1Four checks before a level changes a dose
When was it drawn?
Relative to the last dose. A sample taken while a drug is still absorbing or distributing into tissue can read high, so labels name a sampling time. Find it before you look at the number.
Was the patient at steady state?
Steady state takes several half-lives of unchanged dosing. A level drawn sooner understates where the regimen is heading, unless a loading dose was given.
Free or total?
Most assays report total drug. When protein binding falls (low albumin, uremia, a displacing drug), the active free fraction rises while the total can look low or normal.
Does it fit the patient?
A level that contradicts the clinical picture is a reason to question the sample, its timing or the assay before touching the dose.
2The arithmetic that keeps coming back
2.1Half-life and the road to steady state
Half-life equals 0.693 divided by the elimination rate constant. After n half-lives of constant dosing, the fraction of steady state reached is 1 − 0.5ⁿ. Only half-life sets the pace: a bigger dose raises the plateau but does not get you there any sooner.
2.2Loading dose
A loading dose is the volume of distribution (Vd) multiplied by the concentration change you want. When drug is already on board, that change is the target minus the current level. For a hypothetical drug with a Vd of 40 L and a planned rise of 8 mg/L, the arithmetic gives 320 mg; the figures are illustrative, and real loading doses come from the label or a guideline.
2.3Free versus total
Correction equations estimate free drug from albumin for a few highly bound drugs. They rest on assumptions about binding, so when the laboratory can measure a free concentration, the measured value wins. Binding that saturates as the concentration rises defeats a simple correction altogether.
2.4Autoinduction
A few drugs induce their own metabolism. Clearance climbs over the first weeks of therapy, and a level drawn early overstates later ones at the same dose. The general timing of induction is covered on CYP450 inducers and inhibitors.
3Vancomycin: AUC replaced the trough
| Approach | Target | Status in the 2020 guideline |
|---|---|---|
| AUC-guided dosing | 24-hour AUC/MIC of 400–600 (MIC assumed 1 mg/L) | Recommended over trough-only monitoring |
| Trough-only monitoring | Trough 15–20 mg/L | No longer the target for serious MRSA infections |
Source: Rybak et al., Am J Health Syst Pharm 2020 (as of October 27, 2026). How the AUC is estimated, by Bayesian software or from two levels, is on vancomycin AUC versus trough monitoring.
4Terms the cases lean on
- Therapeutic range
- A concentration band linked to benefit with acceptable toxicity across a population; a starting guide for the individual patient.
- Half-life (t½)
- Time for the concentration to fall by half once distribution is complete.
- Volume of distribution (Vd)
- The apparent volume that relates the amount of drug in the body to its plasma concentration; it drives the loading dose.
- Steady state
- The point where the amount eliminated per interval equals the amount given, so peaks and troughs repeat.
- Trough
- The lowest concentration in a dosing interval, drawn just before the next dose.
- Free fraction
- The unbound share of drug in plasma; the part that reaches receptors and is cleared.
5Questions about drug levels
Which patients need the most care with levels?
Patients whose binding or clearance has shifted: low albumin, changing kidney function, critical illness and advanced age. For older adults, the 2023 AGS Beers Criteria add a list of drugs to avoid or dose-reduce as kidney function declines, which pairs naturally with level-guided dosing.
Is a level drawn before steady state useless?
No, but it is a snapshot of a rising curve. Read it with the half-life in mind, and expect the next level on the same dose to come back higher.
Where does TDM sit on the BCPS outline?
Monitoring, including drug levels, is part of Domain 2, therapeutics and patient management, which makes up 36% of the content outline per the BCPS Examination Specifications (effective September 2024, as of October 27, 2026).
6Levels in context
Decide what each concentration really tells you, and only then what to do with the dose.
Study cases written by our editors, not BPS items. The real BCPS exam is computer-based multiple choice, answered in order with no going back.
Case 1 of 6 · 0 answered
How case rounds work
Pick an option to see the verdict and a note on every option. Keys while the round has focus: 1–5 or A–E answer, ← → move between cases. Nothing is saved or sent anywhere.
