However the initial volume of blood drained is not as important as the amount of on-going bleeding. If the patient remains haemodynamically stable they may be admitted and observed. The colour of the blood is also important - dark, venous blood being more likely to cease spontaneously than bright red arterial blood. Patients admitted for observation who have continuing drainage with no signs of reduction in chest tube output over 4-5 hours should also undergo thoracotomy. The threshold for this is usually stated at around 200-250mls of blood per hour.