Critical values for sodium estimation

Clinical laboratories should have a policy mandating staff involved in analysis of patient samples to convey critical results to clinical staff urgently, usually by telephone. A critical result has been defined as a result "at such variance with normal as to be life threatening, unless something is done promptly, and for which some corrective action could be taken". 

Not all parameters measured in clinical laboratories warrant critical values, but sodium measurement in either plasma or whole blood is one that does. It is of course a matter of judgment what the critical values for sodium measurement (indeed any measured parameter) should be, and this is one of several issues addressed by a recent evaluation study of sodium critical values. 

At the large New York medical center where this study was conducted the quoted reference range for sodium is 135-145 mEq/L (135-145 mmol/L) and chosen critical values are <120 mEq/L (mmol/L) and >155 mEq/L (mmol/L). 

During a 6-month study period, 111,545 sodium measurements were reported. Of these, 613 (0.5 %) were critical results: 166 critically low (i.e. <120 mEq/L) and 447 critically high (i.e. >155mEq/L). These critical results were derived from a total of 172 patients (71 with critically low values and 101 with critically high values). 

A medical response, defined as either reordering of blood estimation or documented treatment of hypo- or hypernatremia, occurred within 4 hours of the result being received in 63 % of hyponatremic cases and 66 % of hypernatremia cases. Overall mortality was 19 % among hyponatremic patients and 48 % among hypernatremic patients. 

Of particular interest was the finding that most (64 %) of the hyponatremic patients had sodium in the range of 155-160 mEq/L. Of the 64 patients with sodium in this range, 36 (56 %) died. In many hospital laboratories 160 mEq/L is chosen as the upper critical value. The evidence of this study suggests that sodium in the range of 155-160 mEq/L is associated with high risk of death and that 155 mEq/L rather than 160 mEq/L might be more suitable as the upper critical level.