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Background: In the laboratory evaluation of suspected paracetamol poisoning, a non-invasive sample type that avoids venepuncture would be an attractive alternative to plasma, particularly in the paediatric setting. Salivary paracetamol measurement has not previously been evaluated in the published medical literature in the setting of deliberate self-poisoning (DSP). Methods: In-house validation experiments (recovery, stability and lower limit-of-detection) were performed on pooled saliva samples using a Roche Acetaminophen assay on a Roche/Hitachi 917 analyser. A clinical study of comparison of paired saliva and plasma samples was also conducted involving adult patients presenting with DSP of paracetamol, the results of which were published previously. Results: The validation experiments using pooled saliva samples showed: (i) mean recovery ( paracetamol concentration 37.5-525 mg/L) 100.01% (+0.02 SD); (ii) precision of repeated assay over 24-h period CV ,4%; (iii) lower limit-of-detection 0.9 mg/L. The clinical study of 21 patients with mean plasma paracetamol concentration of 48 mg/L (range 0 -130) and mean saliva concentration 62 mg/L (range 0 -183) showed good correlation between saliva and plasma concentrations (r 2 ¼ 0.91).

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Keywordsdeliberate self poisoning, emergency department, overdose, paracetamol, quantitative analysis, risk assessment ---------------------------------------------------------------------- WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT• Paracetamol is commonly used in deliberate self poisoning (DSP) and this requires blood sampling to refine risk assessment. If saliva concentrations agreed with plasma concentrations, then this could support the development of non-invasive testing. Our pilot work supports this hypothesis, but was largely confined to nontoxic concentrations. WHAT THIS STUDY ADDS• We found agreement between the indications for treatment of paracetamol DSP based on plasma and saliva paracetamol concentrations. Saliva may hold promise as a non-invasive method to risk stratify paracetamol poisoning. AIMSParacetamol is commonly used in deliberate self poisoning (DSP) and requires blood sampling to refine risk assessment. We aimed to test the agreement between plasma and saliva paracetamol concentrations in the toxic range in DSP. METHODSContemporaneous paired plasma and saliva paracetamol concentrations were measured. Saliva was collected using a Sarstedt Salivette® device and the concentration was measured using a colorimetric method. RESULTS Fifty CONCLUSIONSThe agreement between the indications for treatment of paracetamol DSP based on plasma and saliva paracetamol concentrations extends into the toxic range, but with slightly lower agreement. Saliva may hold promise as a non-invasive method to risk stratify paracetamol poisoning.