Electronic prescribing

Most drug administration is based on the interpretation of a prescription, which may be handwritten or computer-generated. Electronic prescribing, linked with automated drug selection is being increasingly developed in the UK. The advantages claimed for such systems include a reduction in medication error rate and a reduction in medicine supply turnaround times (Goundrey-Smith 2006).

The development of these systems will in no way obviate the need for nurses to maintain and develop their numerical skills. Indeed, new skills may be called for to ensure that all the benefits of advanced technology are experienced by patients.

Contents of a prescription

Nurses’ responsibilities regarding prescriptions

The standards set for registered nurses and midwives (NMC 2008) in relation to prescriptions include:

The dose

The quantity of a drug to be given to the patient is arrived at by a series of steps. In the first place, the prescriber uses his/her knowledge of the drug and the condition when prescribing. The nurse then interprets and correlates the information on the prescription and the label (which may or may not involve a calculation) to determine the actual dosage to be given.

Many drugs are presented in a form in which the required dose is available without the need for a calculation. Examples of such products are shown in Box 1.2.

Even if no calculation as such is involved, the person administering the drug must ensure the correct dose(s) is given to the patient. It should be noted that it is possible to overdose when using a topical preparation especially one that contains a potent corticosteroid (see example on p. 176). This can only be done by careful reading and interpretation of the label on the container with particular reference to the actual dose contained in a given weight and/or volume, and correct interpretation of the prescription. Specialist information may have to be consulted.

Labels on medicine containers

Labels on medicine containers fulfil a number of functions that reflect the needs of all the professions involved in the manufacture, distribution and administration of medicines. The needs of patients in the community are also being increasingly catered for as patient packs become the norm. For example, labels show details of ingredients other than the active drug.

The content of the label on a medicine container is controlled by various legal requirements defined in Directive 92/27/EEC (BMA and RPSGB 2009 p. x). Requirements are specified for both the outer packaging and the inner foil or blister pack. Labelling on the outer package or on other packaging (e.g. bottle containing tablets) is more comprehensive than that on the inner (blister) pack.

The content of a typical label on an outer package is illustrated in Fig. 1.2. Not all the information listed below is illustrated. The focus for the nurse will be the name and strength of the product together with the expiry date. Batch numbers are used to identify the product in the event of any faults which may require a product recall.

Figure 1.2 Typical label on an outer package of foil-packed tablets (note the use of Braille)

Information which may not be of relevance to the nurse in a given situation will appear on many labels. Nurses need to be able to distinguish what information is important. The information required by regulation for the outer package is:

The following particulars normally appear on the blister- or foil-packs of tablets/capsules contained within the main package:

Errors have been known to have been caused by misinterpretation of the packaging. For example:

In order to make the demonstrations and exercises in Chapters 4, 5 and 8 more realistic, labels from actual pharmaceutical products have been reproduced (with permission). For technical reasons, some other labels have been produced “in-house”. In both cases, the images of the labels do not show all the detailed product information required by law. To do so would be impractical owing to pressure on space within the text. The information on all the “labels”, however, is technically correct as it stands and will enable the demonstrations to be followed and the exercises completed.

Drug names

The label on a medicine container will often show two names. For commercial reasons, the manufacturer gives prominence to the proprietary name. Most prescribing policies require the use of the British Approved Name (BAN) or the Recommended International Non-proprietary Name (rINN). Drug names which either sound alike or look alike can cause difficulties, and may even result in errors (McNulty and Spurr 1982a, 1982b).

Drug compounds used in the formulation of medicines

In order to achieve the desired therapeutic outcome, a drug must be combined with other substances to produce a medicine (formulation process). The physical and chemical properties of the drug must be taken into account in this process. In some cases, it may be necessary to chemically modify the parent drug molecule to produce a compound that is fit for purpose.

As an example, morphine is insoluble in water and would not be suitable for formulation into an injection where a rapid and predictable therapeutic response is required. In practice, morphine sulphate (or hydrochloride), which is soluble in water, is used. In most cases, the actual chemical compound used in a formulation will not impact on a nurse’s daily practice. It is the duty of the prescriber to make his/her intention clear on the prescription but nurses must be capable of correctly interpreting prescriptions (and labels on medicines). Prescriptions calling for dexamethasone need particular attention since the modified compounds are not of equal potency to the parent compound. Dexamethasone 1 mg is equivalent to 1.2 mg dexamethasone phosphate or 1.3 mg dexamethasone sodium phosphate. The BNF includes preparations of dexamethasone as shown in Table 1.1.

