VENOUS STASIS AND FOREARM EXERCISE DURING VENIPUNCTURE AS SOURCES OF ERROR IN PLASMA ELECTROLYTE DETERMINATIONS.

A study was made of errors in plasma sodium, potassiu, chloride and calcium determinations caused by venous stasis and forearm exercise during venipuncture. Blood samples were taken from both arms of healthy young adults, one arm being used as a control. Analyses were performed in a routine hospital laboratory and the technicians were unaware of the identity of the samples. Laboratory precision was assessed at the same time.Venous stasis alone for two minutes caused no significant change, but the use of continuous slow forearm exercise for one minute caused mean elevations of 0.73 mEq./1. in potassium, and 0.4 mg. per 100 ml. in calcium, the maximum elevations being 1.1 mEq/1. and 1.3 mg. per 100 ml., respectively. A clinically unimportant but significant increase in sodium values of 1.40 mEq./1. was also observed. Chloride values were not affected. In addition there was a substantial error in calcium determinations in the laboratory, which emphasizes the need for repeated calcium determinations to avoid diagnostic error.The conclusion is reached that forearm exercise must be avoided during venipuncture.