BACKGROUND: Blood loss from diagnostic procedures in critically ill patients needs to be minimized. Traditionally, when drawing blood from arterial lines, the initial sample used to clear the line has been discarded (open method). Use of a temporary reservoir enables this discard sample to be returned to the patient (closed method). METHODS: Critically ill surgical patients were prospectively randomized to the open or closed method of drawing blood from arterial lines. Blood loss to diagnostic sampling was measured in both groups. RESULTS: A comparison study (n = 1657) of these two methods revealed that blood loss to the patient was significantly decreased (P << .01) using the closed method. Mean blood loss per patient per day was 69 mL in the open group (n = 873) vs 35 mL in the closed group (n = 784). CONCLUSIONS: Use of the closed method when drawing blood from arterial lines results in a significant decrease in blood lost to diagnostic procedures.