Therapeutic drug monitoring (TDM) is essential in neurology for optimizing efficacy and minimizing toxicity of medications with narrow therapeutic windows, significant pharmacokinetic variability, or potentially serious adverse effects. The most common neurologic indications include antiepileptic drug levels (phenytoin, lithium, carbamazepine, valproate, lamotrigine), anticoagulant monitoring (warfarin INR, DOAC-specific assays in selected situations), immunosuppressant levels (tacrolimus, cyclosporine, mycophenolate), and a growing list of disease-modifying therapies and biologic agents. Successful interpretation requires understanding when to draw the level (trough vs peak), how to interpret in the context of clinical response, and recognition of drug-drug interactions that distort interpretation. This page covers the major TDM tests in neurology, with focus on the antiepileptic drugs and anticoagulants most frequently encountered.

🔹 Bottom Line: Therapeutic Drug Monitoring

  • Phenytoin: zero-order kinetics — small dose changes can produce big level changes. In hypoalbuminemia, renal failure, valproate co-therapy, or pregnancy: order free phenytoin, not just total.
  • Lithium: trough 12 h post-dose. Toxicity is driven by dehydration, hyponatremia, NSAIDs, ACE-Is, thiazides. Severe toxicity → hemodialysis. SILENT = irreversible cerebellar ataxia after toxicity.
  • Carbamazepine: auto-induces; SIADH/hyponatremia; HLA-B*1502 testing in Asians before initiation.
  • Valproate: hyperammonemic encephalopathy with normal LFTs — treat with L-carnitine. Teratogenic in pregnancy.
  • Lamotrigine in pregnancy: clearance increases 2–3× — levels drop, often requires dose escalation. Valproate doubles lamotrigine; carbamazepine/phenytoin halve it.
  • Warfarin INR 2–3 most indications; 2.5–3.5 for triple-positive APS and mechanical mitral valve. DOACs: routine TDM unnecessary; aPTT/PT unreliable.
  • Natalizumab: JCV antibody index stratifies PML risk. Eculizumab: meningococcal vaccination before initiation. Azathioprine: TPMT testing first.
  • Avoid in MG: aminoglycosides, fluoroquinolones, β-blockers, magnesium — can precipitate crisis.

Antiepileptic Drug (AED) Monitoring

When TDM Is Routinely Useful

  • Suspected toxicity: confusion, ataxia, nystagmus, encephalopathy.
  • Breakthrough seizures: confirm therapeutic levels.
  • Suspected non-adherence: subtherapeutic levels in absence of expected efficacy.
  • Drug interactions: dose adjustment after starting/stopping interacting medication.
  • Pregnancy: pharmacokinetic changes (lamotrigine clearance increases markedly, often requires dose escalation).
  • Renal or hepatic dysfunction.
  • Status epilepticus: confirm therapeutic levels of administered drug.

Phenytoin

Therapeutic Range

  • Total phenytoin: 10-20 mcg/mL.
  • Free phenytoin: 1-2 mcg/mL (~10% of total).

When to Check Free Phenytoin

  • Hypoalbuminemia: phenytoin is highly protein-bound; low albumin → falsely low total but normal free level. Total levels misleading.
  • Renal failure: altered protein binding, total may underestimate free fraction.
  • Drug interactions affecting binding: valproate displaces phenytoin from protein, raising free fraction.
  • Pregnancy: protein binding changes.

Pharmacokinetics

  • Zero-order (saturable) kinetics at therapeutic doses — small dose increases can cause disproportionate level increases.
  • Long half-life (~24 hours); steady state after 5-7 days.
  • Many drug interactions (CYP450 inducer and substrate).

Toxicity Signs

  • Levels 20-30: nystagmus, ataxia.
  • Levels >30: dysarthria, lethargy, confusion.
  • Levels >40: severe encephalopathy, seizures (paradoxical).

🔹 Clinical Relevance: Free Phenytoin When Protein Binding Is Disturbed

The classic teaching that “phenytoin levels >20 mcg/mL cause toxicity” assumes normal protein binding — which is often violated in hospitalized patients, ICU patients, the elderly, and patients on multiple drugs. Total phenytoin underestimates the active drug in hypoalbuminemia: a hypoalbuminemic patient with a “therapeutic” total of 12 mcg/mL may have a free level of 2.5 mcg/mL (toxic), while another with a total of 7 may have an appropriate free level of 1.4 (therapeutic).

  • Order free phenytoin in: hypoalbuminemia, renal failure, valproate co-therapy (displaces phenytoin from protein), pregnancy.
  • Interpret levels in clinical context — seizure control and side effects matter more than the number.
  • The same logic generalizes to highly protein-bound AEDs (valproate) and to pregnancy-driven clearance changes (lamotrigine, levetiracetam, oxcarbazepine).

Lithium

Therapeutic Range

  • Acute treatment: 0.8-1.2 mEq/L.
  • Maintenance: 0.6-1.0 mEq/L.
  • Trough level: 12 hours post-dose; usually morning before dose.

