MLT (ASCP) (Medical Laboratory Technician) Test Prep / Course / Chapter

Therapeutic Drug Monitoring: Definition & Techniques

Rachel Torrens

Rachel obtained a Bachelor of Science in Biology from Grove City College. She then earned her Bachelor's and Master's Degree in Nursing from Thomas Jefferson University. For over 8 years, Rachel has practiced as a Board Certified Family Nurse Practitioner, and taught science to elementary aged students.

Expert Contributor
Christianlly Cena

Christianlly has taught college Physics, Natural science, Earth science, and facilitated laboratory courses. He has a master's degree in Physics and is currently pursuing his doctorate degree.

Certain medications require more invasive techniques to adequately monitor a patient's response. In this lesson, learn which drugs require therapeutic drug monitoring (TDM), and the techniques necessary to perform TDM properly.

The Necessity of Monitoring

Have you ever cooked a hot dog? You throw the hot dog in the pan, warm it through, and pull it out when it looks done. Now, is that process the same for roasting a chicken? Nope, with a roast chicken you put it in the oven for the prescribed time, but before pulling it out, you check its internal temperature to ensure proper cooking. In both instances you are preparing meat, but the chicken requires more invasive monitoring than the hot dog.

The same variation in monitoring is true for medications. Some drugs require no monitoring because researchers have determined a standardized dose that is both safe and effective. Furthermore, these drugs are metabolized and absorbed in a consistent manner by different patients; therefore, the healthcare provider can be confident that the drug, once ingested, will behave a specific way.

For example, Mary has a sore throat. You, the healthcare provider, determine it's strep throat. You prescribe penicillin, using the appropriate dosing schedule for Mary's age. There is no further monitoring necessary. Mary's cured - end of story.

Certain medications, like lithium, require close monitoring to prevent toxicity.
medication image

On the other hand, some drugs require monitoring because they behave differently. Specifically, some drugs have a narrow therapeutic index, meaning they're only safe and effective within a small range. The level of medication that will help the patient is very close to the level that will harm the patient and to the level that will be useless. Indeed, many drugs that require the kind of monitoring we're discussing in this lesson have a high risk of toxicity because of the narrow therapeutic index.

Also, some drugs have significant absorption and metabolism variability, meaning that the drug is processed very differently in different people. Sometimes this is a factor of the patient's age, gender or general state of health.

What Is Therapeutic Drug Monitoring?

Therapeutic drug monitoring (TDM) involves checking a patient's body fluids, most often blood plasma, at a certain time to determine the level of a specific drug. TDM is used in addition to physical examination, patient history, and other factors when determining if the proper dosage of the medication is being given.

In addition to certain types of medications, certain situations require TDM, such as:

  • The patient doesn't seem to have a therapeutic response at the expected dose.
  • The healthcare provider is unsure if the patient is taking the medication as prescribed.
  • The patient is experiencing more side effects from the medication.
  • The healthcare provider suspects interaction of the drug with other drugs.
  • The patient needs to stop taking a medication, requiring a steady weaning off process.

Let's examine the procedure used to successfully perform TDM.

Procedure for Therapeutic Drug Monitoring

TDM is an incredibly useful tool, but like any tool, it must be used properly or it will be ineffective. The biggest issue with TDM is timing. The blood plasma sample must be collected at the proper time and in the proper manner in order for the lab value to hold any significance.

The technician plays a crucial role in the proper collection of a specimen for TDM.

Proper Time

A healthcare provider may use TDM to know when a medication has reached a steady state, which is when the amount of drug being ingested equals the amount being excreted. TDM allows the healthcare provider to track the patient's response to initial doses of drugs that are difficult to manage due to variability and narrow therapeutic indexes.

Determining the steady state of a medication depends on the drug's half-life, or the amount of time it takes for half of the drug ingested to be eliminated from the bloodstream. Usually it takes about five half-lives for a medication to reach steady state.

However, if TDM is being used to monitor response to a medication once a steady state has been reached, then the sample needs to be obtained at trough. Trough means just prior to the next dose of medication. For example if a medication is given every 12 hours and the patient's last dose was at 8pm, then the TDM sample needs to be collected at 7:55 am, just before the next dose of medication.

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Additional Activities

Therapeutic Drug Monitoring Word Scramble Activity

In this activity, you will check your knowledge regarding the definition and techniques of therapeutic drug monitoring presented in the lesson.


For this activity, study the scrambled letters and try to unscramble or rearrange the letters to form a word or phrase that fits the given clues. To do this, you must right-click and print this page. With a pencil and an eraser, neatly write your answers in the blank space provided.

Scrambled Words



  1. _____ is any of a group of narrow-spectrum antibiotics that are synthesized from molds and can be used to alleviate a sore throat.
  2. Medications that have a narrow therapeutic index need to be monitored since these are only safe and effective within a _____ _____.
  3. Patients experiencing more _____ _____ from the medication require TDM.
  4. _____ is a challenge in TDM for blood plasma samples must be collected at the proper time and in the appropriate manner to hold any significance.
  5. The _____ is measured just before the next dose of medication is administered.
  6. Some drugs require that the blood sample remain _____; others require the sample to be kept at a certain temperature.
  7. Normally, it takes about five half-lives for a drug to reach a _____ _____.
  8. Monitoring is needed when a patient doesn't seem to have a _____ _____ at the standard dose.
  9. There is a considerable probability of _____ when the dosage of medications having a narrow therapeutic index is wrongfully administered.
  10. Drugs that have already been given a _____ _____ can be taken without the need for monitoring.

Answer Key


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