What laboratory marker elevation would confirm the diagnosis in a patient with hip pain that worsens at night and improves with activity, suggestive of Paget's disease of the bone?

Elevated Alkaline Phosphatase Confirms Paget's Disease of Bone

The laboratory marker that would confirm the diagnosis in this patient is elevated serum alkaline phosphatase (ALP), which is the hallmark biochemical finding in Paget's disease of bone. 1, 2, 3, 4

Clinical Presentation Analysis

The patient's symptom pattern is characteristic of Paget's disease:

• Pain relieved by activity and worsening at night is atypical for mechanical hip pathology but consistent with Paget's disease, which causes bone pain that can be constant and non-mechanical in nature 1, 2, 4
• Age 72 years fits the typical demographic, as Paget's disease affects 1-2% of the population over age 55 years, with peak incidence in older adults 2, 3
• Hip involvement is common since Paget's disease has high preference for the pelvis, spine, skull, and long bones 3, 4

Diagnostic Laboratory Workup

Primary Confirmatory Test

• Serum alkaline phosphatase (ALP) is the single most important laboratory marker for diagnosis and should be measured routinely 1, 2, 3, 4
• Elevated ALP correlates directly with disease activity and the extent of bone involvement 3, 4
• ALP elevation in Paget's disease reflects increased osteoblastic activity in response to accelerated bone resorption 5, 4

Additional Laboratory Tests to Obtain

• Calcium, 25-hydroxy-vitamin D, phosphate, and parathyroid hormone levels should be measured to exclude other metabolic bone diseases such as osteomalacia or hypophosphatasia 1
• Bone turnover markers (serum procollagen type I N propeptide [P1NP] and C-terminal telopeptide [CTx]) can aid in evaluating metabolic bone diseases, though ALP remains the primary marker 1
• Renal function should be assessed for medication safety considerations 1

Important Diagnostic Caveats

When ALP May Be Normal

• Rare cases of active Paget's disease can present with normal ALP levels 6
• In such cases, diagnosis relies on characteristic radiographic findings and radionuclide bone scan 6, 4
• Treatment response must be monitored clinically and with imaging rather than biochemical markers when baseline ALP is normal 6

Confirming Hepatic vs. Bone Origin of Elevated ALP

• Measure gamma-glutamyl transpeptidase (GGT) to confirm that elevated ALP originates from bone rather than liver 1
• GGT is found in liver but not in bone; concomitantly elevated GGT suggests hepatic origin, while normal GGT with elevated ALP confirms bone origin 1

Imaging to Complement Laboratory Findings

• Plain radiographs showing characteristic features (bone expansion, cortical thickening, coarsened trabeculae) confirm the diagnosis 2, 3, 4
• Radionuclide bone scan is the most sensitive test for assessing disease extent and identifying additional asymptomatic sites 2, 3, 4
• MRI or CT with bone scintigraphy can assess both structural changes and disease activity 1

Common Diagnostic Pitfalls

• Do not assume all elevated ALP is from Paget's disease without excluding hepatic causes, bone metastases, fractures, or other metabolic bone diseases 1
• Do not rely solely on laboratory values - diagnosis should never be made on imaging or laboratory findings alone without clinical correlation 1
• Do not miss polyostotic disease - perform whole-body imaging to map clinically silent but radiologically active lesions 1