Biochemical Marker Tests
Biochemical markers of bone reabsorbtion are simple blood or urine tests that can help determine the amount of bone loss. Bone is composed of a living protein framework and when it breaks down or is formed, bits of this living protein framework appear in the blood or urine. Biochemical marker tests measure the amount of a specific bone protein in the blood or urine. As one loses more bone due to osteoporosis, the bone protein fragments (discussed below) show up in the blood and urine in increased amounts.
The commonly used biochemical markers of bone metabolism in clinical use are explained below:
• Bone Specific Alkaline Phosphatase (BSAP) Levels: Alkaline phosphatase levels acts as a biochemical marker for osteoporosis. This test is done to estimate the rate of bone formation over the entire body. BSAP test measures the amount of alkaline phosphatase in the urine. BSAP is present in the membrane of osteoblast cells and is excreted into the urine during bone reabsorption. High level of bone protein in urine is an indication of increased rate of bone reabsorption and hence osteoporosis. Value of this biochemical marker above the ideal range could indicate that more bone loss is occurring due to high osteoclast activity.
• Osteocalcin: It is a bone protein produced by osteoblasts in bone. During the process of bone reabsorption, a small amount of osteocalcin is released in the blood circulation from the bone. The levels of circulating osteocalcin correlates with bone mineralization. The normal level of osteocalcin in blood among 18 years or above is 9-42 mg/ml. Above this normal level is an indication of excessive reabsorption due to osteoporosis.
• N-telopeptide (NTX) or C-telopeptide (CTX) levels: The natural bone protein products such as N-telopeptide (NTX) or C-telopeptide (CTX) are tested in urine sample, which are the biochemical markers of bone reabsorption or loss of bone. The levels of NTX and CTX in urine, correlates with bone reabsorption, as some amount of these bone protein products are released in urine during bone reabsorption. Ideally for a woman, the NTX level in urine should be around 35. If there is an increase in this level can be attributed to the fact that there is excessive bone reabsorption due to osteoporosis.