1. The safety range is narrow and the therapeutic index is low. The therapeutic concentration range of some drugs is very close to the minimum toxic concentration, and can even overlap, making poisoning extremely easy. Only by adjusting the dosage based on the blood drug concentration detected by TDM can a safe and effective therapeutic effect be achieved. This situation exists in digitalis glycosides, most antiarrhythmic drugs, lithium salts of antimanic drugs, etc. For example, the effective blood drug concentration range of procainamide is 4-10μg/ml. Generally, when it exceeds 10μg/ml, toxic reactions such as hypotension and various severe arrhythmias will occur.
2. Medication aimed at controlling the onset or recurrence of diseases: The purpose of such medication is not to treat existing diseases, but to consolidate the therapeutic effect or control the onset or recurrence. Therefore, most of them require long-term medication for several months or even years. Phenytoin sodium is used to control grand mal epilepsy, and cyclosporine is used to inhibit the occurrence of rejection after organ transplantation. At this point, unless the symptoms recur or toxic reactions occur, it is very difficult to determine the efficacy and whether the dosage is appropriate. Only by controlling the blood drug concentration within the effective concentration range through TDM can the effectiveness and safety of long-term medication be guaranteed.
3. Different therapeutic goals require different blood drug concentrations. For some drugs, the required effective blood drug concentration will change depending on the therapeutic goal. When using digoxin to treat atrial flutter or atrial fibrillation, most patients require a blood drug concentration of about 2ng/ml or higher without toxic reactions. However, when the same blood drug concentration is used to treat chronic congestive heart failure, many patients will experience severe toxic reactions such as arrhythmia. Obviously, in this case, it is very necessary to accurately control the blood drug concentration range required for the therapeutic purpose with the help of TDM.
4. Drug overdose poisoning: For most drug overdose poisoning cases, it is not difficult to diagnose based on their specific toxic reaction manifestations. However, when it is difficult to distinguish the toxic reaction manifestations of a few drugs from the diseases they are used to treat, blood drug concentration tests must be relied upon to assist in diagnosis. For example, when phenytoin sodium is used to treat epilepsy, it can also cause twitching when it is in excessive poisoning. Digitalis can be used to treat heart failure and certain arrhythmias. However, its poisoning can also manifest as aggravated heart failure and various arrhythmias. If the dosage is increased merely based on the symptoms, it will lead to serious consequences. On the other hand, for any drug overdose poisoning, TDM will help monitor the rescue effect and evaluate the prognosis.
