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Purpose: With rising healthcare costs limiting access to care, the judicious use of diagnostic tests has become a critical issue for many jurisdictions. Calcium, magnesium and phosphorus serum levels are regularly performed tests in the emergency department, but their clinical relevance have come into question. Authors sought to determine risk factors that could predict abnormal calcium, magnesium and phosphorus serum levels, as well as identify patients who may need corrective interventions., Methods: A retrospective cohort study was conducted in two academic hospitals in Québec City. Demographic and clinical characteristics of 1008 patients who had serum calcium and/or magnesium and/or phosphorus levels drawn by an emergency physician were collected. Multivariate logistic regression models were fitted to obtain adjusted odds ratios for each risk factor for abnormal calcium or magnesium or phosphorus blood levels, and for a required intervention., Results: Among patients for whom calcium, magnesium and phosphorus were tested in the Emergency Department, the most significant risk factors (OR>2) for electrolytic abnormality were as follows: hypocalcemia - respiratory distress, diuretics (excluding loop and thiazide), anti-neoplastic medication, long QTc, chronic kidney disease (CKD); hypercalcemia - bone pain, vitamin D, hallucinations; hypomagnesemia - diabetes, corticosteroids; hypermagnesemia - poor extremity perfusion, CKD, furosemide; hypophosphatemia - seizure; hyperphosphatemia - phosphate-binders, CKD, peripheral vascular atherosclerotic disease. Of all patients tested, 3.4% received a corrective intervention initiated by the emergency physician. Predictors of intervention on an electrolyte abnormality include poor peripheral perfusion, nausea and chronic obstructive pulmonary disease (COPD)., Conclusion: Emergency physicians can potentially reduce the unnecessary testing of calcium, magnesium and phosphorus blood levels by targeting patients with high-acuity conditions or chronic comorbidities such as CKD, diabetes and COPD., Competing Interests: Simon Berthelot receives a salary award from Le Fonds de Recherche du Québec-Santé. François Rousseau receives a salary award in evidence-based laboratory medicine from Le Fonds de Recherche du Québec-Santé, Le Ministère de la Santé du Québec, and Le CHU de Québec – Université Laval. All authors declare no other relationships or activities that could appear to have influenced the submitted work. All authors have completed the ICMJE uniform disclosure form. The authors report no other conflicts of interest in this work., (© 2021 Lapointe et al.)