The influence of primary care prescribing rates for new drugs on spontaneous reporting of adverse drug reactions

Introduction: Adverse drug reaction (ADR) reporting makes a vital contribution to pharmacovigilance, although the factors that influence the reporting rate remain unclear. The aim of this study was to investigate whether the variation in the rate of reporting of suspected ADRs in different regions of Scotland was explained by differences in local prescribing practice and to quantify the extent of this influence. Methods: Population and primary care prescribing data were obtained for ten geographical areas based on the 15 administrative regions of the National Health Service in Scotland. All reports of suspected ADRs received from within Scotland for 2000 and 2001 were available from the regional monitoring centre (Committee on Safety of Medicines, Scotland). The primary analysis was based on 14 medications that appeared in the 'top ten' list for the frequency of reported ADRs for either year. Reporting rates for each area were expressed both in terms of population (reports per million people) and in terms of estimated exposure to those medications in primary care (reports per 1000 prescriptions). For each analysis, the Pearson correlation coefficient between reporting and prescribing data was calculated using SPSS (TM) software. Results: The 'top ten' medications accounted for 1715 of 2817 (60.9%, 95% Cl 59.1, 62.7) ADR reports but only 2.2 million out of a total of 128 million primary care prescriptions (1.7%). Although there was a 3-fold geographical variation in the per-population ADR reporting rate, there was a close correlation between local reporting of ADRs and prescribing of the index medications (p = 0.66, p = 0.04, respectively). This implies that 44% of the observed variation in reporting rate can be attributed to variation in prescribing within the same population. Discussion: Spontaneous ADR reporting in Scotland over the 2 years studied was highly concentrated on a small number of medications that were under intensive surveillance. Although there was a 3-fold variation in reporting rates from individual geographic areas when corrected for the size of the population, primary care prescribing data showed nearly half of this local variation in reporting rates could be explained by differences in prescribing. This study highlights the importance of considering prescribing practice when interpreting spontaneous ADR reporting data.