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1 2014
Contents
Preface v
How to use your textbook vi
About the companion website ix
Part 1
Basic principles of prescribing 1
1 Introduction: principles of good prescribing 2
2 Revision: pharmacokinetics 4
3 Using the British National Formulary 6
4 Taking a medication history 8
5 Reviewing current medicines 10
Part 2
Drug selection 13
6 How to choose a drug 14
7 How to choose frequency 15
8 How to choose a dose 16
9 How to choose route of administration and formulation 18
10 How to choose duration of treatment, define treatment objectives and measure outcomes 20
11 Assessing suitability of treatment regimens for patients 21
12 Worked example 1: Chapters 6–11 22
13 Worked example 2: Chapters 6–11 23
Part 3
Prescribing for special groups 25
14 Prescribing in liver disease 26
15 Prescribing in renal disease 28
16 Prescribing in children 30
17 Prescribing in the elderly 32
18 Prescribing in pregnancy and breast feeding 34
Part 4
Logistics of prescribing 37
19 How to write a drug prescription 38
20 Communicating with patients about medicines 40
21 Therapeutic drug monitoring 42
22 Dealing with adverse drug reactions 44
23 Avoiding drug interactions (drugs, food and alternative medicines) 46
24 Avoiding prescribing errors 48
iii
Part 5
Specific drug groups 51
25 Using drugs for the gastrointestinal system 52
26 Using drugs for the cardiovascular system I 54
27 Using drugs for the cardiovascular system II 56
28 Using drugs for the cardiovascular system III 58
29 Using drugs for the respiratory system 60
30 Using drugs for the neurological system I 62
31 Using drugs for the neurological system II 64
32 Using drugs for infection 66
33 Using drugs for the endocrine system I 68
34 Using drugs for the endocrine system II 70
35 How to use drugs for the musculoskeletal system 72
36 Using drugs in haematology and oncology 74
37 Using drugs in anaesthesia 76
38 An approach to common prescribing requests I 78
39 An approach to common prescribing requests II 80
Appendix Cross references to Prescribing Scenarios at a Glance 81
Index 82
iv
Preface
Prescribing is a core skill for all doctors, and increasingly for non-medical staff. It is a difficult skill to learn. A great deal of attention has been paid to prescribing skills in recent times, and the new Prescribing Safety Assessment has focussed minds on ensuring that new graduates are ready to prescribe safely. I hope that this book will be helpful to students and doctors learning to prescribe. As with many skills, practice is essential, and you should take every opportunity to plan the exact prescription of a drug for patients that you see. Prescribing Scenarios at a Glance provides useful practical examples to work through that are referenced within this text. This book has been written with new graduates embarking on the Foundation Programme in mind, and therefore focuses on mainly hospital-related prescribing; however, many of the principles extend to primary care situations. A highly practical approach has been taken, but will not describe all possible ways to look at a prescribing problem. Seek out the advice of experienced colleagues, whether doctors, nurses or pharmacists, who can provide guidance to the novice prescriber.
This book is deliberately concise, and may be supplemented by Medical Pharmacology at a Glance which gives a useful summary of drug mechanisms of action.
Acknowledgements
Thanks to Dr Mary Joan Macleod for critical review and suggestions.
Further reading
Joint Formulary Committee (2013). British National Formulary 66th ed. British Medical Association and Royal Pharmaceutical Society of Great Britain, London.
Richards D, Coleman J, Reynolds J, Aronson J (2011). Oxford Handbook of Practical Drug Therapy. Oxford University Press, Oxford.
Nicholson TRJ, Singer DRJ (eds) (2014). Pocket Prescriber. CRC Press, London.
v
How to use your textbook
Features contained within your textbook
Each topic is presented in a double-page spread with clear, easy-to-follow diagrams supported by succinct explanatory text.
Your textbook is full of illustrations and tables.
vi
Section not available in this digital edition
vii
Section not available in this digital edition
viii
About the companion website
ix
Basic principles of
prescribing
Part 1
Chapters
1 Introduction: principles of good prescribing 2
2 Revision: pharmacokinetics 4
3 Using the British National Formulary 6
4 Taking a medication history 8
5 Reviewing current medicines 10
Don’t forget to visit the companion website for this book www.ataglanceseries.com/prescribing to do some practice MCQs and case studies on these topics.
1
1
Introduction: principles of good prescribing
Box 1.1 Prescribing framework
Ideally, you should build up a ‘personal formulary’ of drugs for common situations.
