DRUG
CALCULATIONS
FOR NURSES Third Edition
A STEP-BY-STEP APPROACH
ROBERT LAPHAM BPharm Clin Dip Pharm MRPharmS
Clinical Pharmacist, Sunderland
... [Show More] Royal Hospital, City Hospitals Sunderland NHS Trust, UK
HEATHER AGAR RGN BSC (HONS)
Rheumatology Specialist Nurse, Northumbria Healthcare NHS Trust, UK
First published in Great Britain in 1995 by Arnold
Second edition 2003
This third edition published in 2009 by
Hodder Arnold, an imprint of Hodder Education, an Hachette UK company, 338 Euston Road, London NW1 3BH http://www.hoddereducation.com
© 2009 Robert Lapham and Heather Agar
All rights reserved.Apart from any use permitted under UK copyright law,this publication may only be reproduced, stored or transmitted, in any form, or by any means with prior permission in writing of the publishers or in the case of reprographic production in accordance with the terms of licences issued by the Copyright Licensing Agency. In the United Kingdom such licences are issued by the Copyright Licensing Agency: Saffron House, 6–10 Kirby Street, London EC1N 8TS
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Whilst the advice and information in this book are believed to be true and accurate at the date of going to press, neither the authors nor the publisher can accept any legal responsibility or liability for any errors or omissions that may be made. In particular (but without limiting the generality of the preceding disclaimer) every effort has been made to check drug dosages; however it is still possible that errors have been missed. Furthermore, dosage schedules are constantly being revised and new adverse effects recognized. For these reasons the reader is strongly urged to consult the drug companies’ printed instructions before administering any of the drugs recommended in this book.
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Poison is in everything, and no thing is without poison.The dosage makes it either a poison or a remedy.
Paracelsus (1493–1541)
Medieval physician and alchemist
Preface x
How to use this book xi
Pre-test xii
Basics xii
Per cent and percentages xiv
Units and equivalences xiv
Drug strengths or concentrations xiv
Dosage calculations xv
Moles and millimoles xvi
Infusion rate calculations xvi
Answers xvii
1 First principles 1
Objectives 1
Before we start 1
Sense of number and working from first principles 1
Estimation of answers 3
The ‘ONE unit’ rule 3
Checking your answer: does it seem reasonable? 5
Putting it all together 6
Minimizing errors 7
Part 1: Mathematics 9
2 Basics 9
Objectives 9
Introduction 11
Arithmetic symbols 11
Basic maths 12
Rules of arithmetic 22
Fractions and decimals 25
Roman numerals 35
Powers or exponentials 36
Using a calculator 38
Powers and calculators 42
Estimating answers 42
Contents vii
3 Per cent and percentages 48
Objectives 48
Introduction 49
Per cent and percentages 49
Converting fractions to percentages and vice versa 50
Converting decimals to percentages and vice versa 50
Calculations involving percentages 51
Drug calculations involving percentages 54
How to use the percentage key on your calculator 55
4 Units and equivalences 59
Introduction 60
SI units 60
Prefixes used in clinical medicine 61
Equivalences 62
Conversion from one unit to another 63
Guide to writing units 68
5 Drug strengths or concentrations 71
Introduction 72
Percentage concentration 73
mg/mL concentrations 75
‘1 in …’ concentrations or ratio strengths 77
Parts per million (ppm) 79
Drugs expressed in units 79
Part I1: Performing calculations 81
6 Dosage calculations 81
Introduction 82
Calculating the number of tablets or capsules required 82
Dosages based on patient parameters 83
Ways of expressing doses 86
Calculating drug dosages 87
Displacement values or volumes 91
7 Moles and millimoles 94
Introduction 95
What are moles and millimoles? 96
Millimoles and micromoles 97
Calculations involving moles and millimoles 98
Molar solutions and molarity 101
viii Contents
8 Infusion rate calculations 106
Introduction 107
Drip rate calculations (drops/min) 107 Conversion of dosages to mL/hour 109 Conversion of mL/hour back to a dose 114
Calculating the length of time for IV infusions 117
Part III: Administering medicines 120
9 Action and administration medicines 120
Introduction 121
Pharmacokinetics and pharmacodynamics 122
Administration of medicines 130
Promoting the safer use of injectable medicines 137
10 Infusion devices 140
Introduction 141
Gravity devices 141 Pumped systems 141
Infusion device classification 145
11 Children and medicines 147
Introduction 148
Drug handling in children 148
Routes of administration of drugs 151
Practical implications 152 Useful reference books 156
Approximate values useful in the calculation of doses in children 157
Calculating dosages 158
12 The elderly and medicines 159
Introduction 160
Drug handling in the elderly 160 Specific problems in the elderly 162
