Self-prepared heparinized syringes for measuring ionized magnesium in critical care patients.

We have compared ionized magnesium assays in the Nova 8 electrolyte analyser using dry balanced heparinized syringes and self-prepared heparinized syringes. Thirty blood specimens were obtained into syringes either operator-prepared with liquid sodium heparin or commercially manufactured dry balanced heparinized syringes. There was a good correlation between results from the two syringes. The mean difference between sampling methods was 0.01 mmol litre-1 (95% confidence index -0.05 to 0.08 mmol litre-1). The correlations for sodium, potassium and ionized calcium assays were similarly close. The relationship between sampling methods was close enough to justify the clinical use of self-prepared syringes, with potential economies in clinical costs.