When choosing drugs to use consider the following:
What is the diagnosis?
What are you trying to achieve?
Make a list of possible drug classes that could do this
Compare them according to safety, efficacy, suitability and cost
Select a first-choice drug class for this situation
Compare drugs within the class in the same way
Select a first-choice drug for this situation
When you are treating a patient with this type of problem:
Ensure that you have defined the patient’s problem and specified the therapeutic objective
Consider your first-line drug from your personal formulary (or go through the steps above)
Check suitability of the first-choice drug for this patient: Is it likely to be effective? Is it likely to be safe?
Is the form and dose suitable? Is the duration suitable?
If so, start treatment. If not, reconsider. Would a change to the standard regimen for the drug help?
Would a different drug from the same class be suitable? Do you need to go back to the beginning of the process and select a different drug class for this patient?
Once a drug and regimen is selected, start treatment
Give information to the patient
Monitor/stop treatment as appropriate
Worked examples can be found in Chapters 12 and 13.
2
Part 1 Basic principles of prescribing
Prescribing at a Glance, First Edition. Sarah Ross. © 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd.
Companion website: www.ataglanceseries.com/prescribing
P
rescribing is more than writing a drug order on a chart and requires a subset of competencies involving knowledge, judgement and skill. These skills include medication history
taking, reviewing medicines, choosing a new medicine, assessing the suitability of a drug regimen for a patient, writing a prescription, communicating with a patient about their medicines, monitoring drug effects, and dealing with drug-related problems. Prescribing is currently undertaken in a complex healthcare environment with growing numbers of medicines, ageing patients who have increasing numbers of comorbidities, and dwindling resources. Studies have highlighted adverse drug effects and prescribing errors as significant issues. These issues highlight the need for careful and thoughtful prescribing by all prescribers.
Prescribing well is difficult. Opportunities to practice as a student are limited, and looking back many doctors describe an insufficient emphasis on the practical aspects of prescribing in the undergraduate curriculum. This book is one attempt to help by providing clear, concise guidance on how to prescribe safely and effectively, and should be used in combination with practical examples. It could also be helpful as a guide for new graduates as they learn ‘on the job’.
One of the ways to ensure good prescribing is to use a frame-work such as the one described in Box 1.1, which is based on the World Health Organization Guide to Good Prescribing. The steps are outlined in individual chapters of this book. As you develop from a novice into an expert prescriber, the steps will become automatic; however, there will still be times when it is helpful to deliberately work through each one to ensure a good choice is made.
Another element of learning to prescribe is to watch how established practitioners approach it, and to ask why they have selected a particular treatment regimen. This can give insight into the prescribing process, but be careful to consider their choice critically.
Be wary about drug information, particularly if supplied by the manufacturer for marketing purposes. Where possible, seek out unbiased data.
Further Reading
De Vries TPG, Henning RH, Hogerzeil HV, Fresle DA. (1994) Guide to Good Prescribing: A Practical Manual. World Health Organization, Geneva.
3
Chapter 1 Introduction: principles of good prescribing
4
Part 1 Basic principles of prescribing
2
Revision: pharmacokinetics
Figure 2.1 Pharmacokinetics of a single oral dose.
Prescribing at a Glance, First Edition. Sarah Ross. © 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd.
Companion website: www.ataglanceseries.com/prescribing
P
harmacokinetics describes the way in which drugs are pro-
cessed by the body through absorption, distribution, metab-
olism and excretion. Understanding these processes helps
prescribers choose appropriate routes, doses and frequencies of admission, as well as avoid adverse drug reactions in vulnerable populations.
Absorption
In order for a drug to have its intended effect, it must reach the tissues via the systemic circulation. Bioavailability is the term used to describe the percentage of the administered drug that reaches the circulation. For the intravenous route, the bioavailability is 100%. For the oral route, bioavailability is variable as some of the drug is lost between the gut and the systemic circulation. Absorption is affected by the formulation of the drug, as well as its size, lipid solubility and ionisation. Smaller particle size, higher lipid solubility and weaker ionisation will increase absorption. Absorption is also dependent on factors in the gut (pH, motility) that can be affected by food and illness. Metabolism by gut bacteria, by enzymes in the gut or by the liver (known as first pass metabolism) can reduce the amount of drug that reaches the systemic circulation. For some drugs, such as glyceryl trinitrate (GTN), there is so much first pass metabolism that the oral route is unusable.
