Basic Principles of Prescribing Flashcards
Define:
- Compliance
- Adherence
- Concordance
Compliance
- Extent to which the patient’s behaviour matches the prescriber’s recommendations
- Not as favoured as more paternalistic and patient blaming when termed ‘non-compliant’
Adherence
- Extent to which patient’s behaviour matches recommendations from the prescriber
- Preferred term, based on shared decision making
Concordance
- The doctor and patient must come to an agreement regarding therapeutic decisions
- Can be applied to prescribing: the process of prescribing and medicine-taking based on partnership
- Founded on adequate and well communicated knowledge, empowerment and ongoing problem solving where patient is intimately involved throughout
What is some epidemiology of non-adherence?
As low as 50% of people are non-adherent to long-term therapies (even lower in developing countries)
Conditions such as:
- Asthma**
- HTN
- DM
- HIV/AIDS
- Epilepsy
- Dyslipidaemia
- Multimorbidity
- 30-65yrs > Children > adolescents
This naturally has both serious negative health and financial effects on patients and the NHS
What are some reasons for non-adherence?
Socioeconomic factors e.g. age, gender, finances
Health system factors e.g. poor instructions
Therapy related factors e.g. adverse effects, complex regimens
Patient factors e.g. disagreement with necessity
Condition related factors e.g. dysphagia post stroke
How do we improve adherence?
Health professional led:
- Sympathetic, exploratory approach
- Emphasis on clear communication
- Explaining condition, +/- of Tx, explore preferences and barriers
- Providing relevant literature/patient info leaflets
Encourage people to monitor their own symptoms e.g. home BP monitoring
Switching formulations - to longer acting/modified release, depot injections, smaller tablets, combined preparations, different delivery devices etc
Monitored Dosage Systems (MDS) e.g. Dossette
Close follow up, telephone appts
Counselling, family involvement, psychosocial interventions
What is the Yellow Card scheme?
A site allowing you to report the adverse effects of treatment
- Can encourage own monitoring of this at home
There is even an app
What are options to help with prescription costs?
Rarely needed as 89% of all prescriptions are provided free
Prescription Pre-Payment Scheme:
- If your patient is not entitled to free prescriptions, and they have to pay for more than eleven prescribed medicines each year (or more than three prescribed medicines in three months), then it may be cheaper to purchase a PPC, which spreads the cost of prescriptions across 12 months (or 3 months).
- Can purchase a PPC online or in some pharmacies
NHS Low Income Scheme:
- HC1 form for initial assessment (found online) then
- HC2 certificate (which allows free prescriptions) or an HC3 certificate (which provides some help toward prescription charges)
What is the new medicines service?
Pharmacy-based intervention for those with long term conditions:
- Asthma, CPOD
- T2DM
- Antiepileptic and anticoagulants
- HTN
Semi-structured interview schedule to assess adherence
- 7-14 days after initial dispensing then an additional session at 14-21 days post that
How do you reduce calculation errors when prescribing?
Get an independent check
- Get someone else to perform the calculation you are performing separately and compare answers (dont do it together, or just get someone else to look at your answer)
Sense check
- What is the approximate answer I am expecting?
- Does the dose seem reasonable?
- Ensure the dose does not exceed the adult dose/maximum daily dose
For calculations based on weight
- Ensure they have been weighed recently and that it is accurate
- May be other terms used e.g. actual body weight, total body weight, lean BW, ideal BW, adjusted BW etc
- If not obese or cachexic the go for actual - from the most recent weight
- If obese or cachexic - other formulae need using
- But ultimately depends on whether the drug distributes in water or fat
What are some key conversion factors?
kg - mg - microgram - nanogram = differing by a factor of 1000 each time
What are percentage concentrations?
