Minimisation of salicylate interference in the Glynn and Kendal paracetamol procedure

The Glynn and Kendal technique for plasma paracetamol assay was investigated in an attempt to eliminate its susceptibility to interference from salicylate. Modification of the nitrating reaction so as to produce milder conditions by omission of hydrochloric acid, reduction of nitrite concentration, and limiting the time of reaction to 2 minutes resulted in far less interference. A further improvement was achieved by reading at 450 nm. The proposed modification reduces the interference from a salicylate concentration of 100 mg/dl to the equivalent paracetamol concentration of 12 mg/l, compared with about 70 mg/l in the unmodified method.