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Therapeutic drug levels

Definition

Therapeutic drug levels are lab tests to look for the amount of a drug or medicine in the blood.

Alternative Names

Therapeutic drug monitoring

How the Test is Performed

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.

How to Prepare for the Test

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.

How the Test will Feel

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.

Why the Test is Performed

With most medicines, you need a certain level of the drug 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 drug found in your blood allows your provider to make sure the drug levels are in the proper range.

Drug level testing is important in people taking drugs 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.

Normal Results

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

• Acetaminophen: varies with use
• Amikacin: 15 to 25 mcg/mL (25.62 to 42.70 micromol/L)
• Amitriptyline: 120 to 150 ng/mL (432.60 to 540.75 nmol/L)
• Carbamazepine: 5 to 12 mcg/mL (21.16 to 50.80 micromol/L)
• Cyclosporine: 100 to 400 ng/mL (83.20 to 332.80 nmol/L) (12 hours after dose)
• Desipramine: 150 to 300 ng/mL (563.10 to 1126.20 nmol/L)
• Digoxin: 0.8 to 2.0 ng/mL (1.02 to 2.56 nanomol/L)
• Disopyramide: 2 to 5 mcg/mL (5.89 to 14.73 micromol/L)
• Ethosuximide: 40 to 100 mcg/mL (283.36 to 708.40 micromol/L)
• Flecainide: 0.2 to 1.0 mcg/mL (0.5 to 2.4 micromol/L)
• Gentamicin: 5 to 10 mcg/mL (10.45 to 20.90 micromol/L)
• Imipramine: 150 to 300 ng/mL (534.90 to 1069.80 nmol/L)
• Kanamycin: 20 to 25 mcg/mL (41.60 to 52.00 micromol/L)
• Lidocaine: 1.5 to 5.0 mcg/mL (6.40 to 21.34 micromol/L)
• Lithium: 0.8 to 1.2 mEq/L (0.8 to 1.2 mmol/L)
• Methotrexate: varies with use
• Nortriptyline: 50 to 150 ng/mL (189.85 to 569.55 nmol/L)
• Phenobarbital: 10 to 30 mcg/mL (43.10 to 129.30 micromol/L)
• Phenytoin: 10 to 20 mcg/mL (39.68 to 79.36 micromol/L)
• Primidone: 5 to 12 mcg/mL (22.91 to 54.98 micromol/L)
• Procainamide: 4 to 10 mcg/mL (17.00 to 42.50 micromol/L)
• Quinidine: 2 to 5 mcg/mL (6.16 to 15.41 micromol/L)
• Salicylate: varies with use
• Sirolimus: 4 to 20 ng/mL (4 to 22 nmol/L) (12 hours after dose; varies with use)
• Tacrolimus: 5 to 15 ng/mL (4 to 25 nmol/L) (12 hours after dose)
• Theophylline: 10 to 20 mcg/mL (55.50 to 111.00 micromol/L)
• Tobramycin: 5 to 10 mcg/mL (10.69 to 21.39 micromol/L)
• Valproic acid: 50 to 100 mcg/mL (346.70 to 693.40 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.

What Abnormal Results Mean

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 250 mcg/mL (1653.50 micromol/L)
• Amikacin: greater than 25 mcg/mL (42.70 micromol/L)
• Amitriptyline: greater than 500 ng/mL (1802.50 nmol/L)
• Carbamazepine: greater than 12 mcg/mL (50.80 micromol/L)
• Cyclosporine: greater than 400 ng/mL (332.80 micromol/L)
• Desipramine: greater than 500 ng/mL (1877.00 nmol/L)
• Digoxin: greater than 2.4 ng/mL (3.07 nmol/L)
• Disopyramide: greater than 5 mcg/mL (14.73 micromol/L)
• Ethosuximide: greater than 100 mcg/mL (708.40 micromol/L)
• Flecainide: greater than 1.0 mcg/mL (2.4 micromol/L)
• Gentamicin: 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 5 mcg/mL (21.34 micromol/L)
• Lithium: greater than 2.0 mEq/L (2.00 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 40 mcg/mL (172.40 micromol/L)
• Phenytoin: greater than 30 mcg/mL (119.04 micromol/L)
• Primidone: greater than 15 mcg/mL (68.73 micromol/L)
• Procainamide: greater than 16 mcg/mL (68.00 micromol/L)
• Quinidine: greater than 10 mcg/mL (30.82 micromol/L)
• Salicylate: greater than 300 mcg/mL (2172.00 micromol/L)
• Theophylline: greater than 20 mcg/mL (111.00 micromol/L)
• Tobramycin: greater than 12 mcg/mL (25.67 micromol/L)
• Valproic acid: greater than 100 mcg/mL (693.40 micromol/L)

References

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, Schafer AI, eds. Goldman-Cecil Medicine. 26th ed. Philadelphia, PA: Elsevier; 2020:chap 26.

Nelson LS. Acute poisoning. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 26th ed. Philadelphia, PA: Elsevier; 2020:chap 102.

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