Therapeutic Drug Monitoring (TDM)
Standard Treatment Guidelines and Essential Medicines List for South Africa, Hospital Level, Adults 2019 Edition
Medicines with narrow therapeutic indices and those with variable pharmacokinetics should be monitored regularly to optimise dosing, obtain maximum therapeutic effect, limit toxicity and assess compliance. Prescribing Information for Specific Medicines provides detailed information for specific medicines. TDM sampling for all drugs is to be done only once steady state has been reached (i.e. after 4-5 half-lives).
Lithium
Measure serum levels at about 12 hours after the last dose – i.e. immediately prior to the next dose. Concentrations should be less than 1 mmol/L and should be monitored 6-monthly while on therapy, with more frequent monitoring in the elderly (see Prescribing Information for Specific Medicines for guidance on prescribing lithium).
Aminoglycosides
Peak levels will generally be adequate if dosing is adequate (e.g. gentamicin 5mg/kg/day in a single daily dose) and measuring peak concentrations is not recommended unless the organism has a high minimum inhibitory concentration (MIC) or if the patient is critically ill. Trough concentrations, taken immediately before the next dose, are valuable for identifying potential toxicity. Toxicity may manifest as deafness or renal impairment. Aminoglycosides are relatively contraindicated in renal impairment. Audiology assessment and renal function monitoring is indicated in all patients who develop aminoglycoside toxicity (see Prescribing Information for Specific Medicines for guidance on prescribing amikacin and gentamicin).
Anti-epileptics
Measuring concentrations may be helpful to confirm poor adherence or to confirm a clinical suspicion of toxicity. Routine measurement in patients with well controlled seizures and no clinical evidence of toxicity, is not appropriate. Individual levels may be difficult to interpret – if in doubt, seek assistance from a clinical pharmacologist.
Standard Treatment Guidelines and Essential Medicines List for South Africa, Hospital Level, Adults 2019 Edition