Case 01
A 79-year-old woman with normal renal function takes phenytoin 300 mg daily for epilepsy and has had no seizures. Her total phenytoin level is 8 mcg/mL and serum albumin is 2.0 g/dL. Using the Sheiner-Tozer equation, what is her corrected phenytoin level and the most appropriate action?
Pick an option: the verdict and a note on every option open here.
Best answer: B.
Corrected phenytoin = measured ÷ (0.2 × albumin + 0.1) = 8 ÷ (0.4 + 0.1) = 16 mcg/mL, within the 10–20 mcg/mL range, so the dose should not change. Using the uncorrected 8 mcg/mL ignores the higher free fraction in hypoalbuminemia and could lead to toxicity after a dose increase. The 27 mcg/mL value uses the end-stage renal disease coefficient (0.1 × albumin + 0.1), and 20 mcg/mL drops the 0.1 constant.
- A
- Reading the uncorrected 8 mcg/mL as low ignores the higher free fraction and invites toxicity after a dose increase.
- B
- Correct. 8 ÷ (0.2 × 2.0 + 0.1) gives 16 mcg/mL, inside the 10–20 range, so the seizure-free dose stays.
- C
- 20 mcg/mL comes from dropping the 0.1 constant from the denominator.
- D
- About 27 mcg/mL uses the end-stage renal disease version of the equation, which does not apply with normal kidneys.
Case 02
A 70-kg man with epilepsy is admitted after a breakthrough seizure. His total phenytoin level is 5 mcg/mL (albumin 4.2 g/dL, normal renal function), and the team wants to raise it to 15 mcg/mL with a single IV loading dose. Assuming a phenytoin volume of distribution of 0.7 L/kg, which fosphenytoin dose is most appropriate?
Pick an option: the verdict and a note on every option open here.
Best answer: A.
Loading dose = Vd × (desired − current concentration) = 0.7 L/kg × 70 kg × (15 − 5) mg/L = 490 mg, or about 500 mg phenytoin equivalents. The 750 mg PE answer ignores the level he already has and loads to the full 15 mcg/mL. The 1,400 mg PE answer is the standard 20 mg PE/kg load for a phenytoin-naive patient and would push him into the toxic range; the loading dose depends on Vd, not on clearance or half-life.
- A
- Correct. Loading dose is Vd × change in concentration: 0.7 × 70 × (15 − 5) is 490 mg, about 500 mg PE.
- B
- About 250 mg PE covers only half of the 10 mcg/mL rise he needs.
- C
- About 750 mg PE ignores the 5 mcg/mL already on board and loads to the full 15.
- D
- About 1,400 mg PE is the standard 20 mg PE/kg load for a phenytoin-naive patient and would push him into the toxic range.
Case 03
A 76-year-old woman (64 kg) with permanent atrial fibrillation and HFrEF has taken digoxin 0.125 mg orally every morning at 8 AM for 3 months without a change. At a clinic visit, a digoxin level drawn at 10 AM, 2 hours after today’s dose, is 2.3 ng/mL. She has no nausea, visual change, or palpitations. HR is 74 beats/minute, K 4.4 mEq/L, Mg 2.1 mg/dL, and SCr is stable at 0.9 mg/dL. Which action is most appropriate?
Pick an option: the verdict and a note on every option open here.
Best answer: C.
The Lanoxin (digoxin) prescribing information says that, to allow time for equilibration between serum and tissue, serum digoxin should be sampled just before the next scheduled dose or, if that is not possible, at least 6 to 8 hours after the last dose. A level drawn 2 hours after an oral dose falls in the distribution phase and overestimates the concentration at the site of action. With no signs of toxicity and normal potassium, magnesium and renal function, the right step is to repeat the level at a valid time before changing the dose.
- A
- This acts on a level drawn in the distribution phase, which overstates the tissue concentration. Get a valid level first.
