1. Measurement in Veterinary Pharmacology
CHAPTER 6 - 2
Dr. Dipa Brahmbhatt VMD MpH

2. Dose Calculations
Must know correct amount of drug to administer to a patient
Must be in same system of measurement
Weight conversion factor: 2.2 lb = 1 kg
Remember that drugs can be measured in mcg, mg, g, gr, ml, l, units
Remember that drugs can be dispensed or administered in tablets, ml, l, capsules

3. Dosage of a drug: 2 mg/ kg Weight of animal: 22 lbs
Dose Calculations
Dosage of a drug: 2 mg/ kg Weight of animal: 22 lbs

4. Dose Calculations STEP 1: Convert weight into kg
1 kg = 2.2 lbs
X = 22 lbs
X = (22 lbs/ 2.2lbs) * 1 kg = 10 kg
STEP 2: From dosage to dose
Dosage: 2mg/kg
Dose: 10kg * 2mg/kg = 20 mg

5. Dose Calculations STEP 3: From dose to amount administered 20 mg
Tablets: 40 mg, 80 mg and 100 mg
½ of the 40 mg tablet (assuming that tablet is scored!!)
½ tablet – 40 mg
Solution: 10 mg/ ml
(20 mg/ 10 mg) * 1 ml = 2 ml
2 ml of 10mg/ml drug solution
Total dose: BID for 7 days
Tablet: 7 tablets
Solution: 28 ml of the 10mg/ml solution

6. Dose Calculations Units (U) or International Units (IU)
Insulin, heparin, penicillin
1000 lb cow; 50,000 U/kg; 300,000 U/mL
(1000lb/2.2kg) * (50,000 U/kg) = 22,727,272.73
(22,727,272.73) * (1mL/300,000 U) = 75.8 mls

7. Dose calculations - Verification
Calculating # doses
# doses = total amount of medicine/ strength of each dose
Vet prescribes 200 mg and each dose is 20 mg, # doses? = 200 mg/ 20 mg = 10 doses
Single dose 1 gram, # doses in 10 g = 10 doses
1 dose is 200 mg, # doses in 10 g = 10,000 mg/200mg = 50 doses

8. Dose calculations - Verification
Determining amount in each dose =
Quantity in each dose / # doses
100mg / 20 doses = 5 mg in each dose

10. Solutions
Solutions are mixtures of substances not chemically combined with each other
The dissolving substance of a solution is referred to as the solvent (liquid)
The dissolved substance of a solution is referred to as the solute (solid or particles)
Substances that form solutions are called miscible
Substances that do not form solutions are called immiscible

11. Working with Solutions
The amount of solute dissolved in solvent is known as the concentration
Concentrations may be expressed as ratio strengths: parts per drug per parts per solution: parts (per some amount), weight per volume, volume per volume, and weight per weight
Usually reported out as percents or percent solution
Remember that a percent is the parts per the total times 100

12. Rules of Thumb When Working with Solutions
Parts: parts per million (ppm) means 1 mg of solute in a kg (or L) of solvent
% conc. of 1:1000 epinephrine?
% conc. = (1/1000)*100 = 0.1%
Liquid in liquid: the percent concentration is the volume per 100 volumes of the total mixture (1 ml/100 ml, 5 oz/ 100 oz)
Solids in solids: the percent concentration is the weight per 100 weights of total mixture (60 mg/100 mg, 4.5 g/ 100 g)

13. Rules of Thumb When Working with Solutions
Solids in liquid: the percent concentration is the weight in grams per 100 volume parts in milliliters (dextrose 5% = 5 g/100ml = 5000mg/100ml = 50mg/ml)

14. Percent Concentration Calculations
Pure drugs are substances that are 100% pure
Stock solution (pure drugs) is a relatively concentrated solution from which more dilute solutions are made
Ratio-proportion method: one method of determining the amount of pure drug needed to make a solution
(Amount of drug/amount of finished solution) = (% of finished solution/100% (based on a pure drug))
NB: that the amount of drug used to prepare a solution is added to the total volume of the solvent

15. Percent Concentration Calculations
How much NaCl needed to make 500mls of 0.9% solution
X = (0.9g NaCl/ 100 ml) * 500 ml = 4.5 g (4.5 ml density is 1 mg/ml)
Prepare 1L of 4% formaldehyde fixative solution from 37% stock solution
X = (4%/ 37%) * 1000 = 108 ml of stock solution
1000 ml – 108 mls of stock = 892 ml of solvent

16. Another Way to Determine Volume Concentration
Volume concentration method:
Vs = volume of the beginning or stock solution
Cs = concentration of the beginning or stock solution
Vd = volume of the final solution
C d = concentration of the final solution
Vs x Cs = Vd x Cd

17. Volume concentration method
Vs x Cs = Vd x Cd
How much water must be added to 1L of 90% alcohol to change it to 40%
Vs x Cs = Vd x Cd = 1,000mls x 90% = Vd x 40%
Vd = 2250 mL
2250 mL – 1000 mL = 1250 mL

18. Volume concentration method
How much 1:25 solution of NaCl is needed to make 3 L of 1:50 solution
Convert to %
(1/25)*100 = 4%; (1/50) * 100 = 2%
Vs x Cs = Vd x Cd = Vs x 4% = 3 L x 2%
Vs = 1.5 L

20. Drug Concentrations in Percents
Drug concentrations are sometimes listed in percents
Parts per total = parts (in g) per 100
The front of the vial specifies the concentration (for example, 2% lidocaine)
Use X g/100 ml to determine dose
Dosage: 3mg/ kg; weight: 15 lb dog; 2% lidocaine
(15 lb/ 2.2 lb/kg) * 3mg/kg = mg
20.45 mg / 20 mg/ml = 1.02 mL

21. Reconstitution Problems
Drug is in powder form because it is not stable when suspended in solution
Cefazolin sodium, azithromycin, ampicillin
Such a drug must be reconstituted (liquid must be added to it: sterile water, saline, dextrose)
The label should state how much liquid to add
Powder may add to the total final volume of liquid being reconstituted
Label a reconstituted drug with the date prepared, the concentration, and your initials

22. Reconstitution Problems
Cefazolin sodium: 500 mg + 2mL = 225mg/ml (reconstituted vol. increases to 2.2 mL)
Ampicillin – 25 g (adds 20.5 mL of volume)
104.5 mL (200 mg/mL)
79 mL (250 mg/mL)
41 mL (400 mg/mL)
Patient needs 800mg hence 2 mL