Direct and online continuous monitoring of anaesthetic intake by the patient's blood stream improves practices in the critical procedure of anaesthesia. Several systems have been proposed to address this challenge. Electrochemical techniques are highly appealing because of speed and accuracy, while blood spot sampling and fluidic devices are discouraged in this application, due to the necessity of having constant and continuous monitoring. To overcome this limitation, we developed a sensor suitable for measuring anaesthetics directly in patient's vein. We present here, for the first time, a disposable and low-cost needle-shaped sensor based on a three-electrode electrochemical cell, made of pencil leads and platinum wire. The proposed sensor shows high linearity (99%) in measuring propofol directly in undiluted human serum at the temperature of 37 degrees C in its therapeutic range (30 - 240 mu M), with a limit of detection of 7.2 +/- 3.0 mu M.