Therapeutic drug monitoring (TDM) results are a biological measure of the ability of the patient to clear a drug as long as the patient is adhering to the prescribed dosage. If the patient is not adherent, then TDM can provide evidence of such nonadherence. To properly interpret TDM results, several variables must be considered. First and foremost is the dose the patient is supposed to be taking. The second variable is whether the patient should have achieved a steady-state condition, which means that the amount of drug the patient is taking in a day is equal to the amount the patient is eliminating in the same day so that the level will not change unless changes occur in either the dosing rate or the ability of the patient to clear the drug. For drugs with a half-life of 24 hours, steady state typically occurs when the patient has been taking the same dose for 5 to 7 days (ie, 5 x the half-life). The third variable is the time between when the last dose was taken and the sample was obtained. Ideally, for most drugs, that desired interval is 10 to 12 hours after the last dose was taken assuming the patient is taking the dose daily as different rules apply for depot medications. There are 2 reasons for this stipulation. First, this interval will best reflect the average concentration that the patient achieves over the daily dosing interval on that drug. Second, for most orally administered drugs, the value that is obtained 10 to 12 hours after the last dose will not be influenced by differences in the absorption of the drug. In contrast, if the sample is obtained during the absorption phase, the plasma concentration can be both much higher and less predictable than is the case when absorption is complete. Therefore, these 3 variables must be included when recording TDM results in the chart. As explained in this column, the TDM result without this information is not optimally interpretable. Parenthetically, TDM results do not need to be repeated (unless specific concerns arise as explained in this column), because these results are a measure of the ability of the patient to clear the drug. For depot medications (eg, antipsychotics), the results should be obtained at the end of the dosing cycle just before the administration of the drug and after the patient has ideally received 4 to 5 injections at equal intervals (eg, monthly) to be near steady-state. Parenthetically, TDM at the appropriate fixed interval (usually monthly) with depot medications is much less sensitive to timing issues than TDM of orally administered drugs. This column explains the rationale for obtaining TDM results, principally focusing on orally administered drugs and how to optimally chart and disseminate these results.