Pathology quality assurance: the use of control charts

Breast Screening Quality Assurance Reference Centres have a responsibility to monitor the performance of breast screening services and to feed relevant outcome data back to them. In order to be useful, this feedback has to be meaningful and easily interpretable. Past experience in the West Midlands has shown that this aim was particularly difficult to achieve for pathology data using conventional analytical methods. With this in mind, it was decided to assess the usefulness of control charts.

Control charts demonstrate acceptable and unacceptable variation. Acceptable variation is known as common cause variation and is deemed to be due to natural chance. The only way to reduce this type of variation is to remodel the whole process. Special cause variation is an exceptional variation that needs to be investigated to identify the cause. When examining reasons for special cause variation, the data are first checked for accuracy. The case mix is then reviewed, followed by processes and resources, and finally the individuals involved.

In Figure 1, the dots represent the proportion of Grade 2 breast cancers reported by West Midlands pathology laboratories. The reporting rate in the laboratory represented by the larger dot lies outside the common cause variation limits when compared with other laboratories. Work will be undertaken with this laboratory to identify the reasons for the special cause variation evident in these data.

Figure 1