A 38-year-old woman from rural India presents with progressive bone pain, muscle weakness, and recurrent fractures. Biochemical investigations reveal: serum calcium 7.2 mg/dL, phosphate 2.1 mg/dL, alkaline phosphatase 120 IU/L, and PTH 180 pg/mL. What is the most appropriate next investigation to confirm the diagnosis of osteomalacia?

## Investigation of Choice for Osteomalacia **Key Point:** Serum 25-hydroxyvitamin D [25(OH)D] is the gold standard screening and confirmatory test for osteomalacia. It reflects total body vitamin D stores and is the most reliable indicator of vitamin D status. ### Why 25(OH)D is Superior | Parameter | 25(OH)D | 1,25(OH)₂D | Clinical Use | |-----------|---------|-----------|---------------| | Half-life | 2–3 weeks | 4–6 hours | Long-term status | | Reflects | Total body stores | Active form only | Diagnostic accuracy | | Affected by | Vitamin D intake, sun exposure | PTH, FGF23, kidney function | Specificity | | Diagnostic cutoff | <20 ng/mL (deficiency) | Variable; less reliable | Gold standard | **High-Yield:** In osteomalacia, 25(OH)D is **low** (<20 ng/mL), while 1,25(OH)₂D may be **normal or elevated** (due to secondary hyperparathyroidism driving renal activation). This dissociation is pathognomonic. ### Clinical Pearl **Clinical Pearl:** The patient's biochemical profile—hypocalcemia, hypophosphatemia, elevated PTH, and elevated ALP—is consistent with osteomalacia. The 25(OH)D level will be <20 ng/mL, confirming vitamin D deficiency as the cause. Measurement of 25(OH)D is rapid, non-invasive, and cost-effective. ### Differential Interpretation - **25(OH)D <20 ng/mL** → Osteomalacia (vitamin D deficiency) - **25(OH)D 20–29 ng/mL** → Insufficiency (borderline) - **25(OH)D ≥30 ng/mL** → Adequate (rules out vitamin D deficiency) **Mnemonic:** **25-GOLD** — 25(OH)D is the GOLD standard for vitamin D assessment.