Practice Rational Testing

Investigating hypocalcaemia

BMJ 2013; 346 doi: http://dx.doi.org/10.1136/bmj.f2213 (Published 09 May 2013) Cite this as: BMJ 2013;346:f2213
  1. Fadil M Hannan, clinical lecturer12,
  2. Rajesh V Thakker, May professor of medicine1
  1. 1Academic Endocrine Unit, Oxford Centre for Diabetes, Endocrinology and Metabolism, Nuffield Department of Clinical Medicine, University of Oxford, Churchill Hospital, Oxford OX3 7LJ, UK
  2. 2Department of Clinical Biochemistry, John Radcliffe Hospital, Oxford OX3 9DU, UK
  1. Correspondence to: R V Thakker rajesh.thakker{at}ndm.ox.ac.uk

This article explores how to confirm hypocalcaemia and ascertain its causes

Learning points

  • To confirm hypocalcaemia, calculate serum albumin-adjusted calcium concentrations; in patients who are critically ill or have acid-base disorders, ionised calcium measurements are needed

  • Careful clinical assessment may show common causes of hypocalcaemia, such as post-surgical hypoparathyroidism, chronic kidney disease, or drugs

  • Serum parathyroid hormone measurements are central to investigating and classifying causes of hypocalcaemia; other tests include serum phosphate, magnesium, and creatinine concentrations

  • Serum vitamin D measurements are indicated in patients with inadequate exposure to sunlight, nutritional deficiency, or malabsorption syndrome

A 42 year old woman with active Crohn’s disease presented to her general practitioner for annual review. Serum electrolyte, renal and liver function tests were normal apart from the following: total calcium 2.04 mmol/L (normal range 2.20-2.60 mmol/L); albumin 38 g/L (35-50 g/L); phosphate 0.71 mmol/L (0.80-1.45 mmol/L); and alkaline phosphatase activity 380 U/L (80-290 U/L). She had no history of paraesthesiae, carpopedal spasms, or seizures. Subsequent investigations showed a low serum 25-hydroxyvitamin D (25(OH)D) concentration of 23 nmol/L (normal >75 nmol/L). She was therefore started on oral calcium and cholecalciferol daily. Four months later, the patient presented to the local emergency department with persistent severe diarrhoea, paraesthesiae, carpal spasms, and seizures. Investigations found no causes for the seizures other than hypocalcaemia (ionised plasma calcium 0.82 mmol/L; normal range 1.1-1.30 mmol/L).

What are the next investigations?

  • Confirm hypocalcaemia by measurement of serum albumin-adjusted calcium (albumin-adjusted calcium = total calcium + 0.016 × (40 − albumin)).1 At admission this patient’s serum calcium is 1.56 + 0.016 x (40 – 35) = 1.64 mmol/L

  • In critically ill patients or those with acid-base disorders and symptoms attributable to hypocalcaemia, measure ionised calcium on a blood sample obtained without a tourniquet

  • Define causes of hypocalcaemia by measuring serum parathyroid hormone concentrations.

  • Assess serum magnesium.

  • Other first …

Sign in

Free trial

Register for a free trial to thebmj.com to receive unlimited access to all content on thebmj.com for 14 days.
Sign up for a free trial

Subscribe