General principles 163
13 Sources and interpretation of drug information 165
Introduction 165
Sources of drug information 166
Summary of product characteristics (SPC) 166
Contents ix
Revision test 175
Basics 175
Per cent and percentages 176
Units and equivalences 176
Drug strengths or concentrations 177
Dosage calculations 177
Moles and millimoles 178
Infusion rate calculations 179
Compare your scores 180
Answers to revision test 180
Answers to problems set in chapters 183
3. Percent and percentages 184
4. Units and equivalences 184
5. Drug strengths or concentrations 187
6. Dosage calculations 188
7. Moles and millimoles 195
8. Infusion rate calculations 201
9. Action and administration of medicines 215
Appendices 216
1 Body surface area (BSA) estimates 217
2 Weight conversion tables 221
3 Height conversion tables 223
4 Calculation of Body Mass Index (BMI) 225
5 Estimation of renal function 232
6 Abbreviations used in prescriptions 234
Index 237
Drug treatments given to patients in hospital are becoming increasingly complex. Sometimes, these treatment regimes involve potent and, at times, new and novel drugs. Many of these drugs are toxic or possibly fatal if administered incorrectly or in overdose. It is therefore very important to be able to carry out drug calculations correctly so as not to put the patient at risk.
In current nursing practice, the need to calculate drug dosages is not uncommon. These calculations have to be performed competently and accurately, so as not to put not only the nurse but, more importantly, the patient at risk.This book aims to provide an aid to the basics of mathematics and drug calculations. It is intended to be of use to nurses of all grades and specialities, and to be a handy reference for use on the ward.
The concept of this book arose from nurses themselves;a frequently asked question was: ‘Can you help me with drug calculations?’ Consequently, a small booklet was written to help nurses with their drug calculations, particularly those studying for their IV certificate. This was very well received, and copies were being produced from original copies, indicating the need for such help and a book like this.
The content of the book was determined by means of a questionnaire, sent to nurses asking them what they would like to see featured in a drug calculations book. As a result, this book was written and, hopefully, covers the topics that nurses would like to see.
Although this book was primarily written with nurses in mind, others who use drug calculations in their work will also find it useful. Some topics have been dealt with in greater detail for this reason,e.g.moles and millimoles. This book can be used by anyone who wishes to improve their skills in drug calculations or to use it as a refresher course.
This book is designed to be used for self-study. Before you start, you should attempt the pre-test to assess your current ability in carrying out drug calculations. After completing the book, repeat the same test and compare the two scores to measure your improvement.
To attain maximum benefit from the book, start at the beginning and work through one chapter at a time, as subsequent chapters increase in difficulty. For each chapter attempted, you should understand it a fully and be able to answer the problems confidently before moving on to the next chapter.
Alternatively, if you wish to quickly skip through any chapter, you can refer to the ‘Key Points’ found at the start of each chapter.
A note about drug names
In the past, the British Approved Name (BAN) was used for drugs in the UK. European law now requires use of the Recommended International Non-proprietary Name (rINN) for medicinal substances. In most cases, the old BAN and the new rINN are identical. Where the two differ, the BAN has been modified to the new rINN;for example:amoxicillin instead of amoxycillin.
Adrenaline and noradrenaline have two names (BAN and rINN). However, adrenaline and noradrenaline are the terms used in the titles of monographs in the European Pharmacopoeia and are thus the official names in the member states. The British Pharmacopoeia 2008 shows the European Pharmacopoeia names first followed by the rINN at the head of its monographs (adrenaline/epinephrine);the British National Formulary (BNF) has adopted a similar style.
For a full list of all the name changes, see the current edition of the BNF. Affected drugs that appear in this book will be referred to by their new name (rINN) followed by their old name (BAN) in brackets;for adrenaline, this book will follow the convention used by the British Pharmacopoeia.
Case reports
The journal Pharmacy in Practice highlights real-life medication errors to act as learning points for practitioners. Some of these have been used as Case Reports in this book to illustrate important points to remember.
