Master TDM principles — peak/trough sampling, therapeutic ranges, half-life. Apply to aminoglycosides on ASCP.

ASCPprep.com | Clinical Chemistry
Chapter — Therapeutic Drug Monitoring (TDM)
ASCP Board Exam · Lab-Focused Study Guide
Pharmacokinetics · Clinical Interpretation · Pre-analytical Variables · Practice Questions

1 · Priority Map & Quick Foundation

Topic Priority Why It’s Tested
Trough/peak sampling & steady state timing HIGH Wrong draw time = wrong result; the single most common TDM exam theme.
Digoxin (range, 8-h rule, K⁺/Mg²⁺, DLIS) HIGH Classic toxicity scenario; electrolyte interplay and early-draw trap.
Phenytoin (free vs total, zero-order, SST) HIGH Protein-binding + nonlinear kinetics + pre-analytical trap all in one drug.
Aminoglycosides & Vancomycin (nephro/oto-toxicity) HIGH Narrow index antibiotics; peak vs trough logic, Red Man Syndrome.
Lithium (12-h trough, Na⁺/dehydration) HIGH Renal handling tied to sodium — favorite physiology link.
Immunosuppressants (whole blood, calcineurin) MEDIUM Specimen type (whole blood) and mechanism are reliably asked.
PK basics (Vd, clearance, half-life, bioavailability) MEDIUM Formula recall and concept matching.
Hepatic metabolism (Phase I/II, CYP450, induction) MEDIUM Drug–drug interaction reasoning.
Antiepileptics (VPA, carbamazepine, newer AEDs) MEDIUM Ranges + signature toxicities.
Antineoplastics / pharmacogenomics (TPMT) LOW Occasional recall items (methotrexate rescue, TPMT).
Key Point — Focus vs Skim
Master cold: when to draw (trough = just before next dose; steady state = 5–7 doses), the digoxin and phenytoin traps, narrow-index antibiotics, lithium↔sodium. Skim: exact mechanisms of antineoplastics and antifungals — know the one-liner, not the biochemistry.

Quick Foundation (start here if TDM is new)

  • Why TDM exists: some drugs have a narrow therapeutic index — the helpful dose and the toxic dose are very close, so we measure blood levels instead of guessing.
  • Only free drug works: drug bound to protein (albumin/AAG) is parked; only the unbound fraction reaches the target and causes effect and toxicity.
  • Timing beats the number: a level means nothing without knowing when it was drawn relative to the dose.
  • Trough = lowest level, drawn right before the next dose (checks that the drug stays high enough). Peak = drawn after absorption/distribution (checks it doesn’t get too high).
  • Steady state = drug in ≈ drug out; reached after ~5–7 doses (≈5 half-lives). Don’t judge a regimen before then.
  • The liver packages drugs for disposal (Phase I makes them reactive, Phase II makes them water-soluble); inducing those enzymes clears the drug faster (shorter half-life).
Analogy Think of protein binding like a parking garage. Only the cars on the road (free drug) cause traffic (effect/toxicity). If the garage shrinks (low albumin), more cars hit the road — a “normal total” can still cause a traffic jam.

Why this matters clinically: the same total drug level can be safe in one patient and toxic in another depending on protein binding, electrolytes, organ function, and draw timing.

Concept Plain-English Explanation
Bioavailability How much of a dose actually reaches the blood. IV = 100%; oral is lower (first-pass loss).
Volume of distribution (Vd) How widely a drug spreads into tissue. Fat-loving drugs hide in tissue → big Vd.
Clearance How fast the body removes the drug (volume of blood cleared per unit time).
Half-life (T½) Time for the level to fall by half. Drives dosing interval and time to steady state.
Trough Lowest concentration — drawn just before the next dose.

