Calcium, magnesium, and phosphorus: emergency department testing yield

To investigate how often the ED ordering of stat serum calcium (Ca+2), magnesium (Mg+2), and phosphorus (PO4(-3)) levels affected clinical treatment; to define the diagnoses of patients for whom Ca+2, Mg+2, and PO4(-3) measurements did affect clinical therapy; and to suggest guidelines for more appropriate ordering of these laboratory tests.

A retrospective chart review was performed in an academic teaching hospital. All adult ED patients who had Ca+2, Mg+2, or PO4(-3) laboratory testing during the 9-month study period were included and evaluated for potential clinical impact of an abnormal Ca+2, Mg+2, or PO4(-3) laboratory test.

1.477 patients had Ca+2, Mg+2, or PO4(-3) measured while in the ED during the study period. Of these, 260 patients (17.6%) had a total of 312 abnormal Ca+2, Mg+2, or PO4(-3) values as defined by results exceeding +/- 15% of normal reference values. Of these, only 5 patients (0.3%) received treatment for abnormal values in the ED, while 75 patients (5.1%) were treated once admitted to the hospital. In this study, the only diagnostic groups to whom significant treatment was administered were diabetic patients (Ca+2 and PO4(-3); alcoholic patients (Mg+2); and renal failure patients (Ca+2, Mg+2, and PO4(-3).

These results suggest that stat Ca+2, Mg+2, and PO4(-3) levels seldom affect clinical treatment in the ED. The frequency of ordering these tests may be reduced by obtaining Ca+2, Mg+2, or PO4(-3) measurements only for patients known to be at risk for such abnormalities, based on their existing or suspected diagnoses. The authors suggest obtaining these tests, when indicated, on a "non-stat" basis, with the subsequent laboratory results becoming available in-hospital, where treatment is more likely to occur.