To obtain the maximum benefit from this book, it is a good idea to attempt the pre-test before you start working through the chapters. The aim of this pre-test is to assess your ability at various calculations.
The pre-test is divided into several sections that correspond to each chapter in the book, and the questions try to reflect the topics covered by each chapter. You don’t have to attempt questions for every chapter, only the ones that you feel are relevant to you. Answering the questions will help you identify particular calculations you have difficulty with.
You can use calculators or anything else you find helpful to answer the questions, but it is best to complete the pre-test on your own, as it is your ability that is being assessed and not someone else’s.
Don’t worry if you can’t answer all of the questions. As stated before, the aim is to help you to identify areas of weakness. Once again, you don’t have to complete every section of the pre-test, just the ones you want to test your ability on.
Once you have completed the pre-test and checked your answers,you can then start working through the chapters. Concentrate particularly on the areas you were weak on and miss out the chapters you were confident with if you wish.
It is up to you as how you use this book, but hopefully the pre-test will help you to identify areas you need to concentrate on.
The pre-test consists of 50 questions and covers all the topics and types of questions in the book. Mark your score out of 50, then double it to find your percentage result.
BASICS
The aim of this section is to test your ability on basic principles such as multiplication, division, fractions, decimals, powers and using calculators, before you start any drug calculations.
Long multiplication Solve the following:
1 678 × 465
2 308 × 1.28
Long division
Solve the following:
3 3143 ÷ 28
4 37.5 ÷ 1.25
Basics
Fractions
Solve the following, leaving your answer as a fraction:
5 3
5 ×
9 7
3 12
6 ×
4 16
3 9
7 ÷
4 16
5 3
8 ÷
6 8
Convert to a decimal (give answers to 2 decimal places):
9
10
Decimals
Solve the following:
11 25 × 0.45
12 5 ÷ 0.2
13 1.38 × 100
14 25.64 ÷ 1,000
Convert the following to a fraction:
15 1.2
16 0.375
Roman numerals
Write the following as ordinary numbers:
17 VII
18 IX
Powers
Convert the following to a proper number:
19 3 × 104
Convert the following number to a power of 10:
20 5,000,000
Pre-test
PER CENT AND PERCENTAGES
This section is designed to see if you understand the concept of per cent and percentages.
21 How much is 28% of 250g?
22 What percentage is 160g of 400g?
UNITS AND EQUIVALENCES
This section is designed to test your knowledge of units normally used in clinical medicine, and how to convert from one unit to another. It is important that you can convert between units easily, as this is the basis for most drug calculations.
Convert the following.
Units of weight
23 0.0625 milligrams (mg) to micrograms (mcg)
24 600 grams (g) to kilograms (kg)
25 50 nanograms (ng) to micrograms (mcg)
Units of volume
26 0.15 litres (L) to millilitres (mL)
Units of amount of substance
Usually describes the amount of electrolytes, as in an infusion (see Chapter 7 ‘Moles and millimoles’ for a full explanation).
27 0.36 moles (mol) to millimoles (mmol)
DRUG STRENGTHS OR CONCENTRATIONS
This section is designed to see if you understand the various ways in which drug strengths can be expressed.
Percentage concentration
28 How much sodium (in grams) is there in a 500 mL infusion of sodium chloride 0.9%? mg/mL concentrations
29 You have a 5mL ampoule of dopexamine 1%. How many milligrams of dopexamine are there in the ampoule?
Dosage calculations
‘1 in ...’ concentrations or ratio strengths
30 You have a 10 mL ampoule of adrenaline/epinephrine 1 in 10,000. How much adrenaline/epinephrine – in milligrams – does the ampoule contain?
Parts per million (ppm) strengths
31 If drinking water contains 0.7 ppm of fluoride,how much fluoride (in milligrams) would be present in 1 litre of water?
DOSAGE CALCULATIONS
These are the types of calculation you will be doing every day on the ward. They include dosages based on patient parameters and paediatric calculations.
Calculating the number of tablets or capsules required
The strength of the tablets or capsules you have available does not always correspond to the dose required. Therefore you have to calculate the number of tablets or capsules needed.
32 The dose prescribed is furosemide (frusemide) 120 mg. You have 40mg tablets available. How many tablets do you need?
Drug dosage
Sometimes the dose is given on a body weight basis or in terms of body surface area. The following questions test your ability at calculating doses based on these parameters.