2 · Core Content — Analyte by Analyte

Pharmacokinetic Foundations

Feature Details
Absorption (passive diffusion) Requires the nonionized (hydrophobic) form. Weak acids stay nonionized in low gastric pH → absorbed in stomach.
Bioavailability Fraction of dose reaching circulation unchanged. IV = 100%. Oral reduced by first-pass hepatic metabolism.
Volume of distribution Vd = Dose / Concentration. ↑ lipid solubility → ↑ Vd (sequestered in fat/nerve).
Protein binding Albumin → acidic drugs. α₁-acid glycoprotein (AAG) → basic drugs (quinidine, lidocaine); AAG rises in acute-phase reaction.
Elimination First-order: ΔC/ΔT = −kC. T½ = 0.693 / k. Steady state ≈ 5–7 doses.
Clearance Theoretical volume of blood cleared of drug per unit time. ↓ in cirrhosis (↑ T½) and renal failure.

Let the table anchor the formulas: if a drug is 13% eliminated per hour, k = 0.13/h, so T½ = 0.693/0.13 ≈ 5.3 h.

Hepatic Metabolism (MFO / CYP450)

Feature Details
Phase I Oxidation/reduction/hydrolysis → reactive intermediates (CYP450 mixed-function oxidase).
Phase II Conjugation (glutathione, sulfate, glucuronide) → water-soluble, excretable.
Induction ↑ enzyme synthesis → faster clearance → shorter half-life (e.g., phenobarbital, rifampin, smoking→CYP1A2).
Inhibition ↓ clearance → ↑ levels (e.g., valproic acid raises phenobarbital).
Pharmacogenomics Genetic CYP450 variation explains interindividual dosing differences; TPMT for thiopurines.

Cardioactive Drugs

Drug Range / Key Fact Mechanism / Trap
Digoxin 0.8–2.0 ng/mL; draw ≥8 h post-dose Inhibits Na⁺,K⁺-ATPase → ↑ intracellular Ca²⁺. Low K⁺/Mg²⁺ potentiate toxicity. DLIS falsely elevates immunoassay in neonates/uremia.
Quinidine Antiarrhythmic; binds AAG Natural agent suppressing PVCs.
Procainamide Measure with NAPA NAPA = active metabolite; total activity = parent + NAPA.
Disopyramide Quinidine substitute Used when quinidine side effects excessive.
Pre-analytical — Digoxin Serum; draw ≥8 h after the oral dose (tissue equilibration). Early draw = falsely “toxic.” Beware DLIS (newborns, uremia, pregnancy) on immunoassays.

Antibiotics (Narrow Therapeutic Index)

Drug Toxicity Key Fact
Aminoglycosides (gentamicin, tobramycin, amikacin) Nephrotoxicity + ototoxicity (8th nerve, irreversible) Highly polar → must be given IV/IM (no GI absorption). Monitor peak (efficacy) & trough (toxicity).
Vancomycin Nephrotoxicity; Red Man Syndrome (histamine, infusion-rate) Glycopeptide vs gram-positive cocci; trough drives nephrotoxicity risk.
Pre-analytical — Aminoglycosides/Vancomycin Timing is everything: peak drawn after distribution, trough just before next dose. Mislabeled peak/trough = clinically dangerous misinterpretation.

Antiepileptic Drugs (AEDs)

Drug Range Signature Point
Phenytoin (total) 10–20 µg/mL Zero-order kinetics; free fraction ↑ in low albumin/uremia; SST gel adsorbs → falsely low; gingival hyperplasia.
Phenobarbital — Prodrug = primidone; potent inducer (adjust after 10–15 d); ↑ by valproic acid.
Valproic acid 50–100 µg/mL Inhibits GABA transaminase; enzyme inhibitor.
Carbamazepine — Active metabolite 10,11-epoxide; leukopenia/aplastic anemia.
Oxcarbazepine — Prodrug → licarbazepine.
Levetiracetam (Keppra) — 100% bioavailable; renal, mostly unchanged.
Pre-analytical — Phenytoin Avoid SST (gel adsorbs phenytoin → falsely low). In hypoalbuminemia/uremia, order free phenytoin — a “normal” total can be toxic.

