Related tests: Alkaline phosphatase
What is bone?
Bone is a living, metabolically active tissue, and about 10% of it is remodelled over the course of a year. It is made up mainly of collagen, a protein that forms the framework and gives bone its tensile strength, and calcium phosphate, a mineral complex that makes the structure rigid. This combination of collagen and calcium gives bone both strength and enough flexibility to bear weight and withstand stress. More than 99% of the body’s calcium is contained in the bones and teeth. The remaining 1% is in the blood.
Throughout life, old bone is constantly removed (resorption) and replaced with new bone (formation). During childhood and adolescence, bone formation is faster than resorption. As a result, bones grow and become heavier and denser. Bone formation outpaces resorption until peak bone mass (maximum bone density and strength) is reached, at around the age of 24. After that, resorption slowly begins to outpace formation. Bone loss is very rapid in the first years after the menopause, but continues in the years that follow.
What diseases affect bone?
Osteoporosis. Osteoporosis, or porous bone, is a disease characterised by low bone mass and structural changes in bone tissue. This increases the likelihood of fractures of the hip, spine and wrist. Osteoporosis develops when bone resorption is too fast compared with bone formation. The likelihood of developing osteoporosis increases if peak bone mass is not reached during growth. Both women and men suffer from this disease, which can be prevented and treated.
Paget’s disease.
Paget’s disease is a chronic disorder characterised by enlargement and deformity of the bones. It involves excessive breakdown and formation of bone tissue. The result is weakened bone, with bone pain, arthritis, deformities and fractures. It may be caused by infection with a “slow virus” that remains latent for years before symptoms appear. It may be inherited, since it can affect more than one member of the same family. Paget’s disease is rarely diagnosed before the age of 40. It affects men and women equally.
Bone metastases.
Cancer cells that leave the primary tumour and enter the bloodstream can settle in almost any tissue in the body. Bones are one of the most common sites where these circulating cells settle and grow. Metastases can develop in bones near to or far from the primary tumour. Metastatic bone disease is not the same as primary bone cancer. Primary bone cancer refers to a cancer that starts in the bone. Bone metastases and primary bone cancer differ in their risk factors, treatments and prognosis. Primary bone cancer is much less common than bone metastases. Bone metastases are one of the most frequent causes of pain in cancer patients.
About 50% of cancer patients (except those with skin cancers) develop bone metastases during their illness. Cancers of the breast, prostate, kidney, lung, pancreas, stomach, colon and rectum, thyroid and ovary cause most bone metastases. The spine is the site most often affected by metastases, followed by the pelvis, the upper leg (femur) and the skull.
What tests are used to detect a bone problem?
Bone problems are usually diagnosed with bone densitometry, which uses a special type of X-ray. Ultrasound, which uses high-frequency waves, can also be used.
Bone markers, signals of the bone remodelling process, are sometimes used alongside densitometry during diagnostic assessment. Assessing bone remodelling involves measuring markers of bone resorption, such as telopeptides, and markers of bone formation, such as bone-specific alkaline phosphatase (b-ALP). Bone resorption markers can be measured in urine or blood samples, while formation markers are measured in blood.
Bone markers are most often used to monitor bone disease or the response to treatment. Bone markers can show the response to targeted treatment sooner than X-ray imaging can. In this way, if the response to treatment is not adequate, treatment can be changed.
There is evidence that bone markers can predict the risk of bone metastases in breast or prostate cancer. They can also predict the response to treatment of a bone-loss disease.
1. Which medicines are approved by the FDA (Food and Drug Administration, the US body responsible for food and drug safety) for treating osteoporosis?
Hormone replacement therapy (HRT), alendronate, calcitonin, raloxifene and risedronate.
2. How common is osteoporosis?
It affects more than 28 million Americans, 80% of whom are women.
3. When are these tests requested?
In some cases bone markers are requested together with other bone mineral density tests to diagnose a bone disease. Usually, however, they are requested periodically to monitor patients who have already been diagnosed with a bone disease and are being treated, to check the effectiveness of treatment.