ray absorptiometry (pDXA) measures the bone density in the forearm, finger, and heel. Single-energy X-ray absorptiometry (SXA) measures the bone density in the wrist or heel.
      • Quantitative computed tomography (QCT) scanning measures the bones of the lower (lumbar) spine because these bones change as a person ages. The peripheral QCT (pQCT) scan measures the forearm bone density.
      • Quantitative ultrasound (QUS) uses sound waves to measure bone density at the heel, shin, and finger.
      • Radiographic absorptiometry (RA) scanning uses an X-ray film of the hand and a small metal wedge to calculate bone density.

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      What Do the Bone Mineral Density Test Results Mean?

      Bone mineral density tests measure bone density in the spine, hip, and/or wrist, which are the most common sites of fractures due to osteoporosis. The results of the bone mineral density test are compared to two standards (norms):

      • The age-matched reading, known as the Z-score, compares a person's bone density to what is expected in someone of equivalent age, sex, and size. However, among older and elderly adults, low bone mineral density is common, so comparison with age-matched norms can be misleading.
      • The young-normal reading, known as the T-score, compares bone density to the optimal peak bone density of a healthy young adult (30 years old) of the same sex. The T-score determines fracture risk, which increases as bone mineral density falls below young-normal levels. The T-score, which is a comparison between the solidness (density) of the bones and the bones of the average young healthy population, is measured in standard deviations (SDs).
      • SD is a statistical term that describes variation in a population. According to the World Health Organization's (WHO) diagnostic categories, individuals whose T-score is within 1 SD of the norm are considered to have normal bone density. Scores below the norm are indicated in negative numbers.
      • For most bone mineral density tests, -1 SD equals a 10%-12% decrease in bone density. The risk for broken bones increases by 50%-100% for every SD below the young-normal standard.

      In addition to calculating Z-scores and T-scores, the DXA report may include a FRAX (fracture risk assessment tool) score. FRAX uses the bone density measurement, and other risk factors, to calculate a person's risk of breaking any major bone or hip due to osteoporosis in the next 10 years. FRAX is used for those people who have not yet taken bone-building medication.

      If the probability of a hip fracture in the next 10 years is calculated to be more than 3%, or the probability of a fracture of any major bone due to osteoporosis is greater than 20%, treatment with prescription medication to reduce the risk of bone fracture is often recommended.

      WHO Definitions of Osteoporosis Based on Bone Density Levels

      • Normal: Bone density is within 1 SD (+1 or -1) of the young adult mean.
      • Low bone mass (osteopenia): Bone density is 1 to 2.5 SDs below the young adult mean (-1 to -2.5 SD).
      • Osteoporosis: Bone density is 2.5 SDs or more below the young adult mean (less than -2.5 SD).
      • Severe (established) osteoporosis: Bone density is more than 2.5 SDs below the young adult mean and one or more broken bones (osteoporotic fractures) has occurred.

      With the information from a bone density test, a doctor can identify the degree of bone loss and determine whether a person is at risk for fracture. In general, the lower the bone density (weaker bones), the higher the risk for fracture. Test results help determine which prevention or treatment options are right.