Utility of calcium, magnesium and phosphate testing in the emergency department

To determine how frequently calcium (Ca), magnesium (Mg) and phosphate (PO₄ ) tests change ED patient management.

We undertook a retrospective observational study in an Australian tertiary referral ED. We enrolled adult patients (aged ≥18 years) who presented between 1 January and 30 June 2017 and who had a serum Ca, Mg or PO₄ test ordered and completed during their ED stay. Patient symptoms, medical history, electrolyte levels and ED management changes were extracted from the electronic medical record.

Of the 33 120 adults presented during the study period, 1716 (5.2%, 95% confidence interval [CI] 5.0-5.4) had at least one Ca, Mg or PO₄ test completed in the ED. This included 4776 individual electrolyte tests, of which 776 (16.2%, 95% CI 15.2-17.3) were abnormal. Fifty-six (7.2% [95% CI 5.5-9.3] of patients with abnormal tests, 1.2% [95% CI 0.9-1.5] of all tests) tests were associated with a change in ED management. Twenty-six out of 1683 (1.5%) Ca levels were low with six (23.1%) management changes; 203 (12.1%) were high with 10 (4.9%) management changes. One hundred and twenty-eight out of 1579 (8.1%) Mg levels were low with 33 (25.8%) management changes; 30 (1.9%) were high with no management changes. Two hundred and twenty-five out of 1514 (14.9%) PO₄ levels were low with six (2.7%) management changes; 164 (10.8%) were high with one (0.6%) management change. Fifty (2.9%) patients had management changes despite normal electrolyte levels.

Ca, Mg and PO₄ testing is common. However, the yield of clinically significant abnormal levels is low and patient management is rarely changed. Testing of these electrolytes needs to be rationalised.