Therapeutic Drug Monitoring and Prescribing Calculations

This module covers the calculation skills needed before prescribing: unit conversion, dose-to-volume conversion for liquid formulations, weight-based and paediatric dosing with maximum-dose caps, dilutions and displacement values, infusion rates, maintenance intravenous fluids, and creatinine clearance estimation by Cockcroft-Gault. It ends with a checking routine designed to catch ten-fold errors before a prescription is written.

1. Units, Strengths and Converting a Prescribed Dose into a VolumePreview

   This opening lesson builds the unit and strength fluency that every later dose calculation depends on. It covers conversion between grams, milligrams, micrograms and nanograms, the meaning of percentage, ratio (1 in x) and mg/mL strengths, and how to turn a prescribed dose into a volume of liquid formulation. It then sets out a simple sense-check of the answer, including plausibility against the formulation available and the maximum dose, and notes where strengths, formulations and unit conventions differ between regions and formularies. Written for final-year medical students preparing for finals and the Prescribing Safety Assessment, final-year pharmacy students, and foundation doctors working in UK practice with British National Formulary dosing.

   10 min
2. Weight-Based and Paediatric Dosing, Maximum-Dose Caps, Dilutions and Displacement Values

   This lesson covers the calculations behind weight-based prescribing for children, neonates and adults. It shows how to turn an mg/kg or microgram/kg dose into a total dose and then into a volume to draw up, keeping units explicit at every step. It then shows how a maximum-dose cap overrides the calculated figure, the usual reason a paediatric dose stops rising once a child reaches a certain weight. Worked paediatric and neonatal examples are used, including cases where rounding and measurement error matter. The second half deals with reconstitution: the concentration of the resulting solution after a simple dilution, and displacement values, which explain why the final volume of a reconstituted powder injection is usually greater than the volume of diluent added. It closes with common ten-fold errors and a short checking routine. Doses are teaching examples only; confirm against the current formulary and local policy, as practice varies by region.

   10 min
3. Infusion Rates, Intravenous Maintenance Fluids and Creatinine Clearance: A Checking Routine

   This final lesson in the module covers infusion rate calculations in mL/hour and micrograms/kg/minute, intravenous maintenance fluid volumes with their sodium, potassium and glucose content, and estimation of creatinine clearance using the Cockcroft-Gault equation. It notes where local protocols and formularies differ, and builds a stepwise checking routine that catches ten-fold errors. Teaching examples are used only to illustrate calculation methods; prescribing decisions depend on current local guidance, the prescription, and the patient. The lesson is written for final-year medical students preparing for finals and the Prescribing Safety Assessment, for pharmacy students, and for foundation doctors in UK practice.

   10 min
4. Prescribing: Prescribing for a 2-Year-Old with Fever and Pain
   15 min
5. OSCE station: Prescribing IV Maintenance Fluids for a 70 kg Man Nil by Mouth
   11 min
6. Calculation practice: Prescribing Calculations for Safe Practice: Units, Doses, Fluids and Renal Function
   15 min
7. Quiz: Prescribing Calculations for Safe Practice: Units, Doses, Fluids and Renal Function
   9 min

Applied pharmacokinetics for prescribers: clearance, volume of distribution, half-life, time to steady state and when a loading dose is needed. The module then applies these principles to phenytoin, digoxin and lithium, covering correct sampling times, interpretation of levels including correction of phenytoin for low albumin, the precipitants that push each drug into toxicity, and how each toxicity presents. Doses given are teaching examples, and practice varies by region and formulary.

1. Clearance, Volume of Distribution, Half-Life and Steady State: The Principles Behind a Drug Level

   This lesson sets out the pharmacokinetic principles that underpin therapeutic drug monitoring. It defines clearance, volume of distribution and half-life, and shows how they combine to determine the steady-state concentration and the time taken to reach it. It explains why a loading dose is used when a rapid effect is needed, and why falling renal function and enzyme inhibition push steady-state levels upwards. It closes with the general rules for when to take a trough sample, preparing the ground for phenytoin, digoxin and lithium in the two lessons that follow. Doses quoted in the lesson are teaching examples; the platform's deterministic rules, not the reader, decide whether a prescription is safe.

