Immunohistochemistry (IHC) is applied routinely in surgical and clinical pathology, because it is essential for diagnosis and