Plasma salicylate concentration was monitored in 42 children on long-term salicylate therapy for rheumatoid arthritis. A given dose of salicylate per kg resulted in large variations in plasma levels, both between individuals and for a single individual at different times. The factors responsible for such variations were studied; in 6 cases urinary metabolites of salicylate were analysed. The relation between salicylate dosage and plasma half-life accounts for the fact that small changes in dosage can result in large changes in plasma concentration. The addition of corticosteroid or ACTH therapy results in lower plasma levels of salicylate, and necessitates higher dosage of salicylate. After the introduction of routine monitoring of plasma salicylate, the incidence of toxic symptoms fell sharply.
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