Work out the dose required for the following:
33 Dose = 0.5mg/kg Weight = 64kg
34 Dose = 3mcg/kg/min Weight = 73kg
35 Dose = 1.5mg/m2 Surface area = 1.55 m2 (give answer to 3 decimal places)
Calculating dosages
Calculate how much you need for the following dosages:
36 You have aminophylline injection 250mg in 10mL. Amount required = 350mg
37 You have digoxin injection 500mcg/2mL. Amount required = 0.75mg
38 You have morphine sulphate elixir 10mg in 5mL. Amount required = 15mg
39 You have gentamicin injection 40mg/mL, 2mL ampoules. Amount required = 4mg/kg for a 74kg patient: how many ampoules will you need?
Pre-test
Paediatric calculations
40 You need to give trimethoprim to a 7-year-old child weighing 23kg at a dose of 4mg/kg twice a day.
Trimethoprim suspension comes as a 50mg in 5mL suspension. How much do you need for each dose?
Other factors to take into account are displacement volumes for antibiotic injections.
41 You need to give benzylpenicillin at a dose of 200mg to a 6-month-old baby. The displacement volume for benzylpenicillin is 0.4 mL per 600mg vial.
How much water for injections do you need to add to ensure a strength of 600mg per 5mL?
MOLES AND MILLIMOLES
This section is designed to see if you understand the concept of millimoles. Millimoles are used to describe the ‘amount of substance’, and are usually the units for body electrolytes (e.g. sodium 138mmol/L).
Moles and millimoles
42 Approximately how many millimoles of sodium are there in a 10mL ampoule of sodium chloride 30% injection? (Molecular mass of sodium chloride = 58.5)
Molarity
43 How many grams of sodium chloride is required to make 200ml of a
0.5M solution? (Molecular mass of sodium chloride = 58.5)
INFUSION RATE CALCULATIONS
This section tests your knowledge of various infusion rate calculations. It is designed to see if you know the different drop factors for different giving sets and fluids, as well as being able to convert volumes to drops and vice versa.
Calculation of drip rates
44 What is the rate required to give 500mL of sodium chloride 0.9% infusion over 6 hours using a standard giving set?
45 What is the rate required to give 1 unit of blood (500 mL) over 8 hours using a standard giving set?
Answers
Conversion of dosages to mL/hour
Sometimes it may be necessary to convert a dose (mg/min) to an infusion rate (mL/hour).
46 You have an infusion of dopamine 800 mg in 500 mL. The dose required is 2mcg/kg/min for a patient weighing 60kg.
What is the rate in mL/hour?
47 You are asked to give 500mL of doxapram 0.2% infusion at a rate of 3mg/min using an infusion pump. What is the rate in mL/hour?
Conversion of mL/hour back to a dose
48 You have dopexamine 50mg in 50mL and the rate at which the pump is running is 21mL/hour. What dose – in mcg/kg/min – is the pump delivering?
(Patient’s weight = 88kg)
Calculating the length of time for IV infusions
49 A 500mL infusion of sodium chloride 0.9% is being given at a rate of 21 drops/min (standard giving set).
How long will the infusion run at the specified rate?
50 A 250mL infusion of sodium chloride 0.9% is being given at a rate of
42mL/hour.
How long will the infusion run at the specified rate?
ANSWERS
12 25
1 315,270
2 394.24
3 112.25
4 30
5
6
7
8
9 0.40
10 0.56 (0.5625)
11 11.25
Answers
13 138
14 0.02564
15
16
17 7
18 9
19 30,000
20 5 × 106
21 70g
22 40%
23 62.5 micrograms
24 0.6 kilograms
25 0.05 micrograms
26 150 millilitres
27 360 millimoles
28 4.5g
29 50milligrams
30 1mg
31 0.7mg
32 Three furosemide (frusemide) 40mg tablets
33 32mg
34 219mcg/min
35 2.325mg
36 14mL
37 3mL
38 7.5mL
39 4 ampoules
40 9.2mL
41 4.6mL
42 51.3mmol (rounded to 51mmol)
Sometimes it is necessary to adjust the dose by rounding like this for ease of calculation and administration, as long as the adjustment is not so much that it makes a large difference to the amount.