Psychoactive Drugs

Drug Use Key Fact
Lithium Bipolar (mania) 12-h trough; excretion parallels Na⁺ → dehydration/thiazides ↑ toxicity.
TCAs (imipramine, amitriptyline, doxepin) Depression Cardiotoxic in overdose.
Clozapine / Olanzapine Schizophrenia, bipolar Monitored for compliance & toxicity (seizures/overdose).
Pre-analytical — Lithium Draw 12 h post-dose. Do not use a lithium-heparin tube (falsely high). Hydration status changes the result.

Immunosuppressants (ISDs)

Drug Mechanism / Target Key Fact
Cyclosporine / Tacrolimus Calcineurin inhibition → blocks T-cell activation Whole blood specimen (RBC/FKBP binding); nephrotoxic; immunoassay cross-reactivity.
Sirolimus mTOR inhibition Whole blood.
Mycophenolic acid (MPA) — Prodrug = mycophenolate mofetil; often AUC-monitored.
Pre-analytical — Cyclosporine/Tacrolimus/Sirolimus Whole blood (EDTA), not serum/plasma — drug partitions into red cells. Wrong tube = falsely low.

Antineoplastics & Others

Drug Key Fact
Methotrexate Inhibits dihydrofolate reductase; leucovorin rescue protects normal cells.
Busulfan Alkylator; AUC-monitored to prevent venoocclusive liver disease.
Thiopurines (6-MP, azathioprine) Toxicity depends on TPMT activity (pharmacogenomics).
Theophylline 8–20 µg/mL; smoking induces CYP1A2 → ↑ clearance.
Caffeine Monitored for neonatal apnea.
5-Flucytosine / azoles (voriconazole) Flucytosine peak → myelosuppression; azoles inhibit 14α-demethylase (ergosterol).

3 · Visual Memory Hooks

Mnemonic — “DIG-8-K”: DIGoxin needs 8 hours before the draw, and low K⁺ makes it toxic.
Mnemonic — Aminoglycoside “Nephro + Oto = the 2 N-O’s”: kidneys (Nephro) and ears (Oto / 8th nerve) — and they’re Not Orally absorbed.
Mnemonic — “Lithium = Sodium’s shadow”: wherever Na⁺ goes, lithium follows; dehydration/low Na⁺ → lithium accumulates.
Mnemonic — “FREE drug is the BUSY drug”: only unbound drug works — low albumin frees more drug → toxicity at a “normal” total.

Pattern Triggers

If you see… Think… Because…
Digoxin “toxic” 2 h after dose, patient fine Drew too early Tissue distribution incomplete before 6–8 h.
Digoxin in range + arrhythmia + diuretic Hypokalemia Low K⁺ potentiates Na⁺,K⁺-ATPase inhibition.
Normal total phenytoin + ataxia + low albumin Order free phenytoin ↑ free fraction toxic despite normal total.
Falsely low phenytoin SST gel tube Gel adsorbs the drug.
Falsely high digoxin in a neonate DLIS Endogenous immunoreactive substances.
Tacrolimus run on serum Wrong specimen Needs whole blood (RBC binding).
Patient quits smoking on theophylline Rising level / toxicity Loss of CYP1A2 induction → ↓ clearance.

Commonly Confused Pairs

Pair Distinction
Peak vs Trough Peak = after absorption (toxicity check); Trough = before next dose (efficacy check).
Induction vs Inhibition Induction ↑ clearance, ↓ level/half-life; Inhibition ↓ clearance, ↑ level.
Albumin vs AAG Albumin binds acidic drugs; AAG binds basic drugs and rises in acute-phase.
Primidone vs Phenobarbital Primidone is the prodrug; phenobarbital is the active metabolite.
Total vs Free phenytoin Free reflects active drug; order free when albumin is low/uremia present.

