Calculations
Fita Rahmawati
Using the following Basic Formula for drug
calculations:
D x V = amount to give
H
D = desired dose (i.e. the drug dose
ordered by the physician)
H = on-hand dose (i.e. the drug dose on
the label of the drug container)
V = vehicle (i.e. the form and amount in
which the drug comes)
Ans ; 5 ml
Case 1
A medication order for a patient weighing
70kg calls for 0.25mg of amphotericin B
per kg of body weight to be added to
500ml of 5% dextrose injection.
If the amphotericin B is to be obtained
from a reconstituted injection that contain
50mg per 10ml,
how many milliliters should be added to
the dextrose injection?
Total quantity needed for the patient:
0.25 x 70 = 17.5 mg
Reconstituted solution contain 50mg per
10ml
50 mg 10 ml
17.5mg X
X= 3.5ml
Case 2
An intravenous infusion is to contain
15mEq of potassium ion and 20mEq of
sodium ion in 500ml of 5% dextrose
injection.
Using an injection of potassium chloride
containing 6g per 30ml and 0.9%
injection of sodium chloride, how many
milliliters of each should be used to supply
the required ions?
15 mEq of K+ will be supplied by 15 mEq of
KCl
And 20 mEq of Na+will supplied by 20 mEq
of NaCl
1mEq of KCl = 74.5mg
15mEq of Kcl = 1117.5mg or 1.118g
6g 30 ml
1.118 g X
X= 5.59ml
1mEq of NaCl = 58.5mg
20mEq of NaCl = 1170mg or 1.17g
0.9g 100ml
1.17g X
X = 130ml.
CASE 3
The following is a formula for a desired TPN solution. Using
the source of each drug as indicated, Calculate the amount
of each component required in preparing solution.
TPN solution
formula Component Source
(a) Sodium Chloride 35 mEq Vial, 5 mEq per 2 ml
(b) Potassium Acetate 35 mEq Vial, 10 mEq per 5 ml
(c) Magnesium Sulphate8 mEq Vial, 4 mEq per ml
(d) Calcium Gluconte9.6 mEq Vial, 4.7 mEq per 10 ml
(E) Potassium Chloride 5 mEq Vial, 40 mEq per 20 ml
(f) Folic Acid 1.7 mg Ampul, 5 mg per ml
(g) Multplevitamin infusion 10 ml Ampul, 10 ml
To be added to:
Amino Acid Infusion (8.5 %) 500 ml
Dextrose injection (50 %) 500 ml
CASE 4
A medication order calls for 1000 ml of D5W
to be administered over an 8-hour period.
Using an IV administration set which delivers
10 drops per ml, how many drops per minute
should be delivered to the patient?
Volume of fluid = 1000 ml
8 hour = 480 minutes
1000/ 480 = 2.1 ml per min
2.1 ml/min x 10 (drops per ml) = 21
drops/minute
Calculating Osmolarity of an IV
Admixture
For each component of the admixture, multiply the
volume in milliliters of that component times the
value of mOsm per ml of the component.
Add the mOsm obtained in step [1] for each of the
components in order to determine the total number of
milliosmoles in the admixture.
Add together the volumes of each component in order
to obtain the total volume of the admixture.
Divide the total number of milliosmoles from step [2]
by the total volume from step [3], then multiply by
1,000 to obtain an estimate of the osmolarity of the
admixture in units of milliosmoles per liter.
Example Calculation
DESCRIPTION VOLUME x mOsm/m = mOsm
L
Sterile Water for
500mL x 0.00 = 0.00
Injection
Sodium Bicarbonate
50mL x 2.00 = 100.00
8.4%
Potassium Chloride 10mL x 4.00 = 40.00
Heparin 5,000 units 0.5mL x 0.46 = 0.23
Pyridoxine 1mL x 1.11 = 1.11
Thiamine 1mL x 0.62 = 0.62
Totals [3] 562.50mL [2] 141.96
[Step 4] Osmolarity of Admixture
(141.96 / 562.5) x 1,000 = 252 mOsm / L
Normal osmolarity of blood/serum is
about 300-310 mOsm/L
