The cost figure used in the COMPASS reports is the Ingredient Cost equivalent to England's Net Ingredient Cost (NIC). Using cost as a measure of prescribing is important in terms of monitoring the overall drugs bill, checking spend within prescribing budgets and calculating savings made or potential savings that could be made. However, using costs to measure drug consumption is problematic due to differences in cost of alternative preparations; different national costs levels; fluctuations in currency and changes in prices. Also shifts towards more expensive drugs have more impact on the total level than shifts towards cheaper drugs¹. When measuring changes in drug consumption between two time periods, increases or decreases in the cost of the drug itself may actually give the false impression that changes in prescribing trends have occurred.

1. Guidelines for ATC classification and DDD assignment. WHO Collaborating Centre for Drug Statistics Methodology, Oslo 1998.

A higher cost could be due to a large number of inexpensive agents being prescribed. It may therefore be useful to examine prescribing frequency - particularly when tracking complete courses of treatment such as antibiotic drugs and vaccinations. However, the count of prescription items conceals the wide variation that occurs in the quantity per prescription item. When measuring changes in drug consumption between two time periods, the quantity per item may have changed, but this will not be revealed in the frequency measurement¹.

1. Bogle SM, Harris CM. Measuring prescribing: the shortcomings of the item. BMJ 1994; 308:637-40.

Counting the number of tablets/capsules as a measure of prescribing is useful when comparing different preparations of the same drug, for example, comparing the prescribing of bendroflumethiazide 2.5mg with the 5mg preparation, or comparing the prescribing of the healing strength PPIs with the maintenance strength. However, physical units, number of packages or tablets have limited value when calculating drug consumption for whole drug groups. Weight of active chemical entity (i.e. number of mg etc) has the disadvantage of giving lower potency drugs a disproportionate fraction of the total. Number of tablets is not a good indicator of drug consumption as lower strength preparations contribute more to the total that higher strength preparations¹.

1. Guidelines for ATC classification and DDD assignment. WHO Collaborating Centre for Drug Statistics Methodology, Oslo 1998.

The Defined Daily Dose (DDD) is the assumed average maintenance dose per day for a drug used for its main indication in adults¹. A DDD is defined for both the active ingredient and the route of administration. Therefore, salbutamol tablets have a different DDD from salbutamol inhalers. The DDD is a technical unit of measurement developed to overcome the problems with the traditional units of measurement. Its main function is to allow figures on drug consumption from different sources to be converted into comparable units. Trends in drug consumption for drugs from the same therapeutic group but of different chemical structures and potencies can be compared, both nationally and internationally, using this method. The DDD method does not provide an exact picture of actual use, but rather a rough estimate of drug consumption. This is because the DDD is a unit of measurement and not a recommended dose and it may not be a real dose.

For some drug preparations DDDs have not be defined due to a number of problems:

• Ointments and creams: different quantities are used depending on the size of the area being treated
  
  
• Vaccinations and other 'one-off' treatments: these are not used as maintenance doses
  
  
• Combination preparations: with more than one active ingredient it is difficult to determine for which ingredient the DDD should be defined.
  
  
• HRT and contraceptives: different preparations are given for varying time periods, other products contain mixtures of tablets and patches.

Below are a number of examples of how the number of DDDs is calculated for different preparations.

Number of DDDs = (quantity x strength) / DDD

1. Guidelines for ATC classification and DDD assignment. WHO Collaborating Centre for Drug Statistics Methodology, Oslo 1998.

Generic prescribing rates in Northern Ireland refer to drugs that have been prescribed and dispensed generically. Some drugs prescribed generically cannot be dispensed generically as they are still under patent and until the patent expires other companies cannot produce this drug and market it as a cheaper generic product.

In England two generic prescribing rates are available:

1. Overall generic prescribing rate - Where an item has been prescribed generically, but not necessarily dispensed generically.
2. Dispensed generic prescribing rate - Where an item has been prescribed generically and has been dispensed generically.

In Northern Ireland drugs prescribed generically while still under patent are not recorded as generic prescribing. Therefore NI's generic rate is comparable to England's 'Dispensed generic prescribing rate'.

The major advantage of prescribing generically is of course the cost-savings as generics usually cost considerably less than their brand equivalents. However, it is recommended that certain preparations such as lithium, ciclosporin, multi-ingredient products such as oral contraceptives and modified release preparations should be prescribed by brand name¹. Similarly, patients already well established on either a branded or generic antiepileptic drug should not have their prescription changed².

CSA categorise all prescription items as follows:

The calculation for the generic prescribing rates is:
( G / (G + P) ) * 100

1. Generic medicines - can quality be assured? Drug and Therapeutics Bulletin 1997; 35:9-11.
2. Effective generic prescribing. MeReC Bulletin 1996; 7:37-40.