Diagnostic / Sampling Window

Marker When to Draw Why Best Use
Digoxin ≥8 h post-dose Tissue equilibration Avoid false toxicity
Lithium 12 h post-dose (trough) Standardized trough Dose adjustment
Any drug (trough) Just before next dose Lowest point Ensure efficacy
Steady state After 5–7 doses (~5 T½) In = out Evaluate regimen

4 · Diagnostic Flowcharts

A. Differential — “High digoxin level”

Reported digoxin > 2.0 ng/mL → Check draw time → <6–8 h after dose? → Likely pre-distribution artifact → redraw at ≥8 h
If draw time OK → Check K⁺/Mg²⁺ → low? → toxicity potentiated → correct electrolytes
If neonate/uremic/pregnant → consider DLIS false elevation → confirm with alternate method
⚠️ Common mistake: holding digoxin on a high level drawn 2 h post-dose — the level is artificially high, not toxic.

B. “Phenytoin level looks normal but patient has nystagmus/ataxia”

Total phenytoin 10–15 µg/mL (normal) + toxicity signs → Check albumin → low albumin or uremia?
Yes → free fraction ↑ → order FREE phenytoin → free elevated → true toxicity
No → check tube type → SST? → gel adsorption (would lower, not raise) → consider other cause
⚠️ Common mistake: trusting the total level and increasing the dose — you can push a hypoalbuminemic patient into toxicity.

C. Timing/Marker selection — Peak vs Trough

Question = “is the drug staying high enough / is it accumulating?” → draw TROUGH (just before next dose)
Question = “is the peak getting toxic?” → draw PEAK (after absorption/distribution)
Either way → only interpret at steady state (5–7 doses)
⚠️ Common mistake: judging efficacy from a peak or drawing before steady state.

D. Method selection — Immunoassay vs reference method

Routine TDM → immunoassay (fast, automated)
Free-drug needed (low albumin) → ultrafiltration or equilibrium dialysis (gold standard)
Cross-reacting metabolites (tacrolimus, digoxin/DLIS) → confirm with LC-MS/MS
⚠️ Common mistake: trusting an immunoassay value when metabolites/DLIS cross-react.

E. Specimen check before running TDM

Cyclosporine / tacrolimus / sirolimus → whole blood (EDTA), not serum
Phenytoin → avoid SST (gel adsorbs)
Lithium → avoid Li-heparin tube
⚠️ Common mistake: sending serum for tacrolimus → falsely low → unnecessary dose increase.

5 · 25 High-Yield Facts

# Fact
1 IV route = 100% bioavailability.
2 Only the free (unbound) drug is pharmacologically active.
3 Albumin binds acidic drugs; α₁-acid glycoprotein binds basic drugs.
4 Vd = Dose / Concentration; high lipid solubility raises Vd.
5 T½ = 0.693 / k.
6 Steady state is reached after ~5–7 doses (≈5 half-lives).
7 Trough = drawn immediately before the next dose.
8 Enzyme induction shortens half-life; inhibition prolongs it.
9 CYP450 explains most interindividual metabolism differences.
10 Digoxin therapeutic range = 0.8–2.0 ng/mL.
11 Draw digoxin ≥8 h after the oral dose.
12 Low K⁺ and Mg²⁺ potentiate digoxin toxicity.
13 DLIS falsely elevates digoxin immunoassays in neonates/uremia.
14 Procainamide must be measured with its active metabolite NAPA.
15 Aminoglycosides cause nephrotoxicity and ototoxicity and are not orally absorbed.
16 Vancomycin can cause Red Man Syndrome (histamine, infusion-rate related).
17 Phenytoin total range = 10–20 µg/mL and follows zero-order kinetics.
18 Low albumin/uremia raises free phenytoin — order free level.
19 SST gel adsorbs phenytoin → falsely low result.
20 Valproic acid (50–100 µg/mL) inhibits GABA transaminase and raises phenobarbital.
21 Carbamazepine → active 10,11-epoxide; risk of leukopenia/aplastic anemia.
22 Lithium: 12-h trough; excretion parallels sodium.
23 Cyclosporine/tacrolimus/sirolimus require whole blood; inhibit calcineurin/mTOR.
24 Methotrexate inhibits dihydrofolate reductase; leucovorin is the rescue.
25 Theophylline 8–20 µg/mL; smoking induces CYP1A2 and increases clearance.
Exam Tip — the most testable pattern A drug level can be “in range” yet toxic. The exam loves the combination: wrong draw time, altered protein binding, or electrolyte disturbance. Always ask when was it drawn and what is the albumin/K⁺ before trusting the number.

