Paget’s disease of bone is typically diagnosed through a combination of blood tests and X-rays, often after an incidental finding on routine lab work or imaging done for another reason. Many people with Paget’s disease have no symptoms at all, so the diagnosis frequently comes as a surprise. The process is straightforward, but distinguishing Paget’s from other bone conditions requires a specific sequence of steps.
Blood Tests: The Usual First Clue
The most common initial sign of Paget’s disease is an elevated level of alkaline phosphatase in the blood. This enzyme rises when bone is being broken down and rebuilt at an abnormal rate, which is the core problem in Paget’s. A standard blood panel can pick this up, and it’s often what prompts further investigation.
Total alkaline phosphatase has a diagnostic sensitivity of about 74% for Paget’s disease. That’s useful but not perfect, especially because liver problems can also raise alkaline phosphatase levels. A more targeted version of the test, bone-specific alkaline phosphatase, bumps the sensitivity up to 84% and is better at confirming that the abnormal activity is coming from bone rather than the liver.
Here’s an important caveat: if only one bone is affected (called monostotic disease), total alkaline phosphatase can come back completely normal. A normal result does not rule out Paget’s. In that scenario, bone-specific alkaline phosphatase is the better test to order. Other markers of bone turnover exist, but alkaline phosphatase remains the primary screening tool and is also used later to track whether treatment is working.
X-Rays: Confirming the Diagnosis
The Endocrine Society’s clinical practice guideline recommends plain X-rays of the suspected area as the first imaging step when Paget’s disease is suspected. X-rays alone can often confirm the diagnosis because Paget’s produces a set of distinctive changes that look different from most other bone conditions.
What shows up on X-ray depends on the stage of the disease:
- Early (lytic) stage: A wedge-shaped area of bone loss appears in long bones, often described as a “flame” or “blade of grass” shape advancing along the bone. Cortical thickening along the outer surface of the bone and coarsening of the internal bone structure begin to appear.
- Mixed stage: All four hallmarks are visible at once: advancing bone loss, thickened and coarsened internal bone structure aligned along stress lines, thickened outer bone walls, and overall widening of the bone. In the spine, thickening along all four edges of a vertebral body creates what radiologists call a “picture frame” appearance.
- Late (blastic) stage: The bone becomes densely thickened and enlarged. In the skull, patchy areas of dense bone create a “cotton wool” appearance that is essentially unique to Paget’s disease.
These features, particularly bone enlargement combined with cortical thickening, are highly characteristic. Most other bone diseases destroy bone without making it bigger, which is what makes Paget’s recognizable on a standard X-ray.
Bone Scans: Mapping the Full Picture
Once X-rays confirm Paget’s disease in one location, the next recommended step is a radionuclide bone scan. This involves injecting a small amount of radioactive tracer into a vein and then scanning the entire skeleton. Pagetic bone absorbs the tracer intensely because of its high metabolic activity, lighting up on the scan.
The purpose isn’t to confirm the diagnosis (X-rays already did that) but to find out how many bones are involved. Paget’s can affect a single bone or several, and knowing the full extent of the disease matters for treatment planning. On the scan, Paget’s disease shows intense, uniform tracer uptake throughout the affected bone, often involving just one or a few bones in a recognizable pattern. This differs from bone metastases from cancer, which tend to show scattered focal hot spots in an asymmetric, random distribution across the skeleton.
Telling Paget’s Apart From Bone Cancer
One of the most critical parts of diagnosing Paget’s disease is distinguishing it from bone metastases, since both can produce dense, abnormal-looking areas on imaging. The differences are consistent enough that experienced clinicians can usually tell them apart, but the distinction matters enormously.
On X-rays and CT scans, Paget’s disease enlarges the bone and thickens both the outer cortex and internal structure. Bone metastases, by contrast, either destroy bone (creating holes) or deposit abnormal bone (creating dense spots), but they typically erode the cortex rather than thickening it, and they don’t make the bone bigger overall.
MRI adds another layer of distinction. In Paget’s disease, the bone marrow retains its normal fat content even as the bone remodels around it, creating a mixed signal pattern on imaging. Metastatic cancer replaces the marrow entirely with tumor tissue, producing a more uniform signal change. Paget’s also doesn’t invade through the cortex into surrounding soft tissue, while aggressive metastases frequently do.
In the skull, the “cotton wool” pattern of Paget’s looks different from metastatic lesions, which tend to create well-defined destructive areas without bone expansion.
When a Biopsy Is Needed
Most cases of Paget’s disease are diagnosed without a biopsy. The combination of elevated alkaline phosphatase and characteristic X-ray findings is usually sufficient. However, a bone biopsy is sometimes necessary when imaging findings are ambiguous or when there’s concern about a possible malignant transformation. Paget’s disease carries a small risk of developing into bone cancer (osteosarcoma), and a sudden increase in pain, swelling, or a change in the appearance of a known Pagetic lesion can prompt a biopsy to rule that out.
Hearing Tests for Skull Involvement
When Paget’s disease affects the skull, hearing loss is a common complication. The abnormal bone growth can compress nerves or distort the structures of the inner ear. A simple screening questionnaire (the Hearing Handicap Inventory for the Elderly, Screening Version) can effectively identify which patients need formal hearing testing. A score above 8 on this questionnaire increases the odds of detecting moderate hearing impairment by more than fivefold, and it has excellent specificity: only about 2% of Paget’s patients flagged by the questionnaire turn out to have normal hearing. If you’ve been diagnosed with Paget’s in the skull, hearing screening is a practical step that can catch problems early.
Genetic Testing and Family Screening
Paget’s disease runs in families. Mutations in a gene called SQSTM1 account for about 40% of familial cases, and genetic testing for these mutations is available. For the remaining 60% of families, the responsible genes haven’t been identified yet, though researchers have found several genetic markers associated with higher risk.
A combined test using genetic markers and biochemical measures (like alkaline phosphatase) can reach a sensitivity of 96% for identifying affected family members, though its specificity is moderate at 57%, meaning it will flag some people who don’t actually have the disease. Genetic testing is most useful for screening relatives of people with confirmed Paget’s, particularly first-degree family members over age 40, rather than as a primary diagnostic tool for the general population.
The Typical Diagnostic Sequence
In practice, the path to diagnosis usually follows a predictable order. A routine blood test shows elevated alkaline phosphatase, or an X-ray taken for another reason reveals characteristic bone changes. If Paget’s is suspected from blood work alone, targeted X-rays of the painful or affected area come next. If X-rays confirm the diagnosis, a full-body bone scan maps the extent of the disease. Bone-specific alkaline phosphatase provides a baseline number that your doctor will track over time to monitor disease activity and response to treatment.
The whole process is noninvasive and usually doesn’t take long. Most people get a clear answer from blood tests and X-rays alone, with the bone scan serving as a comprehensive survey rather than a diagnostic hurdle.