Skill Group: Clinical Pharmacology and Hospital Management
Drug Dose Calculation
Before You Start …
Before you begin this lesson, you may want to revisit the last lesson to make sure that you are comfortable with volume and mass conversion, as well as calculating fluid deficits.
Introduction
It is now Tuesday evening at the ER clinic, and that 14 year old cat with CKD that you hospitalized on IV fluids is doing much better and ready to be discharged. A urinary tract infection was diagnosed, along with her CKD, and you would like to send her home on amoxicillin oral liquid to treat it. Today the cat is 8 pounds.
- What is her dose of medication? How many milliliters is that?
- How much do you send home with the owner if you want them to give it to her for 10 days?
While you are preparing to discharge her, a client brings in a 9 year old Labrador retriever who is acutely limping. After examining the dog you determine that there is no cruciate ligament injury and it is likely a sprain. You instruct the owner to rest the dog for a week and want to send carprofen (an NSAID) for pain and inflammation. The dog weighs 72 pounds.
- What is the dose for this dog?
- How many tablets, and of what size, do you send home with the owner for a 7 day supply?
In this lesson, you will learn and practice the skills to answer these questions. Remember, these are every day skills and can be applied across many areas of veterinary medicine. They are important, but are really just basic math, easily done on a piece of paper, or with a calculator.
Learning Outcomes
By the end of this lesson, you should be able to:
- Properly calculate the amount of a given dose of medication in milligram (mg) or microgram (ug) for a patient of a given weight.
- Calculate the number of tablets of a certain size for a patient needing a given dose of medication.
- Calculate the volume of a liquid medication of a given concentration needed to provide a given dose of medication.
Dose Calculation
Hint: Abbreviations of Units
- μg: Microgram; 1 μg = 0.000001 g (10-6)
- mg: Milligram; 1 mg = 0.001 g (10-3)
- g: Gram
- kg: Kilogram; 1 kg = 1000 g (103)
Most medications in veterinary medicine are dosed on a g/kg, mg/kg, or ug/kg scale. That means a gram (or milligram or microgram) of the drug is given for each kilogram of the animal’s bodyweight. In very overweight animals, sometimes a “lean bodyweight” is used, but we will address that later in the curriculum.
Example
You want to give metoclopramide to a 25 kg dog. The Metoclopromide dose for a dog is 0.3 mg/kg every 8 hours (or TID).
Calculation: 0.3 mg/kg x 25 kg = 7.5 mg per dose
Medications are generally available in certain concentrations and sizes in liquid and/or tablet/capsule/etc. form respectively. The actual number of ml or pills given must be determined based on the calculated dose (as above).
If there is a dose range for the patient, calculate the upper and lower dose and determine the best way to dispense a dose based on available medication forms and concentrations.
Example
Metoclopromide is available as 5 mg and 10 mg tablets, as well as 1 mg/ml oral liquid. A 7.5 mg dose is desired:
- In 5 mg tablets: 7.5 mg / 5 mg = 1.5 tablets per dose
- In 10 mg tablets: 7.5 mg / 10 mg = 0.75 tablets per dose
- In 1 mg/ml liquid: 7.5 mg / 1 mg/ml = 7.5 ml per dose
Of the above options, giving 1 ½ tablet is much easier then ¾ of a tablet or a large volume of liquid.
Wrapping Up
In this lesson, you learned to calculate dosses for individuals based on patient body weight standard guidelines. In the next lesson, you will learn about the proper writing and filling of a prescription, which will prepare you well for an opportunity to write and fill your own prescriptions in the Stanton Skills Lab!
Before Moving On …
Use the self-check activity below to practice calculating appropriate drug doses.