A blood sample is needed. Most of the time, blood is drawn from a vein located on the inside of the elbow or the back of the hand.

You will need to prepare for some drug level tests.

• Your health care provider will tell you if you need to change the times you take any of your medicines.
• DO NOT stop or change your medicines without talking to your provider first.

You may feel slight pain or a sting when the needle is inserted. You may also feel some throbbing at the site after the blood is drawn.

With most medicines, you need a certain level of it in your blood to get the proper effect. Some medicines are harmful if the level rises too high and do not work if the levels are too low.

Monitoring the amount of the medicine found in your blood allows your provider to make sure the drug levels are in the proper range.

Therapeutic drug level testing is important in people taking medicines such as:

• Flecainide, procainamide or digoxin, which are used to treat abnormal beating of the heart
• Lithium, used to treat bipolar disorder
• Phenytoin or valproic acid, which are used to treat seizures or other conditions
• Gentamicin or amikacin, which are antibiotics used to treat infections
• Tacrolimus, sirolimus or cyclosporine, which are used to suppress immune system activity against transplanted organs
  

Testing may also be done to determine how well your body breaks down the drug or how it interacts with other drugs you need.

Following are some of the medicines that are commonly checked and the normal target levels:

• Acetaminophen: varies with use
• Amikacin: peak: 20 to 25 mcg/mL (34 to 43 micromol/L); trough: 5 to 10 mcg/mL (9 to 17 micromol/L)
• Amitriptyline: 120 to 150 ng/mL (432.60 to 540.75 nmol/L)
• Carbamazepine: 4 to 12 mcg/mL (17 to 51 micromol/L)
• Cyclosporine: 25 to 525 ng/mL (21 to 436 nmol/L) (12 hours after dose)
• Desipramine: 100 to 300 ng/mL (563.10 to 1126.20 nmol/L)
• Digoxin: 0.8 to 2.0 ng/mL (1 to 2.6 nanomol/L)
• Disopyramide: 2 to 5 mcg/mL (5.9 to 14.8 micromol/L)
• Ethosuximide: 40 to 100 mcg/mL (283 to 708 micromol/L)
• Flecainide: 0.2 to 1.0 mcg/mL (0.5 to 2.0 micromol/L)
• Gentamicin: peak: 5 to 10 mcg/mL (10 to 21 micromol/L); trough: 0.5 to 2 mcg/mL (1 to 4 micromol/L)
• Imipramine: 150 to 300 ng/mL (534.90 to 1069.80 nmol/L)
• Lidocaine: 1.2 to 5.0 mcg/mL (5 to 21 micromol/L)
• Lithium: 0.5 to 1.2 mEq/L (0.5 to 1.2 mmol/L) (12 hours after dose)
• Methotrexate: varies with use
• Nortriptyline: 50 to 150 ng/mL (189.85 to 569.55 nmol/L)
• Phenobarbital: 15 to 40 mcg/mL (65 to 173 micromol/L)
• Phenytoin (total): 10 to 20 mcg/mL (40 to 79 micromol/L)
• Primidone: 5 to 12 mcg/mL (23 to 55 micromol/L)
• Procainamide: 4 to 10 mcg/mL (17 to 42 micromol/L)
• Quinidine: 2 to 5 mcg/mL (6 to 14 micromol/L)
• Salicylate: varies with use
• Sirolimus: 4 to 14 ng/mL (4 to 15 nmol/L) (12 hours after dose; varies with use)
• Tacrolimus: 5 to 25 ng/mL (6 to 30 nmol/L) (12 hours after dose)
• Theophylline: 8 to 20 mcg/mL (44 to 111 micromol/L)
• Tobramycin: peak: 5 to 10 mcg/mL (11 to 21 micromol/L); trough: 0 to 2 mcg/mL (0 to 4 micromol/L)
• Valproic acid: 50 to 100 mcg/mL (347 to 693 micromol/L)

Normal value ranges may vary slightly among different laboratories. Talk to your provider about the meaning of your specific test results.

The examples above show the common measurements for results for these tests. Some laboratories use different measurements or may test different specimens.

Values outside the target range may be due to minor changes or be a sign that you need to adjust your dosages. Your provider may tell you to skip a dose if the values measured are too high.

Following are toxic levels for some of the drugs that are commonly checked:

• Acetaminophen: greater than 2150 mcg/mL (9931653.50 micromol/L)
• Amikacin: peak: greater than 325 mcg/mL (6042.70 micromol/L)
• Amitriptyline: greater than 500 ng/mL (1802.50 nmol/L)
• Carbamazepine: greater than 125 mcg/mL (6450.80 micromol/L)
• Cyclosporine: greater than 7400 ng/mL (581332.80 micromol/L)
• Desipramine: greater than 500 ng/mL (1877.00 nmol/L)
• Digoxin: greater than 2.4 ng/mL (3.107 nmol/L)
• Disopyramide: greater than 5 mcg/mL (14.873 micromol/L)
• Ethosuximide: greater than 1500 mcg/mL (1062708.40 micromol/L)
• Flecainide: greater than 1.0 mcg/mL (2.04 micromol/L)
• Gentamicin: peak: greater than 12 mcg/mL (25.08 micromol/L)
• Imipramine: greater than 500 ng/mL (1783.00 nmol/L)
• Kanamycin: greater than 35 mcg/mL (72.80 micromol/L)
• Lidocaine: greater than 95 mcg/mL (3821.34 micromol/L)
• Lithium: greater than 1.62.0 mEq/L (1.62.00 millimol/L)
• Methotrexate: greater than 10 mcmol/L (10,000 nmol/L) over 24 hours after dose
• Nortriptyline: greater than 500 ng/mL (1898.50 nmol/L)
• Phenobarbital: greater than 450 mcg/mL (216172.40 micromol/L)
• Phenytoin (total): greater than 30 mcg/mL (119.04 micromol/L)
• Primidone: greater than 15 mcg/mL (698.73 micromol/L)
• Procainamide: greater than 12.16 mcg/mL (5168.00 micromol/L)
• Quinidine: greater than 10 6 mcg/mL (1930.82 micromol/L)
• Salicylate: greater than 300 mcg/mL (21752.00 micromol/L)
• Theophylline: greater than 20 mcg/mL (111.00 micromol/L)
• Tobramycin: peak: greater than 102 mcg/mL (215.67 micromol/L)
• Valproic acid: greater than 1050 mcg/mL (1040693.40 micromol/L)

Therapeutic drug monitoring

Bluth MH, Pincus MR, Abraham NZ. Toxicology and therapeutic drug monitoring. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 24.

Clarke W. Overview of therapeutic drug monitoring. In: Clarke W, Dasgupta A, eds. Clinical Challenges in Therapeutic Drug Monitoring. Cambridge, MA: Elsevier; 2016:chap 1.

Diasio RB. Principles of drug therapy. In: Goldman L, Cooney KA, eds. Goldman-Cecil Medicine. 27th ed. Philadelphia, PA: Elsevier; 2024:chap 25.

Nelson LS, Calello DP. Acute poisoning. In: Goldman L, Cooney KA, eds. Goldman-Cecil Medicine. 27th ed. Philadelphia, PA: Elsevier; 2024:chap 96.

Sepulveda, J. Reference intervals and laboratory values: In: Goldman L, Cooney KA, eds. Goldman-Cecil Medicine. 27th ed. Philadelphia, PA: Elsevier; 2024: Appendix, e1-e11.

Updated by: Jacob Berman, MD, MPH, Clinical Assistant Professor of Medicine, Division of General Internal Medicine, University of Washington School of Medicine, Seattle, WA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.