Drug Therapy Monitoring

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Drug Therapy Monitoring


Drug therapy monitoring, also known as Therapeutic Drug Monitoring (TDM), is a means of monitoring drug levels in the blood.


TDM is employed to measure blood drug levels so that the most effective dosage can be determined, with toxicity prevented. TDM is also utilized to identify noncompliant patients (those patients who, for whatever reason, either cannot or will not comply with drug dosages as prescribed by the physician).


Because so many different factors influence blood drug levels, the following points should be taken into consideration during TDM: the age and weight of the patient; the route of administration of the drug; the drug's absorption rate, excretion rate, delivery rate, and dosage; other medications the patient is taking; other diseases the patient has; the patient's compliance regarding the drug treatment regimen; and the laboratory methods used to test for the drug.


TDM is a practical tool that can help the physician provide effective and safe drug therapy in patients who need medication. Monitoring can be used to confirm a blood drug concentration level that is above or below the therapeutic range, or if the desired therapeutic effect of the drug is not as expected. If this is the case, and dosages beyond normal then have to be prescribed, TDM can minimize the time that elapses.

TDM is important for patients who have other diseases that can affect drug levels, or who take other medicines that may affect drug levels by interacting with the drug being tested. As an example, without drug monitoring, the physician cannot be sure if a patient's lack of response to an antibiotic reflects bacterial resistance, or is the result of failure to reach the proper therapeutic range of antibiotic concentration in the blood. In cases of life-threatening infections, timing of effective antibiotic therapy is critical to success. It is equally crucial to avoid toxicity in a seriously ill patient. Therefore, if toxic symptoms appear with standard dosages, TDM can be used to determine changes in dosing.

Drawn blood, used for TDM, demonstrates a drug action in the body at any specific time, whereas drug levels examined from urine samples reflect the presence of a drug over many days (depending on the rate of excretion). Therefore, blood testing is the procedure of choice when definite data are required. However, for adequate absorption and therapeutic levels to be accurate, it is important to allow for sufficient time to pass between the administration of the medication and the collection of the blood sample.

Therapeutic Drug Monitoring: Therapeutic And Toxic Range
Drug Level UseTherapeutic
Values are laboratory-specificConcentration obtained 30 minutes after the end of a 30-minute infusion.
Depends on
Amikacin  mg/
Antibiotic12-25 mg/
Aminophylline ng/
Bronchodilator10-20 mg/ml>20
Amitriptyline ng/
Antidepressant120-150 ng/ml>500
Anticonvulsant5-12 mg/ml>12
Antibiotic10-20 mg/ml>25
Digoxin ng/mlCardiotonic0.8-2.0 ng/ml>2.4
GentamicinAntibiotic4-12 mg/L>12
LidocaineAntiarrhythmic1.5-5.0 mg/ml>5 mg/
Lithium mEq/LAntimanic0.7-2.0 mEq/L>2.0
Nortriptyline ng/
Antidepressant50-150 ng/ml>500
Phenobarbital mg/
Anticonvulsant10-30 mg/ml>40
Phenytoin mg/mlAnticonvulsant7-20 mg/ml>30
Procainamide mg/
Antiarrhythmic4-8 mg/ml>16
Propranolol ng/mlAntiarrhythmic50-100 ng/ml>150
Quinidine mg/mlAntiarrhythmic1-4 mg/ml>10
Theophylline mg/
Bronchodilator10-20 mg/ml>20
Tobramycin mg/
Antibiotic4-12 mg/ml>12
Valproic acid mg/
Anticonvulsant50-100 mg/ml>100

Blood specimens for drug monitoring can be taken at two different times: during the drug's highest therapeutic concentration ("peak" level), or its lowest ("trough" level). Occasionally called residual levels, trough levels show sufficient therapeutic levels; whereas peak levels show poisoning (toxicity). Peak and trough levels should fall within the therapeutic range.


In preparing for this test, the following guidelines should be observed:

  • Depending on the drug to be tested, the physician should decide if the patient is to be fasting (nothing to eat or drink for a specified period of hours) before the test.
  • For patients suspected of symptoms of drug toxicity, the best time to draw the blood specimen is when the symptoms are occurring.
  • If there is a question as to whether an adequate dose of the drug is being achieved, it is best to obtain trough (lowest therapeutic concentration) levels.
  • Peak (highest concentration) levels are usually obtained one to two hours after oral intake, approximately one hour after intramuscular (IM) administration (a shot in the muscle), and approximately 30 minutes after intravenous (IV) administration. Residual, or trough, levels are usually obtained within 15 minutes of the next scheduled dose.


Risks for this test are minimal, but may include slight bleeding from the blood-drawing site, fainting or feeling lightheaded after blood is drawn, or accumulation of blood under the puncture site (hematoma).



Pagana, Kathleen Deska. Mosby's Manual of Diagnostic and Laboratory Tests. St. Louis: Mosby, Inc., 1998.