   10 min
2. Phenytoin: Saturable Kinetics, Albumin Correction and Recognising Toxicity

   Phenytoin is the standard example of a drug whose metabolism becomes saturated within the therapeutic range, so small dose increases can produce large and unpredictable rises in plasma level. This lesson explains the kinetics behind that behaviour, sets out when to take a phenytoin level, shows how to correct a total level for low albumin, and describes how toxicity presents and what precipitates it. Clinical features such as nystagmus, ataxia, dysarthria, confusion and gum hypertrophy are covered, together with the dose changes, interacting drugs and intercurrent illness that push levels up. Levels and doses are given as teaching examples only: always check the current British National Formulary, the prescribing information and local protocols.

   10 min
3. Digoxin and Lithium: Sampling, Interpretation and the Precipitants of Toxicity

   This lesson covers therapeutic drug monitoring of digoxin and lithium: when to take the sample, what the result means, and which clinical changes push each drug towards toxicity. It sets out the timing rules, digoxin sampled at least 6 hours after a dose and lithium at a standard 12 hours after the dose, then explains how to read the number alongside the patient in front of you. Renal impairment, dehydration, hypokalaemia, thiazide diuretics and dose increases each raise steady-state concentrations by a different mechanism, and each produces a recognisable clinical picture. The lesson links every level and every precipitant to the signs it causes, using UK practice and British National Formulary style dosing as the reference point. Doses quoted are teaching examples only.

   10 min
4. ADR case: Ataxia, nystagmus and gum overgrowth after phenytoin dose increase
   15 min
5. ADR case: Nausea, yellow-tinged vision and bradycardia in an 80-year-old woman on digoxin
   15 min
6. ADR case: Lithium toxicity after introduction of bendroflumethiazide
   15 min
7. PK lab: Applied Pharmacokinetics and Therapeutic Drug Monitoring: Phenytoin, Digoxin and Lithium
   15 min
8. Calculation practice: Applied Pharmacokinetics and Therapeutic Drug Monitoring: Phenytoin, Digoxin and Lithium
   15 min
9. Quiz: Applied Pharmacokinetics and Therapeutic Drug Monitoring: Phenytoin, Digoxin and Lithium
   9 min

This module covers once-daily gentamicin and intravenous vancomycin: selecting the dose from weight and renal function, timing and interpreting drug levels, and recognising nephrotoxicity, ototoxicity and vancomycin infusion reactions. Teaching examples follow UK practice and British National Formulary dosing; local nomograms, protocols and formularies vary.

1. Once-Daily Gentamicin: Selecting the Dose and Timing the Level

   This lesson covers once-daily gentamicin dosing in UK practice. You will learn how to choose between actual, ideal and adjusted body weight, estimate renal function with Cockcroft-Gault, and adjust the dose and interval in obesity and renal impairment. It explains when to take the post-dose level, how to read it against the local nomogram, and how the result determines the next dose and timing.

   10 min
2. Intravenous Vancomycin: Loading, Maintenance Dosing and Monitoring

   A practical lesson on prescribing intravenous vancomycin for adults in UK practice. It covers how to calculate a loading dose from body weight, indication and the severity of infection, and why the loading dose is generally not reduced in renal impairment. Maintenance dosing is then worked through using renal function, age and weight to choose a dose and interval. The lesson compares trough-guided and AUC-guided monitoring, setting out the target, the sampling time and the practical drawbacks of each approach, and notes where local policy and formularies differ. The final part deals with rate-related infusion reactions such as flushing, pruritus and hypotension, and how infusion rate, dilution volume and timing of the dose reduce them. Doses quoted are teaching examples only; always follow current prescribing information and local protocols.

   10 min
3. Recognising and Preventing Ototoxicity and Nephrotoxicity: Aminoglycosides and Glycopeptides

   This lesson explains how aminoglycosides such as gentamicin and glycopeptides such as vancomycin can damage the kidney and the ear. It identifies patients at greatest risk, including those with renal impairment, older age, obesity, or concurrent nephrotoxins. It covers baseline assessment, monitoring of renal function and drug levels, dose adjustment by weight and renal function, duration limits, and what to do when a level is high, renal function falls, or a patient reports tinnitus, hearing change, or vertigo. Practice points reflect UK prescribing and monitoring, but local protocols and formularies vary. Content is for education and training, not a substitute for clinical judgment.

