Health Economics 1994;3:47-56. 15 Townsend P, Phillimore P, Beattie A. Health and ... Towards more rational pre- scribing in general practice. London: HMSO ...
ASPECTS OF DRUGS
Buccaling under the pressure: influence of secondary care establishments on the prescribing of glyceryl trinitrate buccal tablets in primary care A J Pryce, H F Heatlie, S R Chapman Although the events described below are real, the names of the people and places have been changed to protect the innocent. Well, actually, so they would give us permission to use their data.
Department of Medicines
Management, Keele University, Keele, Staffordshire ST5 5BG A J Pryce, research assistant H F Heatlie, research assistant S R Chapman, director of prescribing analysis Correspondence to: Ms A J Pryce, Department of Mathematics, MacKay Building, Keele University, Keele, Staffordshire ST5 5BG. BM7 1996;313:1621-4 BMJ
VOLUME 313
Abstract Objective-To determine which characteristics were the best predictors of high rates of prescribing of glyceryl trinitrate buccal tablets. Design-Practice and patient characteristics from 197 practices were examined, and a multiple regression analysis was performed to examine which variables were important in predicting this prescribing. Setting--Former family health services authority (197 practices). Main outcome measure-Volume ofprescribing of glyceryl trinitrate buccal tablets. Results-Four variables contributed significantly to a multiple regression model: the catchment area of the secondary care establishment; the number of partners in a practice; the level of practice deprivation; and whether the practice served an urban or a rural area. The model suggests that the most important variable was the catchment area of the secondary care establishment in which the practice was located. Conclusion-Although only the prescribing of short acting glyceryl trinitrate buccal tablets was studied, an impact of this size on primary care prescribing may have extensive implications for all drug expenditure in primary care. Dr Watson, in the lounge with the British National Formulary Reclined on the chaise longue, Dr Watson ran an interested eye through the November issue of the Association of Cardiologists and Myocardial Experts. He was sipping his second cup of Earl Grey when it caught his eye-the prescribing of glyceryl trinitrate preparations in Buccalshire Family Health Services Authority differed greatly from the rest of the West Midlands region (fig 1)! There was a far greater use of glyceryl trinitrate buccal tablets in Buccalshire: 316 defined daily doses (the average adult maintenance dose of a chemical substance, as defined by the World Health Organisation') per 1000 prescribing units (practice list size adjusted to compensate for patient age2) compared with a mean of 34 defined daily doses per 1000 prescribing units for the rest of the region! He read on, fascinated. This difference did not seem to be explained by higher morbidity rates (fig 2). What was going on? He knew that glyceryl trinitrate was an effective drug for providing both rapid symptomatic relief of and prophylaxis against angina and also that there was no evidence to suggest that the considerably more expensive modified release buccal tablets offer any clinical advantage over the conventional short acting preparations-that is, sublingual tablets and aerosol sprays. Although he knew that the buccal formulation did have a place in prophylaxis against exercise induced angina,' he fondly recalled an analysis of glyceryl trini-
21-28 DECEMBER 1996
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Family health services authority Fig 1 Prescribed defined daily doses of glyceryl trinitrate formulations in family health services authorities of West Midlands region in third quarter of 1994-5. Source: prescribing analysis and cost (PACT) data trate delivery systems which stated that it mattered little which glyceryl trinitrate preparation was used so long as an adequate dose was given.4 Slowly and deliberately he placed the journal on the cocktail cabinet, deep in
thought. There was no alternative-he would have to enlist the help of a statistician. Miss Scarlett, in the study with spss (for Windows version 6) "I suppose that you want me to collect prescribing analysis and cost (PACT) data on glyceryl trinitrate
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angina in family health services authorities of Midlands region in 1993-4. Source: Korner database for
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buccal tablets for all Buccalshire practices (for the quarter ending November 1994), calculate the defined daily doses to measure the volume of prescribing, and then analyse it all?" she said in one breath. "Yes," Dr Watson replied, "and remember to examine independent variables, such as practice and patient characteristics, to control for variations in demand." "Don't worry," she trilled, "I'll include practice characteristics that have previously been identified as potential influences on prescribing, and a measure of morbidity." Playing absent-mindedly with her pencil, Miss Scarlett pondered the problem of controlling for age and sex. The age-sex distribution of the patient population is an important factor in prescribing analysis, and various weightings such as the prescribing unit,' the age, sex, unit and resident temporary prescribing (ASTRO-PU),' and the specific therapeutic group agesex prescribing unit (STAR-PU)6 have been created to account for these variations. However, she mused, all three weightings are based on, at the very least, an entire therapeutic group. Therefore, use of the ASTRO-PU or STAR-PU at subsection level seemed inappropriate. The division of age into under 65 and 65 years and over in a prescribing unit does not accurately mimic the agesex distribution of patients with angina: the prevalence of angina rises dramatically from the age of 45 to 85 and then falls, with men showing a sharper gradient across all age groups.7 She decided to use practice list size as the common denominator. Fundholding has been shown to alter general practitioners' prescribing habits,8 and the number of partners in a practice has been shown to determine the number of drugs prescribed.9 Other factors she considered important in affecting prescribing were the training status of the practice'" and whether or not it was a dispensing practice." Hospital led prescribing also affects prescribing in primary care." A survey of 240 general practitioners in the West Midlands region showed that 76% stated that consultants' influence was either "sometimes great" or C"cgreat." 13 How could we measure this? she wondered.
