PTCE · Order Entry and Processing · 7 min read
Concentrations, Percent Strength and Dilutions
- Differentiate between weight-in-volume, weight-in-weight, volume-in-volume, and ratio strength expressions to calculate doses and active solute quantities.
- Apply the dilution formula C1V1 = C2V2 to determine stock solution volumes, target concentrations, and required diluent amounts.
- Calculate powder displacement volumes and describe reconstitution workflows, documentation requirements, and professional responsibilities under pharmacist supervision.
Foundations of Concentration and Basic Dose Calculations
In daily pharmacy practice, concentration represents the quantity of active drug solute present in a specific volume or mass of total preparation. Pharmacy technicians perform concentration, dilution, and reconstitution calculations under pharmacist supervision, in accordance with applicable law and workplace policy. While the technician handles the mathematical preparation and technical assembly of these orders, the licensed pharmacist is solely responsible for clinical judgment, patient counseling, and the final verification of all prepared orders. To maintain safety and transparency, pharmacy technicians document calculations, diluents, and lot numbers as required by applicable regulations and workplace procedures for pharmacist review.
A concentration expressed in mg/mL indicates the number of milligrams of active ingredient contained in each milliliter of liquid. For example, one milliliter of a 10 mg/mL solution contains exactly 10 milligrams of active drug. To calculate total milligrams of drug in any given volume, multiply the volume in milliliters by the concentration in mg/mL. Conversely, to calculate the volume required for a dose, divide the prescribed dose in milligrams by the concentration in mg/mL. Units of mass and volume can also scale across the metric system: a concentration of 1 gram per liter (1 g/L) is mathematically equal to 1 mg/mL.
Oral liquid concentrations are often expressed in mg/5 mL. While 5 mL is one standardized teaspoonful, doses should be measured in milliliters with an appropriate device rather than household spoons to avoid dosing errors. Whether preparing a commercial liquid or performing non-sterile compounding, understanding this relationship allows the technician to convert between prescribed milligram doses and the exact volume in milliliters that must be administered.
Common mistakes
- Failing to convert units so that C1 and C2, or V1 and V2, match before solving the C1V1 = C2V2 dilution equation.
- Confusing the powder displacement volume (the volume the powder contributes to the liquid mixture) with the physical volume of dry, loose powder in the container.
- Stopping calculations at the stock volume (V1) instead of subtracting V1 from the total final volume (V2) when asked for the required volume of diluent.
- Directing patients to measure oral liquid medications with household teaspoons instead of measuring standardized milliliter doses using an appropriate calibrated device.
Key points
- Pharmacy technicians carry out concentration, dilution, and reconstitution calculations under pharmacist supervision, while the pharmacist maintains exclusive responsibility for clinical judgment, patient counseling, and final verification.
- Concentration measures active solute per total volume or mass; a 1% w/v solution provides 1 g per 100 mL, which equals 10 mg/mL, and multiplying a w/v percentage by 10 yields its concentration in mg/mL.
- Percent strength formats reflect measurement units: w/v represents grams of solute per 100 mL of preparation, w/w represents grams of solute per 100 g of preparation, and v/v represents milliliters of solute per 100 mL of preparation.
- Ratio strength indicates 1 part solute per specified parts total mixture; 1:1,000 w/v equals 0.1% w/v (1 mg/mL, as seen in epinephrine 1 mg/mL), and 1:10,000 w/v equals 0.01% w/v (0.1 mg/mL, as seen in epinephrine 0.1 mg/mL).
- The dilution formula C1V1 = C2V2 requires matching concentration and volume units, showing that solute mass remains constant while adding diluent lowers the final concentration (C2 < C1).
- When volumes are additive, final reconstituted volume equals diluent volume plus powder displacement volume, and varying the volume of diluent added directly changes the resulting drug concentration.
- Technicians document calculations, diluents, and lot numbers, and label reconstituted products with beyond-use dates, storage requirements, and times when required, for final pharmacist review and verification.
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