Rational ordering of serum electrolytes: development of clinical criteria

The purpose of this study was to determine the frequency of electrolyte abnormalities in emergency department patients, to assess the proportion of these abnormalities affecting patient outcome, and to develop a set of clinical criteria to allow selective ordering of serum electrolytes. The ordering physicians completed questionnaires describing the clinical presentations of 1,031 patients on whom electrolytes were requested. For every patient with abnormal electrolyte values, the chart was reviewed to ascertain whether this was a clinically significant electrolyte abnormality (CSEA). The clinicians' responses to the questionnaires were used to construct a clinical criteria set predictive of CSEAs. Five hundred forty-five patients (52.9%) had one or more abnormal electrolytes, but only 161 patients (15.6%) had CSEAs. A set of ten clinical criteria detected 159 of 161 CSEAs (98.8% sensitivity) and would have avoided the ordering of electrolytes in 262 cases (29.9% specificity). At a cost to the patient of $20 per set of electrolytes ordered, detection of 159 CSEAs using the clinical criteria would cost $96.73 per CSEA detected. Detection of the additional two CSEAs by ordering electrolytes on all patients in the series would cost an additional $5,240. If the accuracy of these clinical criteria is confirmed in other settings, they may be useful guidelines for reducing laboratory charges without compromising patient care.