"We must also consider whether a practice is based in an urban or rural location," he continued. "The number of general practice consultations is related to whether the patient lives in a rural or urban area."7 "And deprivation of the patient population has been cited as a potential determinant in variation in prescribing,"'4 added Miss Scarlett. "How can we account for that?" "We can measure patient deprivation using the authority's patient registers and census data from the Office of Population Censuses and Surveys to calculate Townsend scores'5 for each practice, although this method has limitations."'6 "What is the Townsend score?" she asked. "It's a material deprivation index based on four census variables," he explained: "unemployment, car ownership, the level of overcrowding, and the number of owner occupied properties. The index is an unweighted combination of the four scores once the skewness of the unemployment and overcrowding is reduced by means of the natural log transformation. This score performs well in explaining variations in health."'7 The clock on the mantelpiece ticked ominously. "The morbidity of the patient population, as measured by the number of admissions to hospital, must also be included, my dear." He drained his glass, "Can I interest you in a large one?" Colonel Mustard and Miss Scarlett (the morning after), in the study with a large bottle of analgesics Sat side by side at the mahogany desk, the two worked in silence. There were 197 practices in Buccalshire during September to November 1994. Of these, 63 practices did not fall into the four catchment areas and 10 practices were atypical (university health centres and practices dealing only with care of patients with terminal disease). Exclusion of these two subsets of general practitioners resulted in the data for 129 practices being considered by Miss Scarlett. UNIVARIATE ANALYSIS
Golonel Mustard, in the library with the geographical information system (GIS) "Easily, my dear," boomed Colonel Mustard while sipping a large port. "We calculate the Euclidean (or "as the crow flies") distance of each practice from the nearest main acute secondary care unit, and so give each of the four main acute hospitals a catchment area. I accept that this is only a proxy measure and that patients need not necessarily be admitted to the nearest hospital, but the widespread urban geography of Buccalshire Family Health Services Authority would make this a logical assumption."
Miss Scarlett examined the data carefully for clues. Where evidence suggested that the variable showed a trend, the two groups were compared with the Mann-Whitney U test-for example, comparison of the average level of prescribing between fundholding and non-fundholding doctors. Mann-Whitney U tests were also used to compare general practitioners who prescribed glyceryl trinitrate buccal tablets with those who did not-for example, to see whether there was a difference in either the median number of partners or in the median practice list size between these two groups. Univariate analysis of catchment area was by KruskalWallis one way analysis of variance.
Table 1 -Results of univariate analysis of data on 129 general practices in Buccalshire Family Health Services Authority
Factor Median No (lower and upper quartile) of partners Mean (SD)Townsend score* Mean No (SD) of admissions per 1000 practice list size* Median (lower and upper quartile) practice list size
Correlation coefficient (P value)
Average (spread)
Range
Test statistic
P value
2 (1, 5) 0.27 (0.88)
I to 8 -1.82 to 2.73
U = 880.5 NA
0.41 NA
r. = -0.47 (
