Mastering the SI Units System: A Guide to Pharmaceutical Calculations
Welcome to our foundational tutorial on Pharmaceutical calculations. Based on the 13th edition of Howard C. Ansel’s Pharmaceutical Calculations, this guide covers the International System of Units (SI)—the bedrock of modern pharmacy practice.
1. Understanding Metrification
The International System of Units (SI) is a globally recognized system for taking measurements like weight and mass. Formerly known as the metric system, the transition from older units like gallons, feet, and pounds to liters, meters, and grams is referred to as metrification.
Terminology Tip: In a measurement like 12 kilograms, the numerical portion (12) is the denominate number, while the unit (kilograms) is the denomination. We use symbols (like kg) rather than abbreviations.
2. Common Pharmacy Prefixes
Prefixes help us document measurements efficiently. For example, instead of writing 0.002 meters, we write 2 millimeters (milli = 1/1000).
| Prefix | Symbol | Power of 10 | Value |
|---|---|---|---|
| Kilo | kg | 10^3 | 1,000 |
| Deci | dg | 10^-1 | 0.1 |
| Centi | cg | 10^-2 | 0.01 |
| Milli | mg | 10^-3 | 0.001 |
| Micro | mcg/μg | 10^-6 | 0.000001 |
3. Beyond SI: Other Systems
While SI is the gold standard, pharmacists still encounter the Avoirdupois system (measuring weight in pounds/ounces) and the Apothecaries system. Mastering the relationship between these older units and SI is vital for inter-conversion calculations.
4. Practice Problems
Q1: Reduce 0.5kg, 50mg, and 2.5dg to grams and add them.
0.5 kg × 1000 = 500g
50 mg × 0.001 = 0.05g
2.5 dg × 0.1 = 0.25g
Total = 500.3g
Q2: How many 81mg tablets can be prepared from 1kg of Aspirin?
1 kg = 1,000,000 mg
1,000,000 mg / 81 mg = 12,345.67
Answer = 12,345 Tablets (ignoring leftover fraction)
5. Case in Point: Dopamine Infusion
A patient requires 500mcg of dopamine per minute from an infusion containing 400mg in 250ml.
500mcg/min × 60 min = 30,000mcg/hr = 30mg/hr
400mg total / 30mg/hr = 13.33 hours = 13h 20m
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