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[ Home ](https://mdster.com) 2. [ Blog ](https://mdster.com/blog) 3. [ Medical Education ](https://mdster.com/blog?category=medical-education) 4. Indications for TDM: When a Drug Level Changes Clinical Care [ Medical Education ](https://mdster.com/blog?category=medical-education) Indications for TDM: When a Drug Level Changes Clinical Care ============================================================== Use therapeutic drug monitoring to answer a clinical question—not simply to fill in a laboratory value. [ MDster Editorial Team ](https://mdster.com/about) · Oct 08, 2026 · 7 min read · 28 [ Reviewed by Dr. Ali Ragab, MBBCH, MSc, MCAI ](https://mdster.com/medical-reviewers/dr-ali-ragab) [Editorial Policy](https://mdster.com/editorial-policy) | [Corrections Policy](https://mdster.com/corrections) [ Internal Medicine ](https://mdster.com/blog?tag=internal-medicine) [ Medication Safety ](https://mdster.com/blog?tag=medication-safety) [ Clinical Pharmacology ](https://mdster.com/blog?tag=clinical-pharmacology) [ Therapeutic Drug Monitoring ](https://mdster.com/blog?tag=therapeutic-drug-monitoring) ![Indications for TDM: When a Drug Level Changes Clinical Care](https://mdster.com/storage/blog/images/indications-for-tdm-when-a-drug-level-changes-clinical-care.png) Share this article Share this post On this page 1. [ Start With the Decision, Not the Assay ](#start-with-the-decision-not-the-assay) 2. [ Suspected Nonadherence: Interpret the Pattern ](#suspected-nonadherence-interpret-the-pattern) 3. [ Narrow Therapeutic Index: Monitor When Clearance Can Shift ](#narrow-therapeutic-index-monitor-when-clearance-can-shift) 4. [ Troubleshoot Toxicity Versus Inadequate Effect ](#troubleshoot-toxicity-versus-inadequate-effect) 5. [ Make Every Level Interpretable ](#make-every-level-interpretable) 6. [ Key Takeaways ](#key-takeaways) 7. [ Conclusion ](#conclusion) 8. [ Frequently Asked Questions ](#blog-faqs) 9. [ References ](#references-heading) On this page 1. [ Start With the Decision, Not the Assay ](#start-with-the-decision-not-the-assay) 2. [ Suspected Nonadherence: Interpret the Pattern ](#suspected-nonadherence-interpret-the-pattern) 3. [ Narrow Therapeutic Index: Monitor When Clearance Can Shift ](#narrow-therapeutic-index-monitor-when-clearance-can-shift) 4. [ Troubleshoot Toxicity Versus Inadequate Effect ](#troubleshoot-toxicity-versus-inadequate-effect) 5. [ Make Every Level Interpretable ](#make-every-level-interpretable) 6. [ Key Takeaways ](#key-takeaways) 7. [ Conclusion ](#conclusion) 8. [ Frequently Asked Questions ](#blog-faqs) 9. [ References ](#references-heading) Your patient has another seizure despite a prescribed antiseizure medication. Is the dose inadequate, the medication being missed, or the diagnosis wrong? Before increasing the dose, ask whether a **drug concentration would change your next decision**. That question is the best starting point for therapeutic drug monitoring (TDM). [\[1\]](#cite-1 "Reference [1]") Start With the Decision, Not the Assay -------------------------------------- TDM is most useful when drug exposure varies substantially between patients, the consequences of too little or too much exposure matter, and a measured concentration can guide action. The three recurring indications on the ward are suspected nonadherence, safe dosing of a narrow-therapeutic-index drug, and an unexpected mismatch between treatment and clinical response. [\[2\]](#cite-2 "Reference [2]") A level is not a substitute for examining the patient. Before ordering one, decide whether a low result would prompt an adherence discussion or dose review, and whether a high result would change treatment. For most people with well-controlled epilepsy, routine antiseizure drug levels are unnecessary; NICE recommends considering them for uncontrolled seizures, adverse effects, poor adherence, or circumstances such as pregnancy or renal failure. [\[1\]](#cite-1 "Reference [1]") ### Suspected Nonadherence: Interpret the Pattern When seizures recur, a drug level may reveal unexpectedly low exposure. It cannot, by itself, distinguish missed doses