SCRIPT: dosing and calculation

Question 1

Calculation errors - examples

Answer 1

• Miscalculating the concentration of a medicine in a solution.
• Moving the decimal point in the wrong direction when converting dose units, eg. milligrams to grams.
• Over or under estimating the time interval between doses.
• Miscalculating the number of tablets to be administered.
• Miscalculating the rate of administration

Question 2

Common drugs involved in dosing errors

Answer 2

• Insulin
• Warfarin
• Midazolam
• Any medication that comes in different strengths solutions (eg. oral morphine)

Question 3

How to minimise risk of error

Answer 3

• Ask yourself if that dose seems reasonable
• Check that it does not exceed the maximum daily dose in the BNF
• Have second person independently do the calculation, then compare answers

Question 4

Patient weight calculation.

a) TBW/ABW - when useful? - Which drugs use ABW?
b) IBW - why is it useful? - Which drugs may require IBW?
c) Lean body weight
d) What metric may be used in pregnant women?
e) Formula for paediatric weight approximation
f) IV aciclovir is dosed based on what metric?
g) 1 stone = ? kg
h) 1 pound (lb) = ? grams

Answer 4

a) The patient’s weight as measured by scales
- Generally works well provided patient is not obese or cachectic
- Lipid-soluble drugs generally use ABW
- If BNF does not specify which weight to use, use the ABW generally

b) - Accounts for the fact that an obese person will likely have the same volume of circulating blood as a person with the same height and normal BMI
- Useful for water-soluble drugs like gentamicin (if given by ABW, obese patient would likely receive a toxic level of the drug)

c) ABW minus fat weight
d) Early pregnancy body weight (first trimester weight)
e) (Age +4) x 2 = weight in kg

f) Body surface area
- Square route of … [weight (kg) x height (cm) / 3600]
(some anti-cancer drugs also use this)

g) 1 stone is equivalent to 6.35 kg (roughly x6)
h) 450 grams (roughly half it)

Question 5

Percentage concentrations.*

a) % w/w - explain (e.g. hydrocortisone 0.5% w/w)
b) % w/v - explain (e.g. NaCl 0.9% w/v)
c) % v/v - explain (ChloraPrep, containing isopropyl alcohol 70% v/v)

*Note: 1% solutions are 1 g (or ml) per 100 ml

• think: teaspoon can hold 5 g sugar or 5 ml water
  (g ~ ml)

Answer 5

a) % w/w = drug weight / total product* weight
- Example: Hydrocortisone 0.5% w/w
= 0.5 g of hydrocortisone in 100 g of the cream

*Total product = drug + diluent

b) % w/v = drug weight / total volume
- Example: Sodium chloride 0.9% w/v for infusion
(0.9% = 0.9 g in 100 ml)
= 0.9 g NaCl in 100 ml of the infusion

c) % v/v = drug volume / total volume
- Example: ChloraPrep 70% v/v
(70% = 70ml in 100 ml)
= 70 ml ChloraPrep in 100 ml volume (i.e. 30 ml diluent)

Question 6

Drug ratios.

a) Adrenaline 1 in 1000 = ?
b) Adrenaline 1 in 200 000 =

Answer 6

a) 1 g adrenaline per 1000 mls (= 1 mg per ml)

b) 1 g adrenaline per 200,000 mls (= 5 micrograms per ml)

Question 7

Opioid conversions

a) generally converted to what…?
b) where to find in BNF?
c) PO codeine/tramadol to PO morphine
d) PO morphine to SC/IV/IM morphine

Answer 7

a) Equivalent oral daily dose of morphine
b) Guidance > Prescribing in palliative care > Pain management with opioids

c) Divide by 10
- 240 mg codeine = 24 g morphine

d) Divide by 2
- 20 mg PO morphine = 10 mg parenteral morphine

Question 8

Benzodiazepine equivalence.

a) Generally convert to equivalent dose of….?
(eg. for withdrawal/ switching benzo)
b) Where can you find the conversions in the BNF?

Answer 8

a) Diazepam:
- convert to this and gradually reduce to zero if withdrawing benzos
- generally reduce dose by a small amount every 2 - 4 weeks
- may take months in severely dependent patients

b) Treatment summaries > Hypnotics and anxiolytics > Dependence and withdrawal

Question 9

Corticosteroid equivalence.

a) Generally compared with 5 mg dose of…?
b) Issue with this?
c) Patient has been on IV hydrocortisone 50 mg QDS. Convert to oral prednislone
d) Where to find in BNF?
e) Compare dexamethasone with hydrocortisone

Answer 9

a) Prednisolone
b) Only accounts for glucocorticoid effect (neglects mineralocorticoid effects)

c) 50 mg QDS = 200 mg hydrocortisone per day
(1 mg prednisolone = 4 mg hydrocortisone)
… 200 / 4
= 50 mg prednisolone per day (once daily)

d) Treatment summaries > Endocrine system > Glucocorticoid therapy

e) - Dexamethasone - very high glucocorticoid effect with minimal mineralocorticoid effect (very good anti-inflammatory effect with minimal fluid retention)
- Hydrocortisone - high mineralocorticoid effect with lower glucocorticoid effect (more useful for aldosterone deficiency)

Question 10

Infusion rates.

a) 1 litre over 8 hours - equivalent rate in ml/hr

Answer 10

a) 1000 / 8 = 125 ml/hr

Question 11

NAC for paracetamol OD.

a) First you must know the patient’s…?
b) 1st dose - ?
c) 2nd dose - ?
d) 3rd dose - ?
e) Where to find in BNF?*
f) What is the dose of NAC present in 17 ml of 200mg/ml concentrate?

*Note: in BNF, the volume of acetylcysteine concentrate 200mg/ml for IV infusion is given for each of the 3 infusions (based on the dose in mg/kg)

Answer 11

a) Actual body weight (capped at 110 kg)

b) - 150 mg/kg NAC
- Diluted in 200 ml 5% glucose
- Given over 1 hour

c) - 50 mg/kg NAC
- Diluted in 500 ml 5% glucose
- Given over 4 hours

d) - 100 mg/kg NAC
- Diluted in 1000 ml 5% glucose
- Given over 16 hours

e) Poisoning > Paracetamol overdose
f) 200 x 17 = 3400 mg (= 3.4 g)