Evaluation of a calcium, magnesium and phosphate clinical ordering tool in the emergency department

We developed a clinical tool comprising patient risk factors for having an abnormal calcium (Ca), magnesium (Mg) or phosphate (PO₄) level. We hypothesized that patients without a risk factor do not require testing. This study examined the tool's potential utility for rationalizing Ca, Mg and PO₄ ordering in the emergency department (ED).

We undertook a retrospective observational study in a single metropolitan ED. Patients aged 18 years or more who presented between July and December 2019 were included if they had a Ca, Mg or PO₄ test during their ED stay. Demographic and clinical data, including the presence of risk factors, were extracted from the medical record. The primary outcome was a clinically significant abnormal Ca, Mg or PO₄ level (>0.2 mmol/l above or below the laboratory reference range).

Calcium, Mg and PO₄ levels were measured on 1426, 1296 and 1099 patients, respectively. The positive and negative predictive values and likelihood ratios of the tool identifying a patient with a Ca level > 0.2 mmol/l outside the range were 0.05, 0.99, 1.59 and 0.41, respectively. The values for Mg were 0.02, 1.00, 1.44 and 0.35 and those for PO₄ were 0.15, 0.93, 1.38 and 0.57, respectively. The majority of patients not identified as having an abnormal level did not receive electrolyte correction treatment. Application of the tool would have resulted in a 35.8% cost reduction.

The tool failed to predict a very small proportion of patients (approximately 1%) with an abnormal Ca or Mg level and for whom it would have been desirable to have these levels measured. It may help rationalize Ca and Mg ordering and reduce laboratory costs.