   10 min
4. Prescribing: Urinary sepsis: empirical intravenous antibiotics including gentamicin in an obese patient with renal impairment
   15 min
5. OSCE station: Gentamicin Dose Calculation and Monitoring
   11 min
6. ADR case: Vertigo and hearing loss in a patient with infective endocarditis
   15 min
7. ADR case: Flushing, pruritus and hypotension during a rapid intravenous vancomycin infusion in a 45-year-old man
   15 min
8. Calculation practice: Aminoglycoside and Glycopeptide Prescribing: Gentamicin, Vancomycin and Toxicity Monitoring
   15 min
9. PK lab: Aminoglycoside and Glycopeptide Prescribing: Gentamicin, Vancomycin and Toxicity Monitoring
   15 min
10. Quiz: Aminoglycoside and Glycopeptide Prescribing: Gentamicin, Vancomycin and Toxicity Monitoring
    9 min

Laboratory monitoring and interpretation of results after starting or changing common drugs: potassium and creatinine with renin-angiotensin blockade and spironolactone, sodium with carbamazepine, SSRIs and diuretics, and thyroid, liver and lung tests with amiodarone. Also covers interpreting a raised INR and a structured method for answering data-interpretation questions.

1. Potassium and Creatinine Monitoring with ACE Inhibitors, ARBs and Spironolactone: Baseline, Follow-up and Stopping Thresholds

   ACE inhibitors, angiotensin receptor blockers and spironolactone all reduce renal potassium excretion and can raise creatinine, so each needs baseline and follow-up urea, electrolytes and creatinine. This lesson covers when to take baseline bloods, when to repeat them after starting or up-titrating, how much creatinine rise is expected rather than harmful, and the potassium levels that prompt dose reduction, withholding or stopping. It identifies the risk factors that change those decisions, including chronic kidney disease, diabetes, dehydration, NSAIDs and other potassium-sparing drugs, and shows how to record the result and the action. Thresholds vary between local protocols and the British National Formulary, so always check the current formulary and local guidance.

   10 min
2. Sodium Monitoring and Hyponatraemia with Carbamazepine, SSRIs and Diuretics: Mechanisms, Timing and Action Thresholds

   Hyponatraemia is a common and sometimes serious adverse effect of carbamazepine, selective serotonin reuptake inhibitors and diuretics, but the mechanisms are not the same. Carbamazepine and SSRIs act largely through antidiuretic hormone, while diuretics disturb renal salt and water handling and can also cause volume depletion. This lesson explains why sodium falls with each class, what baseline and repeat sodium monitoring is reasonable after starting or increasing a dose, which symptoms suggest clinically important hyponatraemia, and when to withhold, stop or escalate. It also stresses that monitoring intervals and action thresholds vary between local guidelines, formularies and patient groups, so local policy must always be checked. Written for final year medical students, pharmacy students and foundation doctors; pharmacy students do not sit the Prescribing Safety Assessment.

   10 min
3. Amiodarone Monitoring and the Structured Data-Interpretation Method: Thyroid, Liver and Lung Tests, and a Raised INR

   Amiodarone has a very long half-life and is often started with a loading regimen, so its effects and adverse reactions accumulate and persist long after the dose is reduced. This lesson covers the baseline and periodic monitoring amiodarone requires: thyroid function, liver enzymes and bilirubin, chest imaging and respiratory symptoms, eye examination and neurological review, and the action to take when results are abnormal. It then teaches a three-step method for data-interpretation questions, identify the abnormality, name the culprit drug, choose the action, and works it through a raised INR in a patient taking warfarin who has started amiodarone. Doses and monitoring intervals here are teaching examples; always check the current BNF and local protocols, because practice varies by region and formulary.

   10 min
4. OSCE station: Interpretation of Blood Results: Acute Kidney Injury and Hyperkalaemia in a Patient on Ramipril and Spironolactone
   11 min
5. OSCE station: Pharmacology Viva: Amiodarone
   9 min
6. ADR case: Hyperkalaemia and gynaecomastia in a 66-year-old man on ramipril and spironolactone
   15 min
7. ADR case: Hyponatraemia and unsteadiness in a 45-year-old man on carbamazepine
   15 min
8. ADR case: Weight loss, tremor, and atrial fibrillation recurrence in a 68-year-old man on an antiarrhythmic agent
   15 min
9. Quiz: Laboratory Monitoring and Data Interpretation: Renal and Electrolyte Safety Checks, Amiodarone Monitoring and the Structured Prescribing Safety Assessment Approach
   9 min