43 5.85g sodium chloride
44 27.7 drops/min (rounded to 28 drops/min)
45 15.625 drops/min (rounded to 16 drops/min)
46 4.5mL/hour
47 90mL/hour
48 3.98mcg/kg/min (approx. 4mcg/kg/min)
49 7.94 hours (approx. 8 hours)
50 5.95 hours (approx. 6 hours)
OBJECTIVES
At the end of this chapter, you should be familiar with the following:
• Sense of number and working from first principles
• Estimation of answers
• The ‘ONE unit’ rule
• Checking your answer – does it seem reasonable?
• Minimizing errors
BEFORE WE START
Drug calculation questions are a major concern for most healthcare professionals, including nurses and those teaching them.There have been numerous articles highlighting the poor performance of various healthcare professionals.
The vast majority of calculations are likely to be relatively straightforward and you will probably not need to perform any complex calculation very often. But it is obvious that people are struggling with basic calculations.
It is difficult to explain why people find maths difficult, but the best way to overcome this is to try to make maths easy to understand by going back to first principles. The aim is not to demean or offend anyone, but to recall and explain the basics. Maths is just another language that tells us how we measure and estimate, and these are the two key words.
It is vital, however, that any person performing dose calculations using any method, formula or calculator can understand and explain how the final dose is actually arrived at through the calculation.
SENSE OF NUMBER AND WORKING FROM FIRST PRINCIPLES
There is a risk that calculators and formulae can be used without a basic understanding of what exactly the numbers being entered actually mean; consequently there is a potential for mistakes. Working from first principles and using basic arithmetical skills allows you to have a ‘sense of number’ and in doing so reduces the risk of making mistakes.
Indeed, the NMC Standards for Medicines Management (2008) states:
The use of calculators to determine the volume or quantity of medication should not act as a substitute for arithmetical knowledge and skill.
To ensure that when pharmacists qualify they have basic arithmetical skills and this ‘sense of number’, the Royal Pharmaceutical Society of Great Britain has banned the use of calculators from their registration exam. However, this is not to say that calculators should not be used – calculators can increase accuracy and can be helpful for complex calculations.
The main problem with using a calculator or a formula is the belief that it is infallible and that the answer it gives is right and can be taken to be true without a second thought. This infallibility is, to some extent, true, but it certainly does not apply to the user; the adage ‘rubbish in equals rubbish out’ certainly applies.
An article that appeared in the Nursing Standard in May 2008 also highlighted the fact that using formulae relies solely on arithmetic and gives answers that are devoid of meaning and context. The article mentions that skill is required to: extract the correct numbers from the clinical situation; place them correctly in the formula; perform the arithmetic; and translate the answer back to the clinical context to find the meaning of the number and thence the action to be taken.
How can you be certain that the answer you get is correct if you have no ‘sense of number’? You have no means of knowing whether the numbers have been entered correctly – you may have entered them the wrong way round.
For example, if when calculating 60 per cent of 2 you enter:
×2 instead of ×2
You would get an answer of 3.3 instead of the correct answer of 1.2.If you have a ‘sense of number’ you would immediately realize that the answer 3.3 is wrong.
Another advantage of working from first principles is that you can put your answer back into the correct clinical context.
You may have entered the numbers correctly into your formula and calculator and arrived at the correct answer of 1.2 – but what does it mean? You might mistakenly believe that you need to give 1.2 ampoules instead of 1.2mL.If so,you would need to work out the volume to be drawn up which equals 1.2 ampoules – more calculations and more potential mistakes!
All this may seem unbelievable – but these things do happen.
References
NMC. Standards for Medicine Management (2008). Nursing and Midwifery Council, London.
K Wright. Drug calculations part 1: a critique of the formula used by nurses.
Nursing Standard 2008; 22 (36): 40–42.
The ‘ONE unit’ rule
ESTIMATION OF ANSWERS
Looking at a drug calculation with a‘sense of number’ means that we can often have a ‘rough idea’ or estimate of the answer.
