Alkaline Phosphatase in Osteoporosis
Alkaline phosphatase (ALP) is typically not elevated in uncomplicated osteoporosis, as osteoporosis is characterized by low bone mass without defects in bone mineralization. 1
Relationship Between ALP and Bone Metabolism
- ALP is a marker of bone turnover that can help assess bone metabolism, but its levels remain within normal range in typical osteoporosis 1, 2
- Bone-specific alkaline phosphatase (B-ALP) is more specific for bone turnover than total ALP and can be used to evaluate bone metabolism in osteoporosis patients 2
- In some studies, B-ALP levels have shown negative correlation with bone mineral density (BMD) values in specific skeletal sites like femoral neck and Ward's triangle in osteoporosis patients 2
When ALP May Be Elevated in Bone Disorders
Conditions Associated with Elevated ALP
- Osteomalacia (vitamin D deficiency) typically shows elevated ALP due to impaired bone mineralization 3
- Paget's disease of bone presents with markedly elevated ALP levels 4
- Hyperparathyroidism causes increased bone turnover with elevated ALP 1, 4
- Chronic kidney disease-mineral and bone disorder (CKD-MBD) commonly presents with elevated ALP 5
Medication Effects on ALP
- Teriparatide (Forteo), an anabolic agent used for severe osteoporosis, may cause asymptomatic elevation in ALP as a side effect of increased bone formation 6
- This elevation is expected with anabolic therapy and represents increased osteoblastic activity 6
Clinical Significance of ALP in Bone Disorders
- High levels of bone-specific alkaline phosphatase can help diagnose osteomalacia in the setting of vitamin D deficiency, hypocalcemia, or hypophosphatemia 5
- Monitoring ALP levels is recommended in CKD patients every 12 months, or more frequently with elevated PTH 5
- The predictive power of parathyroid hormone (PTH) levels for bone disorders is increased by concomitant consideration of alkaline phosphatase levels 5
Low ALP in Osteoporosis Patients
- Low serum ALP (<30 IU/L) has been found in approximately 9% of patients attending osteoporosis clinics 7
- Persistently low ALP may signal hypophosphatasia, a condition that can be mistaken for osteoporosis but should not be treated with antiresorptive therapy 7
- Hypophosphatasia was diagnosed in approximately 3% of osteoporosis clinic patients with low ALP 7
Clinical Implications
- In uncomplicated osteoporosis, ALP levels typically remain within normal range 1, 2
- Elevated ALP in a patient with suspected osteoporosis should prompt investigation for other metabolic bone disorders 1, 3
- Bone-specific alkaline phosphatase may be more reliable than PTH for assessing bone turnover in CKD patients 5
- Some research suggests that elevated bone alkaline phosphatase in perimenopausal women might predict later development of osteoporosis as reflected in bone fractures 8
Diagnostic Approach When ALP is Elevated
- Measure serum calcium, phosphate, and parathyroid hormone (PTH) levels to evaluate for metabolic bone disorders 1
- Check 25-hydroxyvitamin D level to evaluate for vitamin D deficiency 1
- Consider bone-specific alkaline phosphatase (B-ALP) testing to confirm bone origin of elevated ALP 1, 2
- Radiographic imaging may be necessary to distinguish between different bone disorders 4