Other routes of administration can be affected by other factors, but the same principles apply to crossing physiological membranes (i.e. particle size, lipid solubility and ionisation).
Distribution
Once in the systemic circulation, the drug must reach the target site of action. Within the circulation, many drugs are bound to plasma proteins (primarily albumin). Only the ‘free’, unbound drug will be active or eliminated.
In order to maintain a drug concentration within the therapeutic index (see Figure 2.1), drug dosing must be balanced against elimination.
Various parameters can be measured for a drug, which allow dosing regimens to be planned. These are the volume of distribution, half-life and clearance. Many people find these concepts difficult and confusing. One key to understanding is to remember that these are only theoretical measurements that allow dosing regimens to be calculated. It is useful to know something about what these concepts are and how they relate to each other.
The volume of distribution is a measure of how well drugs penetrate tissues. This again relates to the drug’s size, lipid solubility
and ionisation. One particular physiological membrane with special properties is the blood-brain barrier, which is impermeable to many drugs.
Half-life is a measure of how long it takes for the concentration of drug within the body to fall to 50%. It is proportional to the volume of distribution, with widely distributed drugs taking longer to be eliminated from the body.
Clearance is a measure of how quickly a drug is eliminated from the body, either by metabolism or by excretion, or both. This is inversely proportional to the half-life of the drug.
Metabolism
Many drugs are metabolised, primarily to make them easier for the body to excrete. In general, this process occurs in the liver; however, metabolism also occurs in other organs, including the kidney and the lungs.
Liver metabolism takes two main forms in sequence: phase I and phase II reactions. Phase I reactions generally involve oxidation and the main enzymes are from the cytochrome P450 family. Reduction and hydrolysis reactions also occur. Phase II reactions usually involve conjugation with a compound such as glucuronide or glutathione. The aim of all these metabolic processes is to detoxify drugs and to make them more hydrophilic and ionised to facilitate excretion. Occasionally, metabolites can be toxic, or inactive ‘pro-drugs’ can be administered and activated by metabolism.
Metabolism can be disrupted by drug interactions and disease, and is variable with age and between different genetic groups (see Chapter 23).
Excretion
Most excretion occurs in the kidney, although a significant proportion happens through the biliary system.
Renal excretion is based on the glomerular filtration rate. Those drugs that are lipid soluble will be reabsorbed, whereas those that are hydrophilic and ionized, often as a result of metabolism, will be excreted. Some active secretion of drugs in the proximal tubule may occur. Renal damage can therefore lead to toxicity (see Chapter 15).
Certain drugs are excreted in bile but can be reabsorbed back into the enterohepatic circulation. Drugs that have been metabolised and conjugated tend not to be reabsorbed and can be excreted.
5
Chapter 2 Revision: pharmacokinetics
6
Part 1 Basic principles of prescribing
3
Using the British National Formulary
Figure 3.1 Example monograph from the British National Formulary.
(Source: Joint Formulary Committee 2013, p. 374. Reproduced with permission from the British Medical Association and Royal Pharmaceutical Society of Great Britain.)
Prescribing at a Glance, First Edition. Sarah Ross. © 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd. Companion website: www.ataglanceseries.com/prescribing
What the British National Formulary does and does not tell you
The British National Formulary (BNF) is a widely used reliable source of drug information produced by the British Medical Association and the Royal Pharmaceutical Society of Great Britain.
The BNF aims to provide prescribers with an up-to-date quick reference guide to prescription medicines in the UK. It does not include all medicines available for over-the-counter purchase, or alternative medicines. Guidance is produced using a combination of manufacturers’ literature, regulatory information, clinical literature and published guidelines. The BNF for children is a companion volume that aims to deal specifically with paediatric drug use. Less information is included in the BNF about obstetric usage, treatments for malignant conditions and anaesthesia, which are felt to be covered by other specialist literature.
The BNF will give information about drug options, but generally not about how to choose a drug (see Chapter 6). More recently, it has included guidance from expert bodies. One example is the inclusion of the British Thoracic Society guidelines on the treatment of asthma. Prescribers may also need to refer to guidelines and local formularies.
The BNF provides guidance about dose and frequency, but may quote wide ranges from which the prescriber must choose, relying on general principles of dose selection (see Chapter 8). Specific guidance on starting doses and how to titrate a drug may be given along with suggested dose reductions for elderly patients may be supplied for some drug entries. The BNF also gives information about the drug preparations available that can help you make these decisions. Where dosage is calculated by weight, the specific calculation and/or dose by weight ranges are given.