Quick Interpretation Table

Lab Finding Clinical Interpretation
High digoxin drawn <6 h post-dose Pre-distribution artifact — redraw at ≥8 h.
Normal total phenytoin + toxicity + low albumin Elevated free phenytoin — true toxicity.
Digoxin “in range” + arrhythmia + diuretic Hypokalemia-potentiated toxicity.
Low tacrolimus on a serum tube Wrong specimen — should be whole blood.
Rising theophylline after smoking cessation Loss of CYP1A2 induction → reduced clearance.
Falsely high digoxin in a neonate DLIS interference.

🎴 Flashcards — Active Recall Practice

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📝 Practice Quiz — Test Your Knowledge

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🪤 Trap Questions — Beat the Examiner

Seven questions built on the misconceptions the ASCP examiner exploits. Each answer explains the trap, why the question was written, and the wrong answer most students pick. (7 questions)
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8 · 📖 Read This 2 Hours Before Your Exam

Numbers

  • Digoxin 0.8–2.0 ng/mL
  • Phenytoin (total) 10–20 µg/mL
  • Valproic acid 50–100 µg/mL
  • Theophylline 8–20 µg/mL
  • Steady state = 5–7 doses
  • T½ = 0.693 / k

Timing

  • Trough = just before next dose
  • Digoxin = ≥8 h post-dose
  • Lithium = 12 h post-dose
  • Interpret only at steady state

Pattern Associations

  • Lithium ↔ sodium/dehydration
  • Digoxin ↔ low K⁺/Mg²⁺
  • Theophylline ↔ smoking (CYP1A2)
  • Tacrolimus/cyclosporine ↔ whole blood
  • Procainamide ↔ NAPA

Opposites

  • Induction ↓ level · Inhibition ↑ level
  • Low albumin ↑ free drug
  • IV 100% vs oral <100% bioavailability
  • Peak (toxicity) vs Trough (efficacy)

Confusing Pairs

  • Primidone (prodrug) → phenobarbital
  • Oxcarbazepine → licarbazepine
  • Total vs free phenytoin
  • Albumin (acidic) vs AAG (basic)

Pre-analytical Traps

  • SST gel → phenytoin falsely low
  • DLIS → digoxin falsely high
  • Serum tube → tacrolimus falsely low
  • Li-heparin tube → lithium falsely high

9 · Final Memory Map

Therapeutic Drug Monitoring
Pharmacokinetics
Bioavailability (IV=100%), Vd = D/C, Clearance, T½ = 0.693/k
Free drug = active; albumin (acidic) / AAG (basic)
Steady state 5–7 doses; trough before next dose
Hepatic Metabolism
Phase I (CYP450, reactive) → Phase II (conjugation, soluble)
Induction ↓ half-life · Inhibition ↑ level · TPMT/pharmacogenomics
Cardioactive
Digoxin 0.8–2.0, ≥8 h, low K⁺/Mg²⁺, DLIS · Procainamide+NAPA
Antibiotics
Aminoglycosides: nephro/oto, IV only · Vancomycin: Red Man, trough
Antiepileptics
Phenytoin 10–20, zero-order, free vs total, SST trap · VPA 50–100 · Carbamazepine epoxide
Psychoactive
Lithium 12-h trough ↔ sodium · TCAs cardiotoxic · Clozapine compliance
Immunosuppressants
Cyclosporine/Tacrolimus (calcineurin), Sirolimus (mTOR) — whole blood


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