Te 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. Tis table is an expanded version of that publication and newly revised for 2021-2022 by Steven W. Cotten PhD, DABCC, FAACC, Assistant Professor in Pathology and Laboratory Medicine, University of North Carolina at Chapel Hill.

Drug Acetaminophen Amikacin

Amiodarone Amitriptyline

Busulfan (IV)

Carbamazepine Cyclosporine

Indication Analgesic

Antimicrobial Antiarrhythmic

Antidepressant/ analgesic (neuropathic pain)

Anti-leukemic, Hematopoietic cell transplantation conditioning

Antiepileptic/ mood stabilizer

Therapeutic Range Critical Value Comments 5-20 µg/mL

>200 µg/mL *drawn 4 hours after ingestion

Peak: 15-30 µg/mL Trough: 4-8 µg/mL

0.5-2 µg/mL 125-250 ng/mL 900-1350  MOL/MIN 4-12 µg/mL Immunosuppressant 100-400 ng/mL

>10 µg/mL >2.5 µg/mL >500 ng/mL

*Determination if a concentration is toxic is dependent upon when it is drawn in relation to the time of ingestion of the dose. Multiple serum concentrations will be needed to monitor improvement and removal of drug.

Peak: 30 minutes after end of infusion. Trough: before next dose. Conventional dosing protocol.

Trough concentration. Serum amiodarone levels >2.5 µg/mL had a positive predictive value of 76% for adverse drug events.

Trough concentration. Life threatening cardiac toxicity and/or seizures with concentration >1000 ng/mL.

>1500 µmol/min Area Under the Curve (AUC) calculations based on post-infusion sampling and dosing protocols vary by institution.

>20 µg/mL >500 ng/mL

Trough concentrations. Correlate serum concentration with clinical presentation.

Specific concentration goal dependent upon clinical situation. For concentrations drawn with intravenous therapy, blood should be drawn from site other than that where drug is infusing. (Cyclosporine adheres to plastic.) TDM levels are dependent on transplant type. Blood concentrations can be method (immunoassay or mass spectrometry) dependent.

Digoxin Doxepin

Ethosuximide Everolimus Flecainide

Fluconazole Flucytosine Gentamicin

Inotrope, AV node blocker

Antidepressant Antiepileptic

0.5-2.0 ng/mL* 110-250 ng/mL

40-100 µg/mL

Immunosuppressant 3-8 ng/mL Antiarrhythmic

Antifungal Antifungal


Hydroxyl itraconazole Antifungal Imipramine Itraconazole

Lamotrigine Lidocaine

Lithium Nortriptyline

Phenobarbital Phenytoin

Posaconazole Primidone

Procainamide (PA) (metabolite: NAPA)

Protriptyline Quinidine Salicylate

Antidepressant Antifungal

Antiepileptic/mood stabilizer


Antidepressant/ analgesic (neuropathic pain)

Antiepileptic Antiepileptic

Antifungal Antiepileptic Antiarrhythmic 0.2-1.0 µg/mL

4.0-20.0 µg/mL 25-50 µg/mL

Peak: 5-10 µg/mL Trough: <2 µg/mL

Not established >180-240 ng/mL

>0.5 ug/mL (localized) >1.0 ug/mL (systemic)

1-15 µg/mL 1.5-5 µg/mL

Mood stabilizer Acute: 1-1.6 mmol/L Chronic: 0.6-1.2 mmol/L

50-150 ng/mL

15-40 µg/mL 10-20 µg/mL

>0.7 µg/mL 5-12 µg/mL

PA: 4-8 µg/mL NAPA: 10-20 µg/mL

Antidepressant Antiarrhythmic

Analgesic, antipyresis Anti-inflammatory

Sirolimus Tacrolimus

Theophylline Tobramycin Valproic acid Vancomycin


50-170 ng/mL 2-5 µg/mL

20-100 µg/mL 100-200 µg/mL

>2.5 ng/mL >500 ng/mL

>200 µg/mL >15 ng/mL >1.0 µg/mL

Samples should be drawn >8 hours after last dose. *Concentrations >1.5 ng/mL may be associated with higher mortality.

Trough concentration. Trough concentration.

Trough concentration. Varies by transplant protocol.

Midpoint or trough concentration. Monitoring recommended when given concurrently with medications that may decrease metabolism (increase concentrations).

None established Limited TDM utility except in patients receiving hemodialysis. >100-200 µg/mL Concentration should be a peak drawn 2 hours post dose.

Peak: >12 µg/mL Trough: >2 µg/mL

Peak: 1 hour after infusion. Trough: before next dose. Conventional dosing protocol.

None established Active metabolite of itraconazole. >500 ng/mL

Concentration = imipramine + desipramine (metabolite).

None established Large PK variability. Should be measured within 5-7 after initiation of therapy. >20 µg/mL >6 µg/mL

Concentration can be drawn at any point (from separate IV line).

>2.0 mmol/L >5 mmol/L potentially fatal

>500 ng/mL

>60 µg/mL >20 µg/mL

Serum concentrations may increase in presence of hyponatremia. Concentration: 12 hours after dose.

Trough concentration. Trough concentration. Do not collect before steady state achieved.

Trough concentrations. Toxic >20 µg/mL (lateral nystagmus), >40 µg/mL (decreased mentation). Toxicity may occur at lower concentrations in presence of hypoalbuminemia. Consider free phenytoin.

None established Should be measured within 7 days of initiation therapy. >15 µg/mL

Metabolized to phenobarbital.

>10 µg/mL >40 µg/mL

>500 ng/mL >6 µg/mL

Vertigo, tinnitus 150-300 µg/mL Nausea, vomiting, hyper-ventilation 250-400 µg/mL Toxicity >500 µg/mL

Immunosuppressant 4-20 ng/mL Immunosuppressant 5-20 ng/mL

Bronchodilator Antibacterial

Antiepileptic/mood stabilizer

Antimicrobial Antifungal 10-20 µg/mL

Peak: 4-8 µg/mL Trough: <1.0 µg/mL

50-125 µg/mL

Trough concentrations: General: 5-15 µg/mL Pneumonia: 15-20 µg/mL

1.0-5.5 µg/mL >25 µg/mL >25 ng/mL >25 µg/mL

>12 µg/mL >2 µg/mL

>200 µg/mL

Mid-point or trough concentration. Procainamide monitoring is particularly important in patients who might be fast acetylators (60% to 70% of northern Europeans, and 50% of black and white Americans) and in patients with renal impairment. Procainamide and N-acetylprocainamide levels should always be measured on the same sample.

Trough concentration. Midpoint or trough concentration.

Serum concentration should be used in conjunction with clinical presentation to make decision on therapy. Multiple serum concentrations will be necessary to monitor improvement and removal of drug.

Trough concentration. Whole blood samples. Therapeutic levels can be lower when used in combination with other immunosuppresants. Blood concentrations can be method (immunoassay or mass spectrometry) dependent. Therapeutic levels depend on type of transplant, time post transplant, and other concomitant drug therapy.

Whole blood samples collected as trough. Therapeutic levels can be lower when used in combination with other immunosuppressants. Bias may be present between immunoassay and LC/MS methods.

Pulmonary literature suggest that concentrations 5-15 mg/L may be as efficacious with less toxicity. Trough concentration dependent upon drug formulation.

Peak: 1 hour after end of infusion. Trough: before next dose. Conventional dosing protocol.

Toxicity may occur at lower concentrations in presence of hypoalbuminemia. Consider free valproic acid. Trough concentration preferred.

Trough: >30 µg/mL Monitoring of peaks no longer recommended. Goal trough concentration dependent upon indication. Trough: before next dose.

>6 µg/mL 14 CLR 2021-2022 • MLO • Should be measured within 7 days of initiation therapy.

Ranges are approximate and may vary with 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.

Trough concentration. High concentrations generally associated with increased somnolence/confusion.

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