Quality assessment of drug sales data: the case of antibacterials in Iceland

Two sets of drug sales data, published by the Icelandic Ministry of Health, did not match for antibacterials in 1989. The search for causes turned out to be a project in itself.

To analyze quality problems in the sales data on antibacterials and describe a method for systematic quality assessment of drug sales data.

Documentary analysis based on the following sources: 1) Nordic Statistics on Medicines, 1975-95; 2) Drug Use (Notkun Iyfja), 1975-93; 3) Icelandic Drug Market, 1975-94; 4) Unpublished data from the Icelandic Ministry of Health. The following framework was developed to evaluate the quality of drug sales data: 1. Completeness of registration; 2. Accuracy and degree of completeness of data; 3. Size and coverage of the data source; 4. Data format; 5. Data accessibility, availability and cost.

Four discrepancies were found, two due to changes in DDD, and two larger ones stemming from errors in calculating DDD, resulting in an overestimation of the contribution of the respective products to the total DDD/1000 inhabitants/day. Errors were detected in available sales data at least back to 1980, resulting in sales being overestimated by up to 13%. The reasons for the discrepancies were found mostly under point 2 in the framework.

The errors uncovered by this study indicate a possible low quality of drug statistics which might lead to wrong conclusions about the level and development of sales of drugs. As a tool, the framework might be used for quality assessment of drug sales data.