Diagnosing osteoporosis requires some quality time with a specialist. It's not the sort of thing that can happen in a 15-minute office visit or without some laboratory testing. To get a diagnosis of the health of your bones, you may have to see a osteoporosis specialist. Orthopedic surgeons are usually more interested in bone problems that are repaired with surgery. An osteoporosis specialist is more likely to be an endocrinologist, or an internal medicine or family practice specialist who has developed a long-term interest in treating osteoporosis. Your osteoporosis specialist will apply multiple tests to confirm or rule out osteoporosis as your diagnosis.
1. Dual-energy X-ray absorptiometry (DXA)
A DXA bone density scan is painless. You lie down on a table, and a machine passes back and forth over your body. Inside the table beneath you, a tiny amount of radiation is sent through your skeleton. The X-ray beam is partially blocked by the minerals in your bones. The darker the image of your bones, the more minerals they contain. The "darkness" of the scan is calculated in numerical terms as grams of minerals per square centimeter of bone.
Your bone mineral density is converted to a standard score, to compare it to the bone mineral density of a healthy young woman (T score) or the average bone mineral density of people your same age and sex (Z score). The score is expressed in terms of standard deviations from the mean of the statistical distribution of the score. If your T score is more than 2.5 standard deviations below the mean (if your bone mineral density is lower than the bone mineral density of 90 percent of healthy young women) then you may be diagnosed with established osteoporosis. If your bone mineral density is -1.0 up to -2.5 standard deviations below the mean for a healthy young woman (your mineral content is definitely "below average"), then you may be diagnosed with osteopenia, a condition that may be progressing to osteoporosis.
2. Vertebral fracture assessment (VFA)
If you have lost height or if your upper back is curved forward into a hunched configuration, your doctor will usually order a vertebral fracture assessment (VFA) in addition to the DXA scan. This test looks for fractures in your spine that are not causing pain but that are causing loss of bone. It is conducted on the same table as the DXA scan, and it is also painless and safe. If your VFA test is positive, you will be offered some kind of medication for osteoporosis, no matter what your DXA score is.
This evaluation can also be done with conventional X-ray, but X-rays are much more expensive and expose you to much more radiation.
3. Peripheral DXA
Doctors also test for biochemical markers of bone turnover. These are most accurately done with a bone biopsy, but having a long, long needle bored into your bone is painful and expensive. Most of the time the doctor will order a simple bone test that gives a snapshot of the metabolic health of the bone.
4. Bone formation markers
The ability of osteoblasts to make new bone can be measured through three different tests:
- Serum bone-speficic alkaline phosphatase,
- Serum osteocalcin, or
- Serum P1NP.
Each of these tests is used in a different situation:
- Serum bone-specific alkaline phosphatase is an essential test for anyone taking a bisphosphonate medication. It measures the earliest signs of bone-building activity and can tell the doctor if the dose of bisphosphonate is too high.
- Serum osteocalcin measures the activation of mineral formation in bone. It's a value that shouldn't be too high or too low, another value that tells the doctor whether you are getting the right dose of a medication.
- Serum P1NP is a test that the doctor will order if you are receiving or if the doctor is considering giving you terparatide (Forteo). This test measures collagen in your bloodstream and indicates new bone-building activity.
5. Bone resorption markers
Resorption markers measure how fast bone is being broken down. The higher the level of a resorption marker, the greater the risk of a broken bone.
There are three resorption makers in common use, but your doctor only needs to test for one:
- N-telopeptide (NTX). This marker measures the amount of bone collagen excreted through the urine. When bones are breaking down faster than they are being built back up, NTX accumulates in urine. This test is useful in measuring bone resorption in both men and women.
- C-telopeptide (CTX). This is a similar test, but it measures a different part of the collagen molecule. It can be run on either a blood sample or a urine sample.
- Deoxypyridinoline (DPD). This test measures the crosslinking strength of bone collagen. It is taken from a urine sample.
These tests are useful for finding out if a drug to stop resorption, such as a bisphosphonate, is actually working. It is important to get the tests done at the same laboratory every time to avoid variations in technique or equipment that may interfere with the interpretation of results.