General & BloodNo Fasting RequiredTurnaround: 4–8 hours (same day)

Therapeutic Drug Monitoring (TDM — Serum Lithium & Digoxin Levels)

Also known as: TDM Blood Test, Serum Lithium Level, Serum Digoxin Level, Therapeutic Level Check, Drug Toxicity Blood Test

Parameters Measured in the Report

A breakdown of individual biomarkers and biological reference intervals typically found in your report:

Biomarker	Standard Range	Unit	What It Means
Serum Lithium Level (12-Hour Trough) (Lithium Level)	Therapeutic Maintenance: 0.6–1.0 Acute Mania Target: 0.8–1.2 Toxic: > 1.5 Life-Threatening: > 2.0	mEq/L (or mmol/L)	Mood stabilizing cation; levels >1.5 mEq/L cause severe neurotoxicity, permanent cerebellar ataxia, and acute renal failure.
Serum Digoxin Level (12-Hour Trough) (Digoxin Level)	Therapeutic Heart Failure: 0.5–0.9 Arrhythmia Target: 0.8–2.0 Toxic: > 2.0	ng/mL (or nmol/L = ng/mL × 1.28)	Cardiac glycoside; levels >2.0 ng/mL cause life-threatening ventricular arrhythmias, heart block, and visual halos.

* Note: Reference ranges may vary slightly between testing instruments and reagent kits. Always refer to the reference interval printed on your specific pathology report.

Clinical Focus

Exact pharmacological concentration of prescription medications with a narrow therapeutic index in blood serum.

Sample & Specimen

Venous blood (Serum red-top tube WITHOUT gel separator)

Fasting Protocol

Strict trough timing required: blood must be drawn exactly 12 hours after the previous evening dose (trough level).

Turnaround Time

4–8 hours (same day)

What Is This Test?

Therapeutic Drug Monitoring (TDM) is a specialized clinical chemistry discipline dedicated to measuring circulating medication concentrations in bloodstream. Many foundational medications possess a **narrow therapeutic index (NTI)** — meaning the therapeutic dose required to treat disease is dangerously close to the toxic dose that causes severe organ damage or death. Prominent examples include **Lithium** (the gold-standard mood stabilizer for Bipolar Disorder) and **Digoxin** (a cardiac glycoside used for atrial fibrillation and heart failure). Because individual patient metabolism, hydration, renal clearance, and drug-drug interactions cause blood concentrations to vary widely, precise TDM blood sampling ensures the patient receives therapeutic benefit without sliding into life-threatening toxicity.

Why Did Your Doctor Order This Test?

Physicians routinely prescribe the TDM (Lithium & Digoxin) in the following clinical scenarios:

Routine surveillance and dosage calibration in bipolar disorder patients maintained on chronic oral Lithium therapy

Routine monitoring in cardiac patients taking Digoxin to manage atrial fibrillation with rapid ventricular rate or congestive heart failure

Investigating symptoms of acute or chronic Lithium toxicity: coarse muscle tremors, unsteady staggering gait (ataxia), dysarthria (slurred speech), confusion, or vomiting

Investigating symptoms of Digoxin toxicity: loss of appetite, nausea, vomiting, yellow-green visual color halos (xanthopsia), or dangerous bradycardia/arrhythmias

Changes in patient kidney function (KFT) or addition of interacting drugs (NSAIDs, ACE inhibitors, diuretics) that reduce drug clearance

How to Prepare Before Your Test

Fasting Requirement: Strict trough timing required: blood must be drawn exactly 12 hours after the previous evening dose (trough level).

Blood MUST be drawn as a "12-Hour Trough Level": precisely 12 hours after taking the last evening oral dose (e.g., if the pill was taken at 8:00 PM, blood must be collected at 8:00 AM the following morning before taking the morning dose).
Do not take your morning medication dose until AFTER the blood draw is completed.
Red-top serum tubes WITHOUT separator gel are required, as silicone separator gels can absorb lipophilic drug molecules.

Understanding Your Results

What higher or lower readings generally mean in clinical practice:

If Values Are Higher Than Normal

A Lithium level >1.5 mEq/L or Digoxin level >2.0 ng/mL establishes drug toxicity. Toxic levels require immediate medication cessation, aggressive intravenous fluid hydration, cardiac telemetry monitoring, and in severe cases, hemodialysis (for lithium) or Digoxin-specific Fab antibody fragments (DigiFab).

If Values Are Lower Than Normal

Sub-therapeutic levels (Lithium <0.6 mEq/L or Digoxin <0.5 ng/mL) indicate ineffective dosing or patient non-adherence, putting the patient at risk for bipolar relapse or loss of cardiac ventricular rate control.

Frequently Asked Questions

Why does a lithium or digoxin test have to be drawn exactly 12 hours after the pill?

After you swallow a pill, drug levels surge in your blood for 2 to 4 hours while being absorbed. Testing during this absorption peak gives a falsely terrifying high reading. Waiting 12 hours captures the "trough" (baseline steady state) that clinical guidelines are calibrated to.

Why does dehydration or taking ibuprofen (Advil) cause sudden lithium toxicity?

The kidneys clear lithium in exact competition with sodium. If you become dehydrated or take NSAID pain relievers (like ibuprofen), your kidneys hold onto sodium and mistakenly reabsorb lithium back into your bloodstream, causing toxic spikes.

Related Laboratory Tests

Tests frequently prescribed together or used as follow-up investigations:

Kidney Function Test (KFT / RFT / Renal Panel)

Serum Creatinine, Blood Urea Nitrogen (BUN), eGFR (filtration rate), Uric Acid, and Electrolytes.

Serum Electrolytes (Sodium, Potassium, Chloride, Bicarbonate)

Electrically charged mineral ions: Sodium (Na+), Potassium (K+), Chloride (Cl-), and Bicarbonate (HCO3-).

Complete Blood Count (CBC / Hemogram)

Counts and evaluates red blood cells, white blood cells, hemoglobin, hematocrit, and platelets.

Medical & Diagnostic Disclaimer

This guide is published for educational and patient informational purposes only. It is not a substitute for professional medical evaluation, diagnosis, or clinical advice.

A laboratory result that falls slightly outside standard statistical reference intervals does not automatically indicate disease; conversely, a normal result does not guarantee the absence of medical conditions. Only your attending physician can interpret lab findings in the context of your medical history, physical exam, and symptoms.