Simple techniques of halving, doubling, addition and multiplication can be used. For example:
1 You have: 200mg in 10mL
From this, you can easily work out the following equivalents:
100mg in 5mL (by halving)
50mg in 2.5mL (by halving again)
150mg in 7.5mL (by addition: 100mg + 50mg and 5mL + 2.5mL)
2 You have: 100mg in 1mL
From this, you can easily work out the following:
500mg in 5mL (by addition: 100mg + 100mg + 100mg + 100mg +
100mg and 1mL + 1mL + 1mL +1mL +1mL)
500mg in 5mL (by multiplication: 100mg × 5 and 1mL × 5)
200mg in 2mL (by doubling)
If estimation is not possible, then rely on experience and common sense. If your answer means that you would need six ampoules of an injection for your calculated dose, then common sense should dictate that this is not normal practice (see later:‘Checking your answer – does it seem reasonable?’).
THE ‘ONE UNIT’ RULE
Various methods are available for drug calculations – we will be using the ‘ONE unit’ rule throughout this book. Using it will enable you to work from first principles and have a ‘sense of number’.
The rule works by proportion: what you do to one side of an equation, do the same to the other side. In whatever the type of calculation you are doing, it is always best to make what you’ve got equal to one and then multiply by what you want – hence the name.
The following example will explain the concept more clearly. We will use boxes in the form of a table to make the explanation easier.
If 12 apples cost £2.88, how much would 5 apples cost?
If we have a ‘sense of number’ we can estimate our answer. Six apples would cost half of £2.88 which would be £1.44; 3 apples would cost half of that: 72p. So 5 apples would cost between 72p and £1.44; probably nearer the upper figure – say £1.20, as a guess.
Now let’s do the calculation using the ‘ONE unit’ rule: Write down everything we know:
12 apples cost £2.88
Then write down what we want to know underneath:
12 apples cost £2.88 5 apples cost ?
We will write everything using boxes in the form of a table:
L R
12 apples cost £2.88
5 apples cost ?
The left-hand side (column L) = what you know and what you want to know.
The right-hand side (column R) = the known and unknown.
First calculate how much one of whatever you have (ONE unit) is equal to.This is done by proportion.Make everything you know (the lefthand side or column L) equal to 1 by dividing by 12:
apples =1apple
As we have done this to one side of the equation (column L), we must do the same to the other side (column R):
L R
12 apples cost £2.88
apples = 1 apple cost
Next, multiply by what you want to know; in this case it is the cost of 5 apples.
So multiply 1 apple (column L) by 5 and don’t forget, we have to do the same to the other side of the equation (right-hand side or column R):
Checking you answer: does it seem reasonable?
L R
12 apples cost £2.88
apples = 1 apple cost
5 apples = 1 × 5 = 5 cost × 5 = £1.20
So 5 apples would cost £1.20.
Working from first principles ensures that the correct units are used and that there is no confusion as to what the answer actually means.
Checking with our original estimation: 5 apples would cost between 72p to £1.44; probably nearer the upper figure – say £1.20, as a guess.
Our guess was the correct answer.
The above is a lengthy way of doing a simple calculation. In reality, we would have completed the calculation in three steps:
12 apples cost £2.88
1 apple cost
5 apples cost × 5 = £1.20
CHECKING YOUR ANSWER: DOES IT SEEM REASONABLE?
As stated before, it is good practice to have a rough idea of the answer first, so you can check your final calculated answer.Your estimate can be a single value or, more usually, a range in which your answer should fall. If the answer you get is outside this range, then your answer is wrong and you should re-check your calculations.
The following guide may be useful in helping you to decide whether your answer is reasonable or not. Any answer outside these ranges probably means that you have calculated the wrong answer.
The maximum you should give a patient for any one dose:
TABLETS Not more than 4*
LIQUIDS Anything from 5mL to 20mL INJECTIONS Anything from 1mL to 10mL
*An exception to this would be prednisolone. Some doses of prednisolone may mean the patient taking up to 10 tablets at any one time. Even with prednisolone, it is important to check the dose and the number of tablets.
Always write your calculations down
PUTTING IT ALL TOGETHER
Using all the above principles, consider the following situation: you have an injection of pethidine with the strength of 100mg per 2mL and you need to give a dose of 60mg.
First – have a rough idea of your answer by estimation. By looking at what you have – 100mg in 2mL – you can assume the following:
• The dose you want (60mg) will be
• less than 2mL (2mL = 100mg)
• more than 1mL (1mL = 50mg – by halving)
• less than 1.5 mL (0.5 mL = 25 mg – by halving and addition: 1mL + 0.5mL = 75mg)
• less than 1.25 mL (0.25 mL = 12.5 mg – by halving and addition:
1 + 0.25mL = 62.5mg) [Show Less]