Chapter objectives

. Understand the premise of this book and how it will help you to pass the Prescribing Safety Assessment (PSA).

. Learn how best to work your way through this book and how to follow the basic principles of safe prescribing.

Introduction

Drug prescribing is one of the most important parts of clinical practice. Yet it remains one of the most commonly failed components of undergraduate assessments and accounts for an uncomfortably high proportion of medical errors. To remedy this the PSA has been introduced. The General Medical Council expects that all UK undergraduates will pass this exam during their final year (though at the time of going to press, there is uncertainty over the date of such enforcement).

The exam comprises eight ‘sections’ within which different facets of prescribing are assessed (see Fig. 1.1). Students must complete the assessment within 2 hours. There are 200 marks available; the Prescribing and Prescription Review sections are assessed extensively and carry the most marks (112/200 marks in total).

How to use this Book

Pass the Prescribing Safety Assessment is written specifically for the exam, with one chapter dedicated to each PSA section. This chapter outlines the universal basic principles of prescribing for all sections and includes discussions of two common concepts applicable throughout. Chapter 2 introduces a simple, memorable and fail-safe approach to prescribing (the PReSCRIBER mnemonic) and each subsequent chapter builds on the previous, creating a robust prescribing method for both the PSA and foundation years. Each chapter also discusses the section’s question structure and how to approach it. Questions (structured identically to the exam) conclude each chapter and cover all scenarios suggested for questioning in the PSA blueprint (2012). Finally, two mock exams (which should be completed within 2 hours each) enable consolidation of previous learning.

The chapter order imitates clinical practice and each chapter consolidates the work of previous ones. Knowing the correct diagnosis reflects accurate data interpretation (Chapter 3), which in turn enables the deduction of appropriate management strategies (Chapter 4). Some treatments require communication of specific information to patients (Chapter 5) and others require calculation skills (Chapter 6), before safely prescribing (Chapter 7). Finally, drug monitoring (Chapter 8) attempts to prevent some adverse drug reactions (Chapter 9).

A secondary aim of the book is to summarize the clinical knowledge required to pass the PSA. There is little merit in limiting learning to drug-related data interpretation or management while ignoring the substantial non-pharmacological remainder: this arbitrary distinction does not avail itself in clinical practice, and the PSA will include scenarios where a drug is not to blame. Consequently, Chapters 3 and 4 include concise, yet comprehensive, summaries of data interpretation and management algorithms, with the most common causes emboldened, and drug-related causes (which will of course be over-represented in the exam) emboldened/italicized.

In the exam you will have access to the British National Formulary (BNF) for every section. If in doubt, look it up! Many questions will require even the most capable candidate to look up an answer, so you should become familiar with it during your preparations. The BNF will even tell you what to prescribe in common clinical situations (just look up the symptom or diagnosis in the index), but you will not have time to do this for each question. It is therefore advisable to learn the theory in each chapter and use the BNF as a backup. The BNF includes sections on antibiotic choice for different infections; you can use this or your local antibiotic policy while preparing, but you should know the first line antibiotic choices for common infections. Drug charts have been provided where written prescriptions are required (Chapter 7), so no additional materials beyond a BNF are needed whilst using this book.

Basic Principles for all Prescribing

Every drug prescription must be:

Marks are awarded for each of these points.

Enzyme Inducers and Inhibitors (See Chapters 8 and 9)

Most of this book concerns the effects of drugs on the body (pharmacodynamics). Conversely, what the body does to the drug (known as pharmacokinetics, including absorption, metabolism and excretion) is slightly less relevant in a practical guide to prescribing, as these processes are reasonably stable and thus a drug’s effect is usually predictable. Problems arise, however, when other substances (in this case concomitantly administered drugs) unintentionally alter these complex systems resulting in increased or decreased drug levels and hence altered effects. Thus, from an early stage it is important to recognize that when you prescribe particular drugs (which are also enzyme inhibitors or enzyme inducers) they may affect seemingly unrelated drugs (Table 1.1).

Most drugs are metabolized to inactive metabolites by the cytochrome P450 enzyme system in the liver, preventing them exerting infinite effects. The activity of these enzymes, however, may in turn be altered by the presence of other particular drugs, known as enzyme inducers and inhibitors. An enzyme inducer will increase P450 enzyme activity, hastening metabolism of other drugs with the result that they exert a reduced effect (and thus a patient will require more of some other drugs in the presence of an enzyme inducer). Conversely, an enzyme inhibitor will decrease P450 enzyme activity and, subsequently, there will be increased levels of other drugs (which in the hands of a diligent physician require a reduced drug dose).

The classic example of this is the effect of newly introduced enzyme inhibitors on patients taking warfarin. In particular, the addition of erythromycin (an enzyme inhibitor) can sometimes and unpredictably cause a dangerous rise in international normalized ratio (INR) if the warfarin dose is not decreased; you should be aware of this in patients presenting with excessive anticoagulation (see Chapter...