% w/w
- Percentage weight per weight i.e. defined weight of medicine added to defined weight of dilutent
- In a 1% w/w preparation there is 1 g of drug/chemical in 100 g of the final product
- e.g. hydrocortisone acetate BP cream 0.5% w/w contains 0.5 g of hydrocortisone in 100 g of the cream
- e.g. sodium chloride 0.9% contains 0.9 g in 100 ml, which is equivalent to 900 mg/100 ml. Therefore, 900 mg/100 = 9 mg in 1 ml
% w/v
- Percentage weight per volume
- i.e. weight of medicine/chemical added to volume of dilutent
- In a 1% w/v solution there is 1 g of drug/substance in 100 ml of the final product
- e.g. sodium chloride 0.9% w/v for infusion contains 0.9 g of sodium chloride in 100 ml of the infusion
% v/v
- Percentage volume per volume
- i.e. Volume of of chemical added to volume of dilutent
- In a 1% v/v solution there is 1 ml of liquid drug/chemical in 100 ml of the final product
- e.g. ChloraPrep, containing isopropyl alcohol 70% v/v contains 70 ml of isopropyl alcohol in 100 ml of solution (i.e. 30 ml of diluent)
How do you work with concentrations expressed as ratios?
g: ml i.e.
1: 1000 = 1 g in 1000 ml
1: 10,000 = 1 g in 10 000 ml
So that:
- Adrenaline 1 in 1000 = 1 mg per ml
Adrenaline 1 in 200 000 = 5 micrograms per ml
What information is required when prescribing infusions?
- The name of the active drug
- The total quantity of the active drug to be added to the infusion (if applicable)
- The name and concentration of the diluent (if applicable)
- The total volume of the syringe or infusion bag
- How long the infusion is to be administered over
- The infusion rate (e.g. ml/hour)
- The infusion rate range (usually in ml/hour) (where applicable)
- The intended dose (e.g. microgram/kg/hour) (where applicable for continuous infusions)
- The route of administration
It is useful for you to know how many millitres are delivered per hour based on the volume and total time prescribed:
- e.g. you should know how many ml/hour are delivered if you ask for 1 litre of fluid to be given over 8 hours - hat way you’ll know if the infusion pump is running correctly
Example:
1 litre of fluid to be given over 8 hours is equivalent to
1000 ml infused over 8 hours =
1000/8 = 125 ml/hour
What is a worked example of giving a drug by infusion?
Vancomycin should be diluted to a concentration of up to 5 mg/ml and run at a rate not exceeding 10 mg/minute
- We want to give 1g
1) Dilution
- 1g = 1000mg
- 1000mg in 1000ml = 1mg/ml
- We want 5mg/ml
- Therefore 1000ml/5 = 200ml
- As 200ml bags do not exist, we use 250ml bag
2) Rate
- Run at no more than 10mg/min
- Therefore 1g (1000mg) needs to run over 100 minutes (1000mg/10 = 100min)
- What volume contains 10mg in the bag size we have diluted it to?
- In our 250ml bag we have 1g (1000mg) - 1000mg/250ml = 4mg/ml
- 10mg = 2.5ml
- So solution needs to run at at least 2.5ml/min
- Can convert to hour (x60) = 150ml/hr
How do you calculate the volume of a parentral drug to be administered or added to an infusion?
1) Volume required = (Dose prescribed x volume of solution)/Amount of drug in solution
Example:
- A 12-year-old girl weighing 40 kg is prescribed 800 mg of phenobarbital as a slow intravenous injection, based on an initial 20 mg/kg dose
- Phenobarbital is available on the ward as 1 ml ampoules containing 60 mg/ml
- Dose prescribed = 800 mg
- Volume of solution = 1 ml
- Amount of drug in solution = 60 mg
- Volume required = (800 mg x 1 ml) / 60 mg = 13.3 ml
ALTERNATIVE METHOD:
- Volume required = (dose you want to give/what you’ve got to give) x what it’s in/solute
- (800mg/60mg)x1ml = 13.3ml
Ensure everything has the same units else errors…
How do you calculate percentage change?
Used for dose reductions under certain circumstances i.e. reduce the dose TO 75% (not BY) if creatine clearance 30-50ml/min
A 25% reduction means that you have 75% of the dose remaining, therefore:
- Step 1: First we convert the percentage (X) to a decimal by dividing it by 100 (X/100) - X = 75, 75/100 = 0.75
- Step 2: 0.75 x original dose e.g. 2300mg = 1725 mg twice a day
