Effect of flushing technique on residual anaesthetic drug content in syringes

INTRODUCTION: The practice of reusing the same syringe to administer different medications during anaesthesia for the same patient is common. One concern with this practice is the potential for drug incompatibility. Although flushing the syringe between drugs may address this concern, its effect on drug removal is unknown. The primary aim of this study was to examine the effect of flushing technique on the residual drug content in syringes of different sizes that were initially used for administering propofol and subsequently reused for administering cefazolin and morphine. METHODS: Syringes (3, 5, 10 and 20 mL) were filled with propofol (1%) and emptied. They were then flushed once, twice or three times using varying volumes of water for injection or normal saline (0.9% sodium chloride), before being reused for cefazolin. The procedure was repeated before the syringes were reused for morphine. Finally, morphine was flushed out. The expelled fluid after each flush was collected and analysed using liquid chromatography-tandem mass spectrometry. RESULTS: For the 10- and 20-mL syringes, no residual drug was detected after a single flush, regardless of the flushing fluid, using 50% or 100% syringe volume flushes. For the 3- and 5-mL syringes, no residual drug was detected after flushing once with normal saline or twice with water for injection using 50% or 100% syringe volume flushes. CONCLUSION: A minimum of two 50% syringe volume flushes with normal saline or water for injection ensured complete removal of propofol, cefazolin and morphine, regardless of syringe size. This may enable safer single-patient syringe reuse during anaesthesia, reducing syringe use, lowering costs and promoting environmental sustainability.