Medication Volume Calculation
To administer 2.5mg from a solution containing 18mg per mL, you need to give 0.14 mL (or approximately 0.14 mL, which can be rounded to 0.15 mL for practical administration).
Mathematical Calculation
The calculation follows the standard formula for determining volume when you know the desired dose and concentration:
- Volume (mL) = Desired Dose (mg) ÷ Concentration (mg/mL) 1, 2
- Volume = 2.5 mg ÷ 18 mg/mL = 0.139 mL 1, 2
- Rounded for practical use: 0.14 mL or 0.15 mL 1, 2
Critical Safety Considerations
Always verify the concentration is expressed as mass per unit volume (mg/mL) rather than as ratios or percentages, as these alternative expressions are a major source of dosing errors. 2, 3
- Drug concentrations expressed as ratios (e.g., 1:1000) or percentages (e.g., 2.25%) cause significantly more calculation errors among healthcare providers—only 65.8% of doctors correctly identified drug mass when expressed as percentages, compared to 93.1% when expressed as mg/mL 2
- The mean error rate for dose calculations increases substantially when concentrations are not expressed in standardized mg/mL format 1, 3
Practical Administration Guidance
- Use a 1 mL syringe with 0.01 mL graduations for accurate measurement of small volumes like 0.14 mL 1
- Double-check your calculation before administration, as calculation errors can differ by factors of 4 to 1000 1
- Have a colleague independently verify the calculation for high-risk medications 1, 3
Common Pitfalls to Avoid
- Do not confuse different concentrations of the same drug—for example, epinephrine comes in 1:1000 (1 mg/mL) and 1:10,000 (0.1 mg/mL) concentrations, and using the wrong one results in 10-fold dosing errors 4, 5
- Verify the units match before calculating—ensure both the desired dose and concentration are in the same mass units (both in mg, not mixing mg with grams) 1, 2
- Account for dilution requirements if the medication requires dilution before administration, as this changes the final concentration 4