Postoperative Plasma Paracetamol Levels following Oral or Intravenous Paracetamol Administration: A Double-Blind Randomised Controlled Trial

In day-case surgery paracetamol is commonly given orally preoperatively, or intravenously intraoperatively. In this double-blind randomised controlled trial we investigated which of these methods of administration achieved therapeutic plasma levels most effectively in the early postoperative period. Thirty patients undergoing day case arthroscopy of the knee were randomised to receive either 1.0 g oral paracetamol 30 to 60 minutes preoperatively (20 patients) or 1.0 g intravenous paracetamol intraoperatively (10 patients). Plasma paracetamol levels were measured 30 minutes after arrival in the recovery room. Secondary outcomes included postoperative pain scores, rescue analgesia requirements and duration of stay in the recovery room. All patients receiving the intravenous preparation had plasma levels above the analgesic level compared to less than half (7/20) in the oral group. Mean plasma paracetamol levels were 88.6 µmol/l for the intravenous group and 53.2 µmol/l for the oral group (P=0.0005). There were trends towards reduced rescue analgesia and duration of stay in the recovery room for the intravenous group although not reaching statistical significance. There was no difference in pain scores between groups. Intraoperative administration of 1.0 g of intravenous paracetamol more reliably achieved effective paracetamol levels in the early postoperative period compared to an equal dose given orally preoperatively. Only a minority of patients receiving the 1.0 g oral dose preoperatively had plasma levels in the therapeutic analgesic range.