Introduction

Following discussion of some of the essential elements of calculating doses of parenteral drugs, examples of calculations are presented at the different levels of complexity described in Chapter 3. They are presented in the same three ways as in Chapter 4, namely:

• Fully worked-up calculation demonstrations

• Examples to practise in a similar format

• Examples to practise in abridged form

All answers are given on p. 195.

Interpretation of the prescription and the label of a parenteral preparation follows the same principles as a non-parenteral preparation.

Equipment for measuring the dose

Medicines for injection are presented in either a glass or plastic ampoule (Fig. 5.1), a rubber-capped vial (Fig. 5.2) or a prefilled syringe or a cartridge (Fig. 5.3). The contents are either in the form of a liquid or presented as a powder requiring reconstitution. To aspirate the contents, a syringe and needle of suitable size are selected. Care must be taken to draw up the correct amount into the syringe. It is therefore essential to appreciate the significance of the calibrations on different syringes (Fig. 5.4). The syringe should be held at eye level and air should be expelled. The reading should be taken at the top of the black bung attached to the plunger of the syringe. When drawing up and giving insulin, a syringe (with integral needle), specifically for insulin calibrated in units, will normally be used (Fig. 5.5).

Figure 5.1 Ampoule (glass).

Figure 5.2 Rubber-capped vial.

Figure 5.3 Cartridge.

Figure 5.4 Syringes showing different calibrations.

Figure 5.5 Insulin syringe showing calibrations.

Safety

When administering cytotoxic drugs (and other potentially dangerous compounds), it is essential to follow local safety protocols and procedures.

Overage

When preparing to draw up an injection, it should be noted that ampoules containing a solution for injection always contain more than the nominal volume. For example, a 1 mL ampoule will normally contain a total volume of 1.1 mL. An overage of 0.1 mL is present so as to allow removal of a full 1 mL dose, otherwise it would not be possible to remove 1 mL due to the ‘losses’ of injection solution on the internal surfaces of the ampoule. Larger-volume ampoules contain overages in proportion to the nominal volume.

It follows therefore that it is very important to measure all doses and not simply to draw up the contents of the ampoule. The presence of overages in ampoules is one reason why it is not acceptable to measure doses in terms of, for example, half an ampoule. A 1 mL ampoule with a strength of 10 mg per 1 mL will contain 11 mg in 1.1 mL. Giving a dose of half the contents of the ampoule could introduce a drug error in this case 5.5 mg rather than the required 5 mg. There is no substitute for measuring the dose accurately. It should be noted that labels of Controlled Drugs show the presence of an overage for legal reasons.

Displacement volumes

A number of drugs are unstable in solution (both parenteral and oral) and are therefore supplied in dry powder form. Prior to administration, the powder must be reconstituted using a diluent (or solvent). Water for injections is commonly used but sodium chloride 0.9% solution may be the recommended diluent; on occasion, a specially formulated diluent may be provided. Enough liquid is needed to dissolve (or suspend) the contents to produce a concentration that prevents irritation of body tissues but at the same time is of minimum volume so as to make the injection less painful.

The addition of 5 mL of a diluent to a vial containing 500 mg of a drug in powder form will produce a volume of more than 5 mL due to the presence of the powder (Fig. 5.6). It can be seen in Fig. 5.6 that the powder has added 0.2 mL to the volume of diluent added. The displacement volume (DV) is the volume of liquid displaced by the powder, in this case, 0.2 mL. If the dose to be administered is less than 500 mg, it will be important to take account of the displacement value (Cable 2008).