Audit and internal quality control in immunohistochemistry

Aims-Although positive and negative controls are performed and checked in s urgical pathology cases undergoing immunohistochemistry, internal quality c ontrol procedures for immunohistochemistry are not well described. This stu dy, comprising a retrospective audit, aims to describe a method of internal quality control for immunohistochemistry. A scoring system that allows com parison between cases is described. Methods-Two positive tissue controls for each month over a three year perio d (1996-1998) of the 10 antibodies used most frequently were evaluated. All test cases undergoing immunohistochemistry in the months of April in this three year period were also studied. When the test case was completely nega tive for a given antibody, the corresponding positive tissue control from t hat day was examined. A marking system was devised whereby each immunohisto chemical slide was assessed out of a possible score of 8 to take account of staining intensity, uniformity, specificity, background, and counterstaini ng. Using this scoring system, cases were classified as showing optimal (7- 8), borderline (5-6), or unacceptable (0-4) staining. Results-Most positive tissue controls showed either optimal or borderline s taining with the exception of neurone specific enolase (NSE), where most sl ides were unacceptable or borderline as a result of a combination of low in tensity, poor specificity, and excessive background staining. All test case s showed either optimal or borderline staining with the exception of a sing le case stained for NSE, which was unacceptable. Conclusions-This retrospective audit shows that immunohistochemically stain ed slides can be assessed using this scoring system. With most antibodies, acceptable staining was achieved in most cases. However, there were problem s with staining for NSE, which needs to be reviewed. Laboratories should us e a system such as this to evaluate which antibodies regularly result in po or staining so that they can be excluded from panels. Routine evaluation of immunohistochemical staining should become part of everyday internal quali ty control procedures.