- B
- Holding and halving treats a sampling error as toxicity. She has no symptoms and normal K, Mg and renal function.
- C
- Correct. A level 2 hours after the dose reflects distribution. The label calls for sampling just before the next dose or at least 6–8 hours after.
- D
- Ignoring a high result is also wrong. The level is uninterpretable, so it should be repeated at a valid time, not dismissed.
Case 04
A 78-year-old woman in the ICU takes valproic acid 750 mg twice daily for epilepsy. Her albumin is 2.1 g/dL. Her total valproate level is 52 mcg/mL (reference 50–100), yet she is newly somnolent with tremor; ammonia and liver enzymes are normal, and she has no other sedating drugs. Which action is most appropriate?
Pick an option: the verdict and a note on every option open here.
Best answer: A.
Valproate is highly and saturably bound to albumin, so in hypoalbuminemia the free fraction rises and a ‘normal’ total level can hide toxic free concentrations; a measured free level (target about 5–15 mcg/mL) guides the dose. Raising the dose on the basis of the total level could worsen toxicity. The Sheiner-Tozer equation applies to phenytoin, not valproate, and levocarnitine is indicated for valproate hyperammonemia, which is absent here.
- A
- Correct. Low albumin raises valproate’s free fraction, so a normal total level can hide toxic free levels; a free level guides the dose.
- B
- Raising the dose on the total level alone could deepen toxicity that the low albumin is masking.
- C
- Levocarnitine treats valproate hyperammonemia, and her ammonia is normal.
- D
- Sheiner-Tozer is a phenytoin equation; valproate binding is saturable and needs a measured free level.
Case 05
A 70-year-old woman with CrCl 22 mL/min is receiving vancomycin for MRSA osteomyelitis. Two post-distribution levels give an elimination rate constant (ke) of 0.029 h⁻¹. Assuming no dose changes, approximately how long after starting therapy will vancomycin concentrations reach steady state?
Pick an option: the verdict and a note on every option open here.
Best answer: D.
Half-life = 0.693 ÷ ke = 0.693 ÷ 0.029 ≈ 24 hours, and steady state is reached after about 4–5 half-lives, or roughly 96–120 hours (4–5 days). Choosing 24 hours confuses one half-life with time to steady state, and 2 days covers only two half-lives (about 75% of steady state). This slow accumulation in renal impairment is why a loading dose is given and why AUC-guided levels should not be over-interpreted early.
- A
- 10–12 days would imply a half-life of over 50 hours, about twice what this ke gives.
- B
- Two days is only about two half-lives, roughly 75% of steady state.
- C
- 24 hours is one half-life, not the time to steady state.
- D
- Correct. Half-life is 0.693 ÷ 0.029, about 24 hours, and steady state takes 4–5 half-lives, so roughly 4–5 days.
Case 06
A 29-year-old woman with focal epilepsy started carbamazepine ER 200 mg twice daily, increased to 400 mg twice daily after 1 week. A trough level at week 2 was 9.5 mcg/mL. At week 6 she has a breakthrough seizure; pill counts confirm adherence, she takes no other medications, and the trough level is now 4.1 mcg/mL. Which explanation and action are most appropriate?
Pick an option: the verdict and a note on every option open here.
Best answer: D.
Carbamazepine induces its own CYP3A4 metabolism over 2–4 weeks (autoinduction), shortening its half-life and lowering levels below the 4–12 mcg/mL range despite adherence, so the dose is increased and the level rechecked. Confirmed pill counts argue against nonadherence. A reproducible clinical event (a seizure) makes laboratory error unlikely, and saturable Michaelis-Menten kinetics describe phenytoin, whose levels rise rather than fall.
- A
- Saturable kinetics describe phenytoin, whose levels climb rather than fall.
- B
- A breakthrough seizure matching the low level makes laboratory error unlikely.
- C
- Pill counts confirm adherence, so the falling level has another cause.
- D
- Correct. Carbamazepine induces its own metabolism over 2–4 weeks, so levels fall despite adherence; raise the dose and recheck.
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