AN AUDIT OF THERAPEUTIC DRUG-MONITORING SERVICE PROVISION BY LABORATORIES PARTICIPATING IN AN EXTERNAL QUALITY ASSESSMENT SCHEME

Therapeutic drug monitoring services were investigated in a questionna ire sent to all subscribers to the United Kingdom National External Qu ality Assessment Scheme for Therapeutic Drug Assays. Questions were po sed on assay availability and use, target ranges, and reporting proced ures for digoxin, lithium, phenytoin, phenobarbitone, carbamazepine, t heophylline, and valproic acid. One hundred fifty-seven laboratories r eplied and, except for lithium, 45% reported in mass units, 34% in mol ar units, and 22% a mixture of mass and molar units. Target ranges for lithium, digoxin, carbamazepine, and phenobarbitone mere highly varia ble but ranges for phenytoin, theophylline, and valproic acid were mor e consistent. Immunoassay was the most popular methodology although hi gh-performance liquid chromatography was commonly used for anticonvuls ants. Paper copies of results were provided by 93% of laboratories, 40 % reported by telephone, 12% by fax, and 25% by computer. Additional d ata, mainly dose, time of last dose, and duration of therapy were requ ested by 55% to 67% of laboratories. Grades of staff authorizing resul ts ranged from nurses to senior consultants, and collaboration with ph armacists occurred in 26% of laboratories. Most laboratories provided a daily analytical service and 73% offered a 24-hour emergency service . This audit unexpectedly identified use of a wide range of target con centrations, particularly for digoxin and lithium.