This newsletter summarizes information on therapeutic drug monitoring (TDM) provided at the recent Antech Medicine Consultants Conference with Dr. Dawn M. Boothe of Texas A&M University.

• Time of patient sampling for TDM. Whether peak or trough drug concentrations are measured is influenced by the need for evaluating efficacy (trough) or safety/toxicosis (peak). If only a single sample will be collected, trough samples are generally preferred for consistency over time. Further, the time of peak plasma drug concentration (PDC) is more difficult to establish.



• Importance of elimination half-life (t½). Drugs with short t½ times should have two samples drawn (e.g. diazepam, thyroxine), whereas those with long t½ times need only a single sample (e.g. bromide, most dogs on phenobarbital).



• Sample handling. Remember that blood samples for TDM should be placed in plain red top tubes (RRT) and not in serum separator tubes (SST).



• Concept of steady state concentrations. Steady state PDCs occur when drug input and elimination by metabolism and/or excretion are equilibrated. Although PDCs change somewhat during the dosing interval, these changes remain consistent at steady state (i.e. both the peak and trough concentrations stay consistent).
  
  With repeated drug dosing, PDCs will reach 50% of their steady state concentration at one t½, 75% at two t½, 87.5% at three t½, etc. Thus, steady state concentrations are attained after 4-5 t½ when the drug is administered with a fixed dosing regimen.
  
  Evaluation of a drug's efficacy is often inappropriate until steady state has been reached, because only then will the maximum peak and trough concentrations and clinical response have been achieved.