Pharmacy calculations play a crucial role in the daily responsibilities of pharmacy technicians.
As part of the Pharmacy Technician Certification Board (PTCB) exam, students must demonstrate proficiency in performing a variety of pharmaceutical calculations.
We’ll break down key areas of PTCB calculations, providing essential formulas, techniques, and tips to help you prepare effectively for the exam.
Types of PTCB Calculations
The PTCB exam covers various types of pharmaceutical calculations. These calculations often involve understanding and applying medical dosages, concentrations, and conversion formulas to ensure patient safety and accurate dispensing of medications.
The core areas to focus on include:
1. Basic Math Skills: Proficiency in basic arithmetic, fractions, decimals, percentages, and ratios is foundational to pharmacy calculations.
2. Dosage Calculations: This includes determining correct doses based on patient weight, age, or body surface area.
3. Concentrations and Dilutions: Understanding how to calculate the strength of solutions and how to dilute or concentrate medications.
4. Alligations: A method for solving problems related to mixing solutions of different concentrations.
5. Conversions: Converting between units, such as milligrams to grams, milliliters to liters, or even from metric to household systems.
Dosage Calculations
One of the most critical sections on the PTCB exam involves calculating dosages. You may encounter situations where you need to calculate the correct amount of medication based on a doctor’s prescription and the patient’s body weight.
The most commonly used formula for weight-based dosing is:
Dose=Prescribed DoseWeight in kg×Patient’s Weight\text{Dose} = \frac{\text{Prescribed Dose}}{\text{Weight in kg}} \times \text{Patient’s Weight}Dose=Weight in kgPrescribed Dose​×Patient’s Weight
For example, if the prescribed dose is 10 mg/kg and the patient weighs 70 kg, the total dose would be:
10 mg/kg×70 kg=700 mg10 \, \text{mg/kg} \times 70 \, \text{kg} = 700 \, \text{mg}10mg/kg×70kg=700mg
Concentration and Dilution Calculations
Concentration refers to the strength of a solution, usually expressed as a percentage (e.g., 5% solution) or ratio (e.g., 1:100). Calculating concentration is vital when preparing solutions or adjusting the strength of a compound.
The formula used for calculating concentration is:
Concentration=Amount of SoluteTotal Volume of Solution×100\text{Concentration} = \frac{\text{Amount of Solute}}{\text{Total Volume of Solution}} \times 100Concentration=Total Volume of SolutionAmount of Solute​×100
In dilution problems, the C1V1 = C2V2 formula is often used to adjust the concentration of a solution:
C1×V1=C2×V2C_1 \times V_1 = C_2 \times V_2C1​×V1​=C2​×V2​
Where:
- C1C_1C1​ = initial concentration
- V1V_1V1​ = initial volume
- C2C_2C2​ = final concentration
- V2V_2V2​ = final volume
Alligations
Alligations help with mixing solutions of different strengths to achieve a desired concentration. It involves balancing the higher and lower concentrations against the desired final concentration.
Here’s how to set up an alligation grid:
- Write the higher and lower concentrations on the left.
- Write the desired concentration in the middle.
- Subtract diagonally to find the ratio of the amounts needed for each concentration.
Example: To make 100 mL of a 15% solution from 20% and 10% stock solutions:
- Subtract 10% from 15% (get 5)
- Subtract 15% from 20% (get 5)
- The two stock solutions will be mixed in a 1:1 ratio.
Conversion Formulas
Conversions are a frequent part of PTCB calculations. You’ll need to convert between metric units (mg, g, mL, L) and household measures (teaspoons, tablespoons, etc.).
A few key conversions to memorize are:
- 1 gram (g) = 1,000 milligrams (mg)
- 1 liter (L) = 1,000 milliliters (mL)
- 1 teaspoon (tsp) = 5 mL
- 1 tablespoon (tbsp) = 15 mL
To convert units, you’ll often use dimensional analysis, which allows you to cancel out units and ensure that your answer is in the correct form.
For example, converting 2 grams to milligrams:
2 g×1,000 mg1 g=2,000 mg2 \, \text{g} \times \frac{1,000 \, \text{mg}}{1 \, \text{g}} = 2,000 \, \text{mg}2g×1g1,000mg​=2,000mg
Common Pitfalls in PTCB Calculations
Understanding common mistakes in pharmacy calculations can help you avoid them on the test. Here are a few areas where students often go wrong:
1. Incorrect Unit Conversions: Not carefully checking units can lead to wrong answers, especially when converting between systems (e.g., from metric to household measures).
2. Misinterpreting Dosage Instructions: Pay close attention to the units in dosage calculations—whether the prescribed dose is based on milligrams, grams, or another unit.
3. Overlooking Dilution Steps: Dilution problems require careful application of the dilution formula. Be mindful of when to dilute versus concentrate a solution.
Tips for Success on the PTCB Practice Test
1. Practice, Practice, Practice: The more you practice pharmaceutical calculations, the more confident you’ll feel during the exam. Use sample PTCB test questions to reinforce key concepts.
2. Memorize Key Formulas: Ensure you have all essential formulas memorized, including dosage, concentration, alligations, and conversion formulas.
3. Use a Calculator: A basic calculator is allowed on the exam. Make sure you’re comfortable using it for complex calculations, especially for dilution problems or converting units.
4. Double-Check Your Work: Always take a moment to double-check your calculations to ensure accuracy, especially for dosage and concentration problems, where a small error can lead to significant consequences.
Conclusion
Preparing for the PTCB exam requires mastering a wide range of pharmaceutical calculations, from dosage to dilution, and everything in between. With the right preparation and understanding of key formulas, you’ll be well-equipped to tackle the calculations portion of the test with confidence.
Make sure to practice regularly, focus on accuracy, and remember the importance of these calculations in ensuring patient safety and medication accuracy.
