Bone is composed of a living protein framework upon which mineral crystals are placed. As bone breaks down, bits of this living protein framework appear in the urine. Tests of bone breakdown, called bone resorption tests, measure the amount of one specific bone protein in the urine (or in the blood) and thus gauge the current rate of bone breakdown. Urinary markers of bone breakdown (known as markers of bone resorption) are simple urine tests that can help determine if you are currently losing bone or not. Such bone breakdown testing can also indicate if your bone-building program is effective at reducing and normalizing the bone breakdown process.
As bone is broken down certain bone protein by-products are excreted in the urine. Measurement of the amount of these bone breakdown by-products can determine the rate of bone breakdown. A high rate of bone breakdown is strongly suggestive of current, ongoing bone loss and a greater risk for osteoporotic fracture. A low rate of bone resorption would be one that is similar to that of the ideal young adult for whom bone breakdown and bone build-up should be in balance. Most ongoing bone loss is associated with high bone turnover (high turnover osteoporosis). There is, however, also a situation of “low turnover osteoporosis.” Here, bone testing shows that bone breakdown is low, as in healthy young adults, but the rate of new bone formation is even lower.
The two most widely used bone resorption markers are the deoxypyridinoline crosslinks test (known as Dpd) and the N-Telopeptides test (known as NTx). Both of these are simple tests done on a urine sample (either a 24-hour urine sample or a random urine sample taken at the lab). Your physician can order either of these tests.
The standard laboratory range for the NTx test is very wide and not helpful. Ideal is an NTx urine level in the high 30s or low 40s. The healthy average for pre-menopausal women, for instance, is around 36 bone collagen equivalent units/mmol creatinine. Values above the ideal range could indicate that bone loss is occurring. In our studies, we like to see more than a 30% reduction in these markers, or normalization to young adult levels.
The second urine test for bone resorption, the Dpd Test, is also good to use, and your doctor will give you collection instructions. Again, the laboratory test result range is wide, but a desired level is in the 4s or a low 5.
In addition to the NTx and Dpd, three other tests look at calcium levels to determine whether calcium is being lost from the bone: the 24-hour urine calcium excretion test, the serum blood calcium test, and the ionized blood calcium level test. The first test looks at how much calcium is being excreted in the urine—usually, among other things, this is a sign that the body is too acid. For this test, you collect all your urine over 24 hours for laboratory analysis to measure the amount of calcium in the urine. The second and third tests require a blood draw to determine the level of calcium in the blood. Of the two, the ionized calcium level test is more precise. Although blood calcium stays within a fairly tightly controlled range, small variations in blood calcium levels can provide useful information for your physician.
It’s never too late
It is never too early nor too late to begin building and rebuilding bone, and these tests can, at times, help assure that you are on the right path or uncover a hidden cause for ongoing bone loss. Remember, however, that both urine bone resorption tests exhibit wide within-subject day-to-day variation. Even with proper collection, shipping, and processing, there is a substantial day-to-day variation in these urine bone breakdown markers. No single test result should be taken as a “bad sign” or an indication that all is not well; instead, the various tests should be used in concert to get an accurate picture of current bone health.