Therapeutic drug monitoring in the Caribbean - abstract

ABSTRACT

For many drugs there is a close relationship between plasma concentration and biological/therapeutic effects. Laboratory services for estimating plasma drug concentrations (Therapeutic Drug Monitoring or TDM) are now provided in most major hospitals of the developing world, including most of the Caribbean, for three major reasons 1) the variable but generally long lag period in the transfer of new technology, 2) the perceived cost of the new technology, 3) the dearth of clinical pharmacologists, the specialists most closely involved in the provision and interpretation of plasma drug levels. A TDM service was set up in Barbados in 1982, in the Clinical Pharmacology Laboratory at the Queen Elizabeth Hospital using the Syva Enzyme Mediated Immuno Assay/spectophotometric system. Eight drugs are now routinely assayed, using (since January 1987) a flourescence polarisation immuno assay (Abbott), which permits rapid estimation and a same day service, facilitating immediate feedback of emergency and out-patient results. Drugs assayed include the top four anti-epileptics (phenytoin, phenobarbitone, carbamazepine and valproate), two cardiac drugs (digoxin and quinidine), theophylline and gentamycin. Assay services (including pharmacokinetic interpretation and advice on dose regimes) are provided for hospital in-and out-patients, private and polyclinic patients and, on request, to other Caribbean countries, i.e. countries of the Organisation of Eastern Caribbean States (OECS) and occasionally Jamaica. In Trinidad TDM services are provided for anti-epileptics at the Port-of-Spain General Hospital. Review of anti-epileptic TDM in Barbados indicates that drug management is usually far less than optimal. Because of its saturation kinetics, phenytoin is particularly difficult to achieve correct therapeutic levels and good results with, only 25 percent of assays falling in the recommended therapeutic range. Improved patient compliance and doctor education both require the concerted attention of the health services if the present poor epilepsy management is to be improved. TDM is less costly than perceived if the service 1) uses a system where equipment is provided by the Company, with a contract for purchase of a minimal number of kits, rather than purchase of equipment, at high capital costs, and having to pay for service; 2) is centralised, providing large runs and minimising cost of controls, etc. Current costs per test for anti-epileptics to our lab (e.g. carbamazepine US$3.50) are comparable with basic cost to QEH of hormone assays (e.g. prolactin US$2.50, testosterone US$3.50, cortisol US$2.30). These costs are, however, multiplied by about four for private patients, or six or more by the commercial labs, where routine assays, e.g. amylase and blood count costs US$6.00 and $15.00 respectively. Since most other lab test costing must include capital equipment and service costs, the comparisons become even more favourable. Finally, assay costs are negligible compared to annual Drug costs for all relevant drugs except phenobarbitone. A select number of drug assays provide invaluable guidance in treating difficult patients with drugs which have a narrow therapeutic/toxic ratio. Anti-epileptic assays in particular are increasingly recognised to be as valuable in treating epilepsy as blood sugars in treating diabetics. No major hospital in the Caribbean or elsewhere should be without a Clinical Pharmacologist or a TDM service, providing assays for anti-epileptics, gentamycin and digoxin at least (AU)