A 38-year-old woman with chronic cholestasis due to primary biliary cirrhosis presents with bone pain, muscle weakness, and easy bruising. Serum calcium is 7.2 mg/dL (normal 8.5–10.5), alkaline phosphatase is elevated, and prothrombin time is prolonged. Which investigation is most appropriate to confirm the suspected fat-soluble vitamin deficiency?

Explanation

## Clinical Context The patient presents with a triad of findings suggestive of **vitamin D deficiency**: hypocalcaemia, elevated alkaline phosphatase (bone turnover), and muscle weakness. The chronic cholestasis (impaired bile salt secretion) impairs absorption of all fat-soluble vitamins, but the clinical presentation is most consistent with vitamin D deficiency. ## Why Serum 25-Hydroxyvitamin D is the Gold Standard **Key Point:** Serum 25-hydroxyvitamin D [25(OH)D] is the **most specific and sensitive marker** of vitamin D status because: 1. It reflects both dietary intake and cutaneous synthesis 2. It has the longest half-life (~2–3 weeks) among vitamin D metabolites 3. It is the substrate for renal 1α-hydroxylase, which produces the active form 1,25-dihydroxyvitamin D 4. Normal range: 30–100 ng/mL (75–250 nmol/L); deficiency is <20 ng/mL **High-Yield:** In cholestasis, vitamin D malabsorption leads to secondary hyperparathyroidism and bone disease. 25(OH)D measurement is the diagnostic gold standard. ## Differential Diagnosis of Fat-Soluble Vitamin Deficiencies in Cholestasis | Vitamin | Deficiency Signs | Best Diagnostic Test | Why Not This One? | |---------|------------------|----------------------|-------------------| | **D** | Hypocalcaemia, elevated ALP, bone pain, muscle weakness | Serum 25(OH)D | ✓ Correct answer | | **A** | Night blindness, xerophthalmia, Bitot's spots | Serum retinol | Retinol is unstable; not routinely measured in acute deficiency | | **E** | Neurological (ataxia, neuropathy), haemolysis | Serum alpha-tocopherol | Alpha-tocopherol is not the presenting feature here | | **K** | Prolonged PT/INR, bleeding | PT/INR (functional test) | Phylloquinone levels are difficult to measure and not standard | **Clinical Pearl:** Although this patient has prolonged PT (suggesting vitamin K deficiency), the **clinical presentation is dominated by vitamin D deficiency** (hypocalcaemia, bone pain, muscle weakness). Vitamin K deficiency would present primarily with coagulopathy and bleeding, not metabolic bone disease. ## Why Other Investigations Are Suboptimal - **Serum retinol-binding protein:** Reflects protein status, not vitamin A deficiency specifically; not the diagnostic test of choice - **Serum alpha-tocopherol:** Vitamin E deficiency is rare in cholestasis and does not explain hypocalcaemia or elevated ALP - **Serum phylloquinone (vitamin K1):** Difficult to measure; PT/INR is the functional test for vitamin K status, not direct vitamin K levels