CHAPTER 2 Pharmaceutical Calculations

# SYSTEMS OF MEASURE

Summary of conversion between metric, apothecaries’ and avoirdupois systems:

Per the United States Pharmacopeia, 1 grain = 64.8 mg.

## METRIC SYSTEM

### Prefixes

kilo- | 10^{3} |
1 thousand (1000) times the basic unit |

hecto- | 10^{2} |
1 hundred (100) times the basic unit |

deka- | 10 | 1 ten (10) times the basic unit |

deci- | 10^{−1} |
1 tenth (0.1) times the basic unit |

centi- | 10^{−2} |
1 hundredth (0.01) times the basic unit |

milli- | 10^{−3} |
1 thousandth (0.001) times the basic unit |

micro- | 10^{−6} |
1 millionth times the basic unit |

nano- | 10^{−9} |
1 billionth times the basic unit |

pico- | 10^{−12} |
1 trillionth times the basic unit |

# INTERPRETATION OF MEDICATION ORDERS

## Example 1:

A prescription for prednisone 5 mg should be taken as follows:

# Example 1:

How many grams of drug should be used to prepare 120 grams of a 2% w/w solution in water?

# Example 2

What is the percentage strength (w/v) of a solution of drug if 40 mL contain 5 grams?

# Example

## Solution

The coal tar (active ingredient) is added to a diluent (petroleum) currently containing no coal tar.

# Example

First, determine how many kcal the patient needs from each component:

Next, convert these kcals into grams:

Then, calculate how many milliliters are needed from each stock solution:

## For females

If the individual is obese or not within 30% of their ideal body weight, other methods of calculating creatinine clearance should be used. Ideal body weight (IBW) or adjusted body weight (ideal body weight plus 40% of obese weight) instead of actual body weight in the Cockcroft-Gault equation will provide a better estimate of creatinine clearance.

# CALCULATIONS FOR PEDIATRIC DOSES

Various pediatric formulas have been used historically to calculate APPROXIMATE pediatric dosages.