from a mistimed sample, changed clearance, an interacting medication, or an incorrect account of the dosing schedule. Ask when the last doses were taken, review refills and recent medication changes, and compare the result with any prior concentration obtained during good seizure control. [\[2\]](#cite-2 "Reference [2]") Do not label a patient ‘nonadherent’ from one result. A detectable level establishes recent exposure, not reliable dosing over previous weeks; a low level needs interpretation against timing and the prescribed regimen. Use the finding to investigate barriers to treatment rather than automatically escalating a dose that may become excessive once regular dosing resumes. [\[2\]](#cite-2 "Reference [2]") > **Clinical Pearl:** In a patient with breakthrough seizures, the most informative comparison may be with *their own* concentration when seizures were controlled—not a population reference interval. [\[2\]](#cite-2 "Reference [2]") Narrow Therapeutic Index: Monitor When Clearance Can Shift ---------------------------------------------------------- Lithium illustrates why scheduled monitoring and event-triggered monitoring both matter. Measure levels after initiation and dose changes, then continue periodic surveillance; check sooner when illness, dehydration, declining renal function, or an interacting drug changes the risk of accumulation. NSAIDs, diuretics, and renin–angiotensin-system drugs warrant particular attention. Obtain a consistently timed sample, commonly **12 hours after the last lithium dose**, and assess renal function alongside the result. [\[3\]](#cite-3 "Reference [3]") For serious MRSA infections treated with IV vancomycin, the monitoring question is whether exposure is sufficient without unnecessary nephrotoxicity. Consensus guidance favors **AUC-guided monitoring**, targeting an AUC over 24 hours of 400–600 mg·h/L when the assumed MIC is 1 mg/L. Do not treat an isolated trough of 15–20 mg/L as the universal goal: that older surrogate can produce excessive exposure, and the evidence for this AUC target comes primarily from serious MRSA infection. [\[4\]](#cite-4 "Reference [4]") Phenytoin presents a different trap. Its saturable metabolism means a small dose increase can produce a disproportionate rise in concentration. In renal or hepatic disease or hypoalbuminemia, the **free concentration** is more informative than total concentration because the unbound fraction increases. A seemingly low total level must not trigger a reflex dose increase in a symptomatic patient. [\[5\]](#cite-5 "Reference [5]") - Recheck exposure when a dose, interacting drug, renal function, or clinical condition changes. [\[6\]](#cite-6 "Reference [6]") - Specify the analyte that answers the question: total versus free phenytoin, or vancomycin concentrations used to estimate AUC. [\[7\]](#cite-7 "Reference [7]") - Do not apply a population ‘therapeutic range’ without considering symptoms and sampling time. [\[2\]](#cite-2 "Reference [2]") Troubleshoot Toxicity Versus Inadequate Effect ---------------------------------------------- Suppose a patient taking digoxin develops nausea and a new arrhythmia during acute kidney injury. Check a digoxin concentration to inform the assessment, but obtain it just before the next dose or **at least six hours after the last dose**; an earlier sample can mislead during distribution. Review the ECG, renal function, potassium, magnesium, and interacting drugs. A concentration alone neither confirms nor excludes clinically important toxicity. [\[8\]](#cite-8 "Reference [8]") The reverse problem also deserves restraint: persistent symptoms do not prove inadequate exposure. In uncontrolled seizures, a level may help distinguish low exposure from treatment failure despite adequate exposure; it does not establish that more drug will work. Reassess the diagnosis, clinical course, and adverse effects before changing therapy. [\[1\]](#cite-1 "Reference [1]") ### Make Every Level Interpretable Before acting on a result, document four details: 1. **The clinical question:** adherence, toxicity, inadequate effect, or changed clearance. 