Simple monitoring of low salicylate concentrations in capillary blood. An appraisal of ultraviolet spectrophotometric methods.

The differential ultraviolet spectrophotometric determination of salicylate according to Williams et al. (11) and the single extraction ultraviolet spectrophotometric method of Stevenson (9) are subject to interference by serotonin. Probably as a result of this interference, the results are erratic when low concentrations of salicylate are monitored in capillary blood. Owing to a peculiarity of the ultraviolet absorbance spectrum of serotonin, there is no appreciable interference by serotonin, if the double extraction method of Stevenson (9) is used, in which the spectrophotometry is carried out in phosphate buffer of pH 6.86. With due precautions and standardization, reliable results are obtained, so that this comparatively simple and rapid method can be recommended wherever capillary blood sampling (finger tip, the heel in pediatrics) is preferable to venous blood sampling. Pharmacokinetical results, obtained by this method, are given for three human subjects after a single oral dose of 500 mg aspirin.