Drug Dose

Calculations-1

1
Bioavailability(F):

2
Volume of distribution(V):

V(L/kg) =

3
4
•

5
•

6
 Case scenario:
Oral Digoxin is to be used in a
patient with congestive cardiac
failure. The patient’s age is 63
years and he weighs 84 kgs. 0.9
ng/mL is the target plasma
concentration(CP ). Creatinine
clearance of the patient is 56
mL/min(CLCR).

Bioavailability= 0.7
Volume of distribution,
V=5.9L/kg

Calculate the loading dose and

maintenance dose.
7
Answer:
Loading dose = volume of distribution(V) × target plasma conc.(CP)
Bioavailability(F)

= (5.9 L/kg×84 kgs) ×0.9 ng/mL

0.7
= 637.2 μg
≈ 0.637mg
0.637/0.125 ≈ 5 tablets

8
 Digoxin clearance (CLD) = 0.33× weight + (0.9× creatinine clearance)
= 0.33 × 84 + (0.9 × 56)
= 78.12 ml/min

Maintenance dose = Digoxin clearance (CLD) × Target plasma conc.(CP )

Bioavailability(F)

= (78.12 mL/min ×0.9ng/mL)

0.7
= 100.44ng/min
= 100.44×60×24 ng/day
= 144633.6 ng/ day
≈ 0.144 mg/day ≈ 1 tablet

9
 2. Dose in renal impairment
The normal therapeutic adult dose of streptomycin is 1000 mg per day. Its clearance is
almost entirely by glomerular filtration. A male patient weighing 70 kgs & aged 50 years
with renal disease has serum creatinine 2.4 mg/dl. What shall be the corrected
streptomycin dose in such a situation?

Use the following additional information

1. Normal creatinine clearance= 100ml/min

2. Cockcroft Gault's formula = (140 - age in years) × weight in kg

72×(Serum creatinine)
In males multiply by 1 and in females multiply by 0.85

10
 Corrected dose = normal dose × patient’s Creatinine Clearance
normal Creatinine Clearance

Patient’s creatinine clearance is calculated using

Cockcroft Gault's formula

=(140 - Age in years) × Weight in kg /(72 × Serum creatinine)

= (140-50) × 70/(72 × 2.4)

= 36.45 ml/min

Corrected dose = 1000 × (36/100)

= 360 mg per day.

11
 3. Dopamine drip
A patient is admitted to casualty with sudden onset chest pain. On examination he
has cold clammy skin and is sweating. The body weight is 60 kg. SBP is 74 mmHg.
Bilateral basal crepts are present. ECG is suggestive of Myocardial infarction.
An i.v. drip of dopamine has to be set for this patient to treat cardiogenic shock.

Dose rate = 5 μg/ kg/ min

One ampoule = 200 mg/ 5 ml
1 ml = 16 drops

12
1.Calculate the dose of dopamine required
5μg/kg/min×60 kg = 300μg/ min

2. Calculate the volume of dopamine required

(How much ml can be administered in a minute)
200 mg---------- 5 ml
300 μg---------- ? ml = 0.0075 ml/min

3. Which I.V. fluid is to be used ?

5% dextrose for the compatibility and stability of dopamine.

4. Calculate the rate of infusion.

= Volume(ml) of solution to be infused × No of drops/ml
Time taken to administer (min)

13
Answer:
Dopamine 5 ml is added to 500 ml 5% dextrose solution.
Total volume of the solution = 505 ml

300 μg 1min
200mg ?

Total duration (mins) in which 505 ml is to be infused

= 200mg/(300 μg/min)
= 200 mg/0.3 mg/min
= 666.6 mins

Rate of dopamine infusion = (505 ml×16 drops/ml)/666 mins

= 12 drops/min.
14
 4. Iron deficiency anaemia
A lady aged 45 years and weighing 50 kg attended the clinic with signs
and symptoms of anaemia. On investigation, she was found to have
microcytic hypochromic anaemia. She was started on oral iron therapy. As
she developed severe nausea and vomiting, the physician decided to start
parenteral iron preparation (Iron sucrose 20mg/ml). Her Hb% was 8gm%.

a) What is the total dose of iron required to raise it to 12 gm%?

b) How will you administer it?

15
a)Total iron dose required
= 4.4 × body weight(kg)×[normal Hb – patient’s Hb(gm%)]
= 4.4 × 50 × (12-8)
= 880mg 20mg---------1ml
= 880/20 880mg--------- ?
= 44 ml of iron sucrose is required
≈ 9 ampoules

b) It is administered with 100ml normal saline as i.v. infusion.

It is given as a maximum of 200mg/dose not more than 3 times per week; dose
must be 24 hours apart.

16
• Therefore, 1st dose of 100mg and subsequently 4 doses of 200mg are
given on alternate days.

• First dose
100mg(5ml) of iron sucrose in 100ml of normal saline

Initial dose- 25mg in 25ml over 15mins

Remainder of first dose- 75mg in 75ml over 30 mins

• Subsequent doses (4 doses on alternate days)

200mg(10ml) of iron sucrose in 100ml of normal saline over 30 mins.

17
 Calculating dose in Paediatrics

For children >2 years of age

1.Young's formula
Paediatric dose = Adult dose × Age in years
Age in years+12

2. Dilling’s formula
Paediatric dose = Adult dose × Age (years)
20

18
For children up to 2 years of age

3. Clark's formula

Paediatric dose = Adult dose × Body weight(kg)

70

19
 5. Paediatric dosing
Paediatric dose of amoxicillin is 30 mg/kg/day in three divided doses. The weight of
the child is 14 kg. Drug available in syrup form contains 250 mg/5ml. Calculate the
dose to be given in ml.

Total dose required per day = 30×14 mg/day

= 420 mg/day

250 mg 5ml
420 mg (x) ml
(x) = 8.4 ml/day
Therefore 2.8 ml of syrup is to be given three times a day. 20
Body Surface Area

Body surface area (BSA) which is based on both height and weight, is a more
precise index than body weight alone for adjustment of drug doses.

Nomograms are to be used to find the BSA of a child with given height and weight.

Individual dose = BSA (m2) × Adult dose

1.73

21
 6. BSA for dosage calculation

BSA (Body Surface Area) of a 10-year-old boy is 1.14 m2. The recommended adult
dose of rifampicin is 600 mg OD. Calculate the dose for the boy based on BSA.

Paediatric dose = BSA of the child × (Adult dose)

1.73
= (1.14/ 1.73) 600
= 395 mg OD

22