CRITICAL VALUES FOR THERAPEUTIC DRUG LEVELS The concept of critical values for drug levels was originally developed by the late Daniel M. Baer, MD, and first published in the April 1982 issue of MLO. This table was reviewed by Rajasri Chandra, MS, MBA and lists the critical values for common therapeutic drugs updated in 2025 by reputed laboratories in the United States. Ranges are approximate and may vary by laboratory and/or assay. Proper interpretation of therapeutic drug concentrations requires that the specimen be drawn at an appropriate time in relation to drug administration. Drug
Indication
Acetaminophen1
Analgesic
Amikacin1
Antimicrobial
Amiodarone1
Therapeutic Range
Critical values for therapeutic drugs are drug levels in a patient’s body that are dangerously high or low, potentially leading to serious or life-threatening effects. These values fall outside the therapeutic ranges and often require immediate clinical attention. The purpose of monitoring is as follows: • Efficacy (drug is at a high enough level to be effective) • Safety (not so high that it becomes toxic)
Critical Value
Method
10.0-30.0 µg/mL
>150.0 µg/mL 4 hrs. after ingestion
Peak: 20.0-35.0 µg/mL
Peak: >40.0 µg/mL
Trough: <8.0 µg/mL
Trough: >10. µg/mL
Antiarrhythmic
0.5-2.0 µg/mL
>2.5 µg/mL
Quantitative Liquid Chromatography-Tandem Mass Spectrometry
Amitriptyline1
Antidepressant/ analgesic (neuropathic pain)
80.0-200.0ng/mL
>500.0 ng/mL
Quantitative Liquid Chromatography-Tandem Mass Spectrometry
Carbamazepine1
Antiepileptic/ mood stabilizer
Free: 1.0-3.0 µg/mL
Free: >4.0 µg/mL
Free: Ultrafiltration Followed by Homogeneous Microparticle Agglutination Immunoassay
Total: 4.0-12.0 µg/mL
Total: >15.0 µg/mL
Total: Homogeneous Microparticle Agglutination Immunoassay
Cyclosporine A1
Immunosuppressant
2-hour post dose: Kidney transplant: 800-1700 ng/mL Liver transplant: 600-1000 ng/mL
None established
Quantitative Liquid Chromatography-Tandem Mass Spectrometry
Digoxin1
Antiarrhythmic
0.6-1.2 ng/mL
>4.0 ng/mL
Electrochemiluminescent Immunoassay
Doxepin + Nordoxepin1
Antidepressant
50-150 ng/mL
>500.0 ng/mL
Liquid Chromatography Tandem Mass Spectrometry (LC-MS/MS)
Ethosuximide1
Antiepileptic
40.0-100.0 µg/mL
>150.0 µg/mL
Enzyme-Multiplied Immunoassay Technique (EMIT)
Everolimus2
Immunosuppressant
3.0-8.0 ng/mL
>30.0 ng/mL
Liquid Chromatography Tandem Mass Spectrometry (LC-MS/MS)
Flecainide1
Antiarrhythmic
0.2-1.0 µg/mL
>1.0 µg/mL
Liquid Chromatography Tandem Mass Spectrometry (LC-MS/MS)
Fluconazole3
Antifungal
5.0-20.0 µg/mL
None established
Quantitative High Performance Liquid ChromatographyTandem Mass Spectrometry
Flucytosine1
Antifungal
Peak: >25 µg/mL
>100.0 µg/mL
Liquid Chromatography Tandem Mass Spectrometry (LC-MS/MS)
Gentamicin1
Antimicrobial
Peak: 3.0-12.0 µg/mL
Peak: >12.0 µg/mL
Trough: <2.0 µg/mL
Trough: <2.0 µg/mL
Itraconazole1
Antifungal
Trough: >0.5 µg/mL (localized) >1.0 µg/mL (systemic)
Not established
Quantitative High Performance Liquid ChromatographyTandem Mass Spectrometry
Lamotrigine1
Antiepileptic/mood stabilizer
3-15.0 µg/mL
>20.0 μg/mL
Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS)
Lidocaine1
Antiarrhythmic
1.0-5.0 µg/mL
>6.0 µg/mL
Enzyme-Multiplied Immunoassay Technique (EMIT)
Lithium1
Mood stabilizer
0.5-1.2 mmol/L
>1.6 mmol/L
Colorimetry
Nortriptyline1
Antidepressant/ analgesic (neuropathic pain)
70-170 ng/mL
>500.0 ng/mL
Liquid Chromatography Tandem Mass Spectrometry (LC-MS/MS)
Phenobarbital1
Antiepileptic
60-40 µg/mL in adults 15-30 µg/mL in children
>60.0 µg/mL
Kinetic Interaction of Microparticles in a Solution (KIMS)
Total: 10.0-20.0 µg/mL
Total: >30.0 µg/mL
Free: 1.0-2.0 µg/mL
Free: >2.5 µg/mL
>0.7 µg/mL (Trough)
Not established
Phenytoin1 Posaconazole1
Antiepileptic
Antifungal
Spectrophotometry Kinetic Interaction of Microparticles in Solution (KIMS)
Turbidimetric Immunoassay
Total: Kinetic interaction of Microparticles in a Solution (KIMS) Free: Membrane Separation/KIMS Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS)
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