Therapeutic Drug Monitoring

What is Therapeutic Drug Monitoring?

Therapeutic Drug Monitoring (TDM) is a practice of medication dosing that uses laboratory testing to adjust the dosage of a medication after an initial dose has been administered. It is used to ensure that the concentration of a drug in the patient's bloodstream remains within a specified therapeutic range. TDM also plays an important role in evaluating the safety of certain medications and helps to determine an appropriate dosage regimen.

When is Therapeutic Drug Monitoring Used?

In general, TDM is used in the following situations:

• When the drug to be monitored is highly toxic or has a narrow therapeutic index.
• When the drug to be monitored has a short half-life.
• When there is a need to ensure that the drug concentration is in the therapeutic window.
• When the patient has an altered metabolic rate or altered kidney function.
• When monitoring drug/drug interactions.

Preparation for Therapeutic Drug Monitoring

There are few things that need to be done prior to performing TDM:

• Assess the patient and obtain a detailed medication history.
• Review laboratory results to assess the patient’s renal and hepatic function.
• Measure total body weight.
• Determine the indication for drug treatment.
• Identify any allergies or sensitivities the patient may have.
• Determine the patient’s age as it may affect the drug’s metabolism.
• Identify any drug/drug interactions that may affect the patient’s response.

Procedure for Therapeutic Drug Monitoring

The following steps are typically involved in a Therapeutic Drug Monitoring procedure:

1. Obtain the patient’s clinical history.
2. Establish the target concentration range for the drug (e.g., 10-15 mcg/ml for a drug with a narrow therapeutic index).
3. Collect the appropriate specimen, typically a blood sample.
4. Analyze the specimen using a validated method such as High Pressure Liquid Chromatography (HPLC), Liquid Chromatography with Mass Spectrometry (LC/MS) or Gas Chromatography (GC).
5. Interpret the test results.
6. Adjust the dosage as necessary.

Types of Therapeutic Drug Monitoring

TDM can be used to monitor any number of drugs. Some of the more common drugs monitored using TDM are:

• Antiepileptics, such as phenytoin and carbamazepine.
• Antidepressants, such as amitriptyline and citalopram.
• Immunosuppressants, such as cyclosporine and azathioprine.
• Anti-arrhythmics, such as amiodarone and sotalol.
• Antibiotics, such as vancomycin and gentamicin.

Risks of Therapeutic Drug Monitoring

There are few side effects associated with TDM, but some include:

• Incorrect dosage if the patient's drug levels are not monitored accurately.
• Unnecessary expense if a drug or dosage is not monitored closely.
• Delayed treatment if the drug levels are not monitored promptly.

Why is Therapeutic Drug Monitoring Important?

TDM is essential to ensure that the correct dose of a drug is maintained in the patient’s bloodstream, and to ensure that the drug doesn’t reach a toxic level. It is also important to ensure that drug/drug interactions don’t occur, or that pre-existing metabolic conditions don’t affect the therapeutic effect of the drug.