Table 1.1 Different preparations of dexamethasone

Importance of labels (to nurses)

Although medicine labels have to meet certain legal requirements, not all the information given on a label is relevant to the nurse when administering a medicine. In order to achieve safe and accurate medicine administration, labels must be read and interpreted with the same degree of care that would be given to any clinical information (e.g. blood glucose level) used in patient care. Particular points to bear in mind when reading the label are as follows:

Weight (mass)

Volume

The abbreviation ‘mL’ for millilitre is the printing convention used in the British National Formulary (BNF) (BMA and RPSGB latest issue) and is an acceptable abbreviation in clinical practice. The term litre should be written in full. Abbreviations such as “l” or “L” are open to misinterpretation.

Length

The above units of mass and volume are used in expressing the strengths of medicines. Units of length are used in calculating doses of certain drugs (e.g. opioids via syringe driver, certain dermatological preparations).

The metric system is in daily use in the UK in all aspects of life. Its use in clinical work should not present problems provided the guidance outlined below is followed:

Other methods of expressing strengths of medicines

The need for a standard system of weight and volume is fundamental to the practice of medicine. There are occasions, however, when the strength of a medicine cannot be expressed directly in units of weight and volume. For some products, the strength may be expressed as the number of parts (by weight) of the active ingredient (drug) contained in a given volume (mL). For example, the strength of epinephrine (adrenaline) injection is generally expressed as 1 in 1000 (1 g in 1000 mL) or 1 in 10 000 (1 g in 10 000 mL).

The reasons for the use of this method of expressing strength are traditional. In practice, this method is more convenient, and probably safer, than the more usual system where the weight in a given volume is expressed as milligrams or micrograms per 1 mL. This is because a very wide range of dosage regimens of this potent drug is used.

It is, however, important to understand how such an expression as 1 in 1000 can be converted to mg per mL or micrograms per mL. “1 in 1000” means that 1 g of the active drug is contained in 1000 mL of the injection solution, and:

Dividing by 1000, we get

Converting mg to micrograms, we find that:

A similar approach can be followed to show that 1 mL of a 1 in 10 000 solution contains 100 micrograms.

Units of activity

The strengths of some medicines obtained from natural biological (or semi-synthetic) sources are sometimes expressed in units of activity. Biological assays are used to standardise these products, since chemical methods cannot be applied. Examples of medicines where the strength is expressed in units are given in Table 1.2.

Table 1.2 Examples of drugs expressed in units of activity

* May be expressed in units or micrograms.

Since the drugs in Table 1.2 marked with an asterisk may be prescribed in either units or micrograms, care is needed in interpreting prescriptions for these products (see BNF for strengths of available products).

Nurses will rarely be required to know the quantitative relationship between the units of activity and units of the metric system. The metric system equivalents for some units are given in the BNF, e.g. 1000 units is equivalent to 1 mg of dornase alfa. Local prescribing policies will normally determine how prescriptions should be written, i.e. in units of activity or units of mass (e.g. micrograms). When prescribing drugs in units of activity, the word “unit(s)” should not be abbreviated to “u” since, if badly written, it may be mistaken for a zero, which could introduce a major error.

Percentages

The strength of most medicines is expressed as grams, milligrams, micrograms, etc., together with the volume in millilitres if it is a liquid medicine. For topical medicines and certain large-volume injections, the strength of the ingredients is expressed as a percentage (see also pp. 22–26):

The term “percentage” means “parts per 100”.

So, for example, 10% means 10 parts per 100.

For a mixture of two solids, the percentage is expressed as weight in weight (w/w), e.g.:

If, however, the preparation is a solid dissolved or suspended in a liquid, the percentage is expressed as weight in volume (w/v), e.g.:

Where the product is a liquid diluted in another liquid, the percentage is expressed as volume in volume (v/v), e.g.:

In a few situations, a liquid is dispersed in a solid, in which case the percentage is expressed as volume in weight (v/w), e.g.:

When considering a medicine, it is important to recognise that the active ingredient is normally evenly distributed throughout the product.

Molarity

Another important way of expressing the strength (concentration) of a medicine is the use of molarity. Since moles and millimoles are used mainly in parenteral therapy, the subject of molarity is addressed in Chapter 6.

Metric/imperial system equivalents

Although all medicine strengths are expressed using the metric system, there may be occasions where there is a need to make conversions, for example, of body weight, between the imperial (non-metric) system and the metric system. These conversions are included in the BNF and BNF for children.