2. **The dosing history:** dose, last administration, recent changes, and interacting medicines. 3. **The sample context:** collection time and whether the result reflects the intended sampling point. 4. **The patient context:** symptoms, renal function, and—when relevant—albumin and electrolytes. [\[2\]](#cite-2 "Reference [2]") Key Takeaways ------------- - Order TDM when its result can change a decision, not merely because an assay exists. [\[2\]](#cite-2 "Reference [2]") - Use levels to investigate suspected nonadherence, but never infer a dosing history from one number. [\[1\]](#cite-1 "Reference [1]") - Monitor narrow-therapeutic-index drugs when exposure or clearance changes; match the assay to the drug’s pharmacology. [\[6\]](#cite-6 "Reference [6]") - When efficacy and toxicity disagree with the reported dose, check timing and clinical context before adjusting treatment. [\[8\]](#cite-8 "Reference [8]") Conclusion ---------- A useful drug level resolves uncertainty; an uninterpreted number creates it. Ask what you will do with the result, collect the right sample, and let the patient’s clinical state lead the final decision. [\[2\]](#cite-2 "Reference [2]") Frequently Asked Questions ---------------------------- ### Can one low antiseizure medication level prove missed doses? No. Review sample timing, recent dosing, interactions, and changes in clearance before interpreting a low concentration as poor adherence. [\[2\]](#cite-2 "Reference [2]") ### When should I check lithium outside routine monitoring? Check when a dose or interacting medication changes, renal function worsens, or an intercurrent illness or dehydration raises concern for accumulation. [\[6\]](#cite-6 "Reference [6]") ### Why request free rather than total phenytoin? Renal or hepatic disease and hypoalbuminemia can increase the active unbound fraction, making total concentrations misleading. [\[7\]](#cite-7 "Reference [7]") ### Does a digoxin level within the reference range rule out toxicity? No. Interpret it with symptoms, ECG findings, renal function, electrolytes, and the time since the last dose. [\[8\]](#cite-8 "Reference [8]") References (8) ------------------ 1. 1. [ NICE. Epilepsies in children, young people and adults: NG217, monitoring and review. ](https://www.nice.org.uk/guidance/ng217/chapter/principles-of-treatment-safety-monitoring-and-withdrawal) [↩](#cite-ref-1-1 "Back to text") 2. 2. [ pubmed.ncbi.nlm.nih.gov/18397299 ](https://pubmed.ncbi.nlm.nih.gov/18397299/) [↩](#cite-ref-2-1 "Back to text") 3. 3. [ NICE. Bipolar disorder: assessment and management, CG185. ](https://www.nice.org.uk/guidance/cg185/chapter/recommendations) [↩](#cite-ref-3-1 "Back to text") 4. 4. [ Rybak MJ, et al. Therapeutic monitoring of vancomycin for serious MRSA infections. ASHP/PIDS/SIDP/IDSA consensus guideline, 2020. ](https://www.idsociety.org/practice-guideline/vancomycin/) [↩](#cite-ref-4-1 "Back to text") 5. 5. [ dailymed.nlm.nih.gov/dailymed/lookup.cfm?setid=3bc82d23-3950-f181-e063-6394a90ad565&version=3 ](https://dailymed.nlm.nih.gov/dailymed/lookup.cfm?setid=3bc82d23-3950-f181-e063-6394a90ad565&version=3) [↩](#cite-ref-5-1 "Back to text") 6. 6. [ dailymed.nlm.nih.gov/dailymed/fda/fdaDrugXsl.cfm?setid=a8f37c66-0b88-4f7c-e053-2995a90a4dea&type=display ](https://dailymed.nlm.nih.gov/dailymed/fda/fdaDrugXsl.cfm?setid=a8f37c66-0b88-4f7c-e053-2995a90a4dea&type=display) [↩](#cite-ref-6-1 "Back to text") 7. 7. [ dailymed.nlm.nih.gov/dailymed/lookup.cfm?setid=0aac9dcd-c4df-84d9-e063-6294a90a0376 ](https://dailymed.nlm.nih.gov/dailymed/lookup.cfm?setid=0aac9dcd-c4df-84d9-e063-6294a90a0376) [↩](#cite-ref-7-1 "Back to text") 8. 8. [ DailyMed. Digoxin tablets: prescribing information. ](https://dailymed.nlm.nih.gov/dailymed/lookup.cfm?setid=e104c3c5-e4b8-7dbd-e053-2995a90ae339) [↩](#cite-ref-8-1 "Back to text") Study pathway Turn reading into Internal Medicine performance ------------------------------------------------- - System‑based, exam‑style questions + explanations - Smarter review to reinforce weak topics - Build confidence with measurable progress Free 5-day trial No credit card required. 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