Makindo Medical Notes

Related Subjects: |Ferritin |CEA |ESR |CRP |ALP |LDH |HbA1c |Alpha Fetoprotein |Anti-Hu ab |Biochemical Lab values

⚠️ Do NOT request 1,25-dihydroxyvitamin D (calcitriol) to assess vitamin D status. The correct test for deficiency, rickets, or osteomalacia is 25-hydroxyvitamin D. 💡 1,25-(OH)₂ D can be normal or even elevated in deficiency due to secondary hyperparathyroidism.

📖 About

• 25-hydroxyvitamin D (25-OH D): Main circulating form, long half-life (~2–3 weeks), best indicator of overall vitamin D stores.
• 1,25-dihydroxyvitamin D (1,25-(OH)₂ D, calcitriol): Active form, short half-life (hours), tightly regulated by PTH, calcium, phosphate, and fibroblast growth factor 23 (FGF23).
• Levels of 1,25-(OH)₂ D do not reflect vitamin D reserves. They may be normal or raised in deficiency due to secondary hyperparathyroidism.
• Extra-renal production of 1,25-(OH)₂ D can occur in sarcoidosis, TB, lymphoma, and granulomatous diseases → can cause hypercalcaemia.

🎯 Clinical Use of Each Test

• 25-OH Vitamin D: Test of choice for suspected deficiency, osteomalacia, rickets, falls, osteoporosis, malabsorption, chronic illness.
• 1,25-(OH)₂ Vitamin D:
  • Investigation of unexplained hypercalcaemia (esp. granulomatous disease or lymphoma).
  • Evaluation of rare inherited disorders (e.g. 1-α hydroxylase deficiency, vitamin D–resistant rickets).
  • Assessment in advanced chronic kidney disease (where 1-α hydroxylation is impaired).

🧾 Sample Requirements

• 5 mL venous blood in gold-top serum separator tube (or rust-top in the Acute Unit).

🌟 Summary

25-OH Vitamin D = correct test for deficiency, osteomalacia, rickets. 1,25-(OH)₂ Vitamin D = only useful in specific scenarios (renal failure, granulomatous disease, rare inherited rickets). 💡 Ordering the wrong test is a common source of wasted investigations and diagnostic delay.