A list of preparations by route of administration is given, but again you may need to choose between several options. The intravenous additives appendix in the BNF gives information on suitable solvents. For some drugs more detailed information about volumes and timing are given, but this is not universal. This information may be available in the literature provided with the drug, or from a pharmacist.
How to find the information you need
Newer versions of the BNF contain the major information in a single monograph. These are arranged by chapter (usually by system), with various levels of subheadings. Some drugs are discussed in more than one chapter, in which case it will be cross-referenced. Drug–drug interactions are listed in Appendix 1 of the BNF. The new version also lists the current adult advanced life support algorithm and emergency drug doses for use in non-hospital settings.
Older versions of the BNF may still be found in many workplaces. In these versions, information about specific drugs in liver disease, renal impairment, pregnancy and breast feeding are in separate appendices at the back.
The electronic BNF is arranged in a similar manner, but in an online format.
A typical monograph (Figure 3.1) will list the indications, cautions, side effects and dose (by indication), followed by information on specific preparations. The preparations section contains other useful information, such as symbols indicating that this is a prescription only medicine (POM), that this is a controlled drug (CD), this is not available for prescription on the NHS (NHS with a strikethrough), a symbol indicating that this preparation is less suitable for prescribing, and the black triangle indicating that this preparation is relatively new and so under heightened scrutiny (this indicates that adverse effects should be reported using the yellow card reporting system).
It is worth looking at the ‘notes for prescribers’ that often precede drug monographs. These may contain guidance from the National Institute for Health and Care Excellence (NICE) or the Committee on Safety of Medicines (SMC). Practical advice on various aspects of drug choice or use is outlined. Further advice on particular issues is given in the introductory chapters: ‘guidance on prescribing’ and ‘emergency treatment of poisoning’. These include information on legal aspects (including prescribing controlled drugs) and on specific patient groups such as those requiring palliative care.
Other sources of information
At times it is necessary to use other sources of information. The BNF is an extremely useful guide for prescribers, but does not always include all the information required for good prescribing decisions.
Guidance on drug choice may be available in a local formulary or protocol, from national expert groups or organisations such as NICE. Clear summaries of evidence for benefit and risk may be given in this guidance or in publications such as Clinical Evidence from the British Medical Journal (BMJ) Group; however, this can be challenging to source and interpret for an individual patient.
Minimal information about the pharmacokinetics of a medicine is included in the BNF. If necessary, you should consult other sources such as the Schedule of Product Characteristics (SPC) produced by the manufacturer (and available at www.medicines.org.uk) or specific sources such as lists of detailed hepatic metabolism pathways. Other guidance, such as The Renal Drug Handbook, may give more information on using specific drugs in particular patient groups. Lists of side effects are not comprehensive and again the SPC may provide more information than the BNF. Side effects are generally listed in order of frequency and by body system, but those thought to be important may be listed first despite being rare. It can be difficult to assess the frequency of side effects from the BNF; these tend to be more clearly set out in the SPC. Drug–drug interactions may be recognised that are not listed in the BNF, and other sources of information such as Stockley’s Drug Interactions may give different lists. However, it is likely that the BNF will cover most of the information needed in most situations.
7
Chapter 3 Using the British National Formulary
8
Part 1 Basic principles of prescribing
4
Taking a medication history
Figure 4.1 Medicines reconciliation form.
Prescribing at a Glance, First Edition. Sarah Ross. © 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd. Companion website: www.ataglanceseries.com/prescribing
Taking a comprehensive history
An accurate and comprehensive medication history is essential for safe and rational prescribing. Omissions and inaccuracies in the medication history may lead to medication errors in as many as two-thirds of patients admitted to hospital. Remember that medicines may be the cause of symptoms, may mask clinical signs and may alter the results of investigations.
The same skills are needed in taking a medication history as in other elements of gathering information. It is usually best to use the term ‘medicines’ when talking to patients rather than ‘drugs’, as the latter can have inappropriate connotations of illegal substances. Start with open questions, focussing down to more specific questions as needed. Closed questions may be effectively employed in clarifying particular elements of the medication history.
A good medication history should include:
Current medicines prescribed – this should include the name, dose, frequency and route of administration along with the indication, duration of therapy and any difficulties experienced
