Formulary of Drugs
A formulary is a list of medicines. It is a collection of formulas for the compounding and testing of medications. A reference book frequently referred to as the “pharmacy bible” is the Martindale and the British Pharmacopoeia.
Most hospital institutions will use a prescription formulary. The main function of a prescription formulary is to specify which medications can be prescribed by which specialist. The development of prescription formularies is based on evaluations of efficacy, safety, and cost-effectiveness of drugs. Depending on the individual formulary, it may also contain additional clinical information, such as indications for the use of the drug, side effects, contraindications and recommended doses.
Drug Calculations
Patients vary in their need for drugs. They also vary in the amount of drugs needed. Providers can and do make mistakes in calculating the correct amount needed by their patients because there is great variation in not only the units of measurement of doses of drugs, but also in the way concentrations of drugs are represented.
Units of Drug Doses
Some drug doses are given in grams (g), milligrams (mg) or micrograms (mcg). Others are given in litres (L) or milliliters (ml or mL) of known concentrations, or in millimoles (mMol or mmol), units or international units. It is advisable that these units be written clearly.
Calculation of Doses Against Body Weight or Surface Area
Drug doses are usually prescribed according to body weight (per kg) or occasionally according to body surface area (per m2). When body surface area is used in drug dosing, it is important to determine the surface area of the patient before making the calculation.
Mistakes can sometimes be made whilst converting the doses from microgram to milligram or even to grams. It is therefore important to know how the various units relate to each other (1 gram = 1,000 mg; 1 mg = 1,000 mcg). It is recommended not to use decimal points in drug dosages e.g. 0.1 mg of fentanyl is better written as 100 mcg of fentanyl. So when calculating the fentanyl requirement for a 75 kg man where the recommended dose is 2 mcg/kg, the total dose required is 150 mcg of the drug.
Pediatric Doses
Pediatric doses of drugs in particular need to be carefully calculated. Sometimes the drug is prescribed according to body surface area (per m2). Under these circumstances, it is important to determine the body surface area of the child from his height and weight using standard acceptable conversion formulae (see Table 9.1) and tables.
Table 9.1
The two most common formulae for determining body surface area from the weight and height of a patient
Calculation in Concentrations
Most times the concentrations are expressed as weight/volume, eg fentanyl is 50 mcg / ml. So the volume of fentanyl required in the 75 kg man mentioned earlier is 3 ml of the 50 mcg/ml fentanyl (as 3 ml will contain 150 mcg).
Drug concentrations may sometimes be expressed as percentages (%). They can mean weight/weight (w/w %), weight/volume (w/v %), volume/volume (v/v %) or part/part percentages. The denominator in each fraction could either stand for the solvent, or the solution as a whole. When presented in this way a 5 % solution means there are 5 g of the drug in 100 g of solution (water is a frequently used solvent). As the density of water is 1 g/ml, it also means 5 g of the drug in 100 ml of the solution.
Local anaesthetics are always quoted as percentages e.g. 0.75 % ropivacaine. It means there are 0.75 g in 100 g of solution or 0.75 g in 100 ml or 750 mg in 100 ml or 7.5 mg per ml.
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