BACKGROUND: Placement of enteral feeding tubes is an important clinical issue. Previous studies suggest that paracetamol absorption is very fast after jejunal administration. The aim was to determine whether paracetamol serum concentration measured by immunoassay can determine the tip position of the feeding tube. METHODS: Thirty-three critically ill patients requiring enteral nutrition with either gastric or post-pyloric feeding tubes were enrolled prospectively in the surgical intensive care unit of a university hospital. Paracetamol was administered in the feeding tube (15 mg/kg) after a baseline blood sample (T0). Thereafter, 8 blood samples were taken between 2.5 and 240 minutes. Paracetamol was analyzed using an automated homogenous immunoassay. RESULTS: The patients did not differ with respect to age or severity of disease. Peak paracetamol concentrations were significantly higher after post-pyloric administration with 42.6 +/- 13.5 versus 20.5 +/- 7.5 mg/L (p < .0001). Time-to-peak paracetamol concentration was significantly shorter with post-pyloric tubes (median, 5 versus 60 minutes; p < .0001). The receiver operating characteristic (ROC) curves showed the highest sensitivity and specificity at 5 minutes with 94.1% and 100%, respectively, for discriminating between gastric and post-pyloric location. CONCLUSIONS: Because of paracetamol's rapid absorption after jejunal administration, the test seems to be a safe and inexpensive alternative to X-ray control for assessment of the enteral feeding tube location. Its value in clinical practice remains to be established.