Neurologic Indications and Toxicity

  • Lithium is used in bipolar disorder; neurologists encounter toxicity (tremor, ataxia, confusion).
  • Mild toxicity (1.5-2.0): coarse tremor, lethargy, GI upset.
  • Moderate (2.0-2.5): confusion, ataxia, myoclonus, hyperreflexia.
  • Severe (>2.5): seizures, coma, cardiac arrhythmias.
  • SILENT (Syndrome of Irreversible Lithium-Effectuated Neurotoxicity): persistent cerebellar ataxia and other neurologic features following lithium toxicity.

Risk Factors for Toxicity

  • Dehydration, hyponatremia (lithium reabsorption mirrors sodium).
  • NSAID, ACE inhibitor, thiazide diuretic use.
  • Renal dysfunction.

Management of Toxicity

  • Mild: hold lithium, hydration.
  • Severe: hemodialysis (lithium is dialyzable).

Carbamazepine

  • Therapeutic range: 4-12 mcg/mL.
  • Auto-induces its own metabolism over first 4-6 weeks — levels often drop after initial dosing.
  • Toxicity: diplopia, ataxia, nystagmus, drowsiness; SIADH; hyponatremia.
  • Severe: leukopenia, hepatotoxicity (monitor CBC, LFTs).
  • HLA-B*1502 testing in Asian populations before initiation (risk of Stevens-Johnson syndrome).

Valproate

  • Therapeutic range: 50-100 mcg/mL.
  • Free fraction usually adequate but highly protein-bound; consider free level in similar situations to phenytoin.
  • Toxicity: tremor, encephalopathy (with hyperammonemia), hepatotoxicity, pancreatitis.
  • Valproate-induced hyperammonemic encephalopathy: confusion with normal liver enzymes; ammonia elevated; reversible with discontinuation; treat with L-carnitine.
  • Monitor: ammonia, LFTs, platelets, lipase.
  • Pregnancy: teratogenic; avoid if possible.

Lamotrigine

  • Therapeutic range: 3-15 mcg/mL (often not routinely measured; titration is critical).
  • Slow titration mandatory to avoid Stevens-Johnson syndrome.
  • Valproate doubles lamotrigine levels (use lower lamotrigine dose).
  • Carbamazepine, phenytoin halve lamotrigine levels.
  • Pregnancy: lamotrigine clearance dramatically increases (sometimes 2-3 fold) — levels drop; dose escalation often needed.

Other AEDs

  • Levetiracetam: TDM rarely necessary (wide therapeutic window, predictable kinetics); levels available if needed for adherence or pregnancy.
  • Topiramate: TDM not standard; clinical monitoring for cognitive effects.
  • Lacosamide: TDM not standard.
  • Brivaracetam: TDM not standard.
  • Phenobarbital: 15-40 mcg/mL therapeutic range; cognitive side effects at higher levels.
  • Oxcarbazepine: metabolite (10-monohydroxy-derivative) monitored if needed; hyponatremia common.
  • Cannabidiol: levels available; primary indication TDM is in setting of valproate co-administration (CBD increases valproate levels).

Anticoagulant Monitoring

Warfarin — INR

  • INR (International Normalized Ratio): standardized prothrombin time ratio.
  • Therapeutic range: 2.0-3.0 for most indications (atrial fibrillation, DVT/PE, mechanical mitral or aortic valves in some cases).
  • Higher range (2.5-3.5): triple-positive APS, mechanical mitral valve.
  • Many drug-drug and food-drug interactions.
  • Time in therapeutic range (TTR) >65% target.
  • Reversal: vitamin K, prothrombin complex concentrate (PCC) for severe bleeding.

Direct Oral Anticoagulants (DOACs)

  • Routine TDM not necessary for most patients.
  • DOAC-specific assays available but limited indications:
    • Bleeding emergency.
    • Need for urgent surgery.
    • Suspected accumulation in renal failure.
    • Suspected non-adherence.
    • Extreme body weight.
  • Dabigatran: diluted thrombin time, ecarin clotting time.
  • Rivaroxaban, apixaban, edoxaban: anti-Xa levels (drug-specific calibration).
  • aPTT and PT may be normal even on therapeutic DOAC levels — do NOT rely on these.
  • Reversal: idarucizumab for dabigatran; andexanet alfa for rivaroxaban/apixaban; PCC otherwise.

Heparin

  • Unfractionated heparin: aPTT 1.5-2.5× control, or anti-Xa 0.3-0.7 IU/mL.
  • Low molecular weight heparin: anti-Xa 0.5-1.0 IU/mL (4 hours post-dose) — measure in obesity, renal failure, pregnancy.

Immunosuppressants and Disease-Modifying Therapies

Tacrolimus

  • Used in MG, organ transplant.
  • Therapeutic range varies by indication (typically 5-15 ng/mL).
  • Trough level.
  • Toxicity: tremor, headache, posterior reversible encephalopathy syndrome (PRES), nephrotoxicity, hypertension.

Cyclosporine

  • Less common in modern neurology.
  • Trough level monitoring.
  • Similar PRES, hypertension, nephrotoxicity risks.

Mycophenolate

  • Used in autoimmune neuropathies, MG.
  • TDM not routine; clinical and CBC monitoring.

Azathioprine / TPMT

  • Check TPMT (thiopurine methyltransferase) activity or genotype before initiating azathioprine.
  • TPMT deficiency → severe myelosuppression with standard dosing.

Biologic Disease-Modifying Therapies — JCV, Antibody Monitoring

Natalizumab (MS, Crohn)

  • JCV antibody index: stratifies PML risk.
  • Higher index + longer natalizumab duration + prior immunosuppression → highest PML risk.
  • Anti-natalizumab antibodies in some patients reduce efficacy.

Ocrelizumab / Rituximab / Ofatumumab

  • CD19/20 B-cell count monitoring during therapy.
  • Hepatitis B serology before initiation.

Eculizumab

  • Meningococcal vaccination before initiation (functional asplenia-like risk).
  • Complement activity (CH50, AH50) monitoring.

Efgartigimod / Rozanolixizumab

  • FcRn antagonists for MG.
  • Monitor IgG levels (will decrease).

Special Situations

Pregnancy — AED Monitoring

  • Pharmacokinetic changes during pregnancy:
    • Lamotrigine clearance increases markedly.
    • Levetiracetam clearance increases.
    • Oxcarbazepine clearance increases.
    • Phenytoin: total decreases (more protein binding changes), free level more reliable.
  • Pre-pregnancy baseline level + monthly monitoring during pregnancy is standard practice.
  • Dose adjustment as levels fall to maintain seizure control.
  • Levels rebound post-partum; reduce dose to avoid toxicity.

Renal or Hepatic Dysfunction

  • Most AEDs affected differently; dose adjustments per drug.
  • Phenytoin: free level monitoring critical.
  • Levetiracetam: dose adjusted for CrCl.
  • Lacosamide: dose adjusted for severe renal failure.

Polypharmacy

  • CYP450 inducers (carbamazepine, phenytoin, phenobarbital, rifampin) lower levels of co-administered drugs.
  • CYP450 inhibitors (valproate, isoniazid, fluoxetine, fluconazole) raise levels of co-administered drugs.
  • Anticipate dose adjustments when starting/stopping interacting drugs.

When NOT to Order TDM

  • Patient clinically well-controlled, no adverse effects, no concern for adherence.
  • Drugs with wide therapeutic window and predictable response (most AEDs other than the “classical” few).
  • Routine monitoring “just to check” — guideline-driven monitoring is rare for most newer AEDs.

Pitfalls and Pearls

  • Phenytoin: zero-order kinetics; small dose changes → big level changes.
  • Hypoalbuminemia + phenytoin: order free level.
  • Lithium toxicity: dehydration, NSAIDs, ACE inhibitors, hyponatremia drive risk.
  • SILENT: irreversible cerebellar ataxia post-lithium toxicity.
  • Carbamazepine: auto-induction; SIADH; HLA-B*1502 testing in Asians.
  • Valproate + hyperammonemic encephalopathy: treat with L-carnitine.
  • Lamotrigine + valproate: doubles level; use lower lamotrigine dose.
  • Lamotrigine + pregnancy: clearance markedly increases; dose escalation needed.
  • Levetiracetam: TDM rarely needed; predictable kinetics.
  • Warfarin INR: 2-3 most indications; 2.5-3.5 triple-positive APS, mechanical mitral.
  • DOAC routine TDM: not necessary; aPTT/PT unreliable for DOACs.
  • DOAC-specific assays: bleeding, urgent surgery, renal failure, suspected non-adherence.
  • Tacrolimus / cyclosporine: PRES, hypertension, nephrotoxicity.
  • Natalizumab: JCV antibody index stratifies PML risk.
  • Eculizumab: meningococcal vaccination before initiation.
  • Azathioprine: TPMT testing before initiation.
  • Pregnancy: monitor AED levels monthly; lamotrigine, levetiracetam especially.
  • TDM goal: optimize clinical outcome, not normalize a number.

References

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  2. Pennell PB, Karanam A, Meador KJ, et al. Antiseizure medication concentrations during pregnancy: results from the Maternal Outcomes and Neurodevelopmental Effects of Antiepileptic Drugs (MONEAD) Study. JAMA Neurol. 2022;79(4):370-379.
  3. Steyerberg EW, Hopman WP, Vermeulen-Brink HJ. Therapeutic drug monitoring of lithium. Br J Clin Pharmacol. 1989;28(2):153-162.
  4. Reilly RF. The pharmacology of warfarin. Mayo Clin Proc. 2007;82(11):1379-1382.
  5. Tomson T, Battino D, Perucca E. Teratogenicity of antiepileptic drugs. Curr Opin Neurol. 2019;32(2):246-252.
  6. Connors JM. Testing and monitoring direct oral anticoagulants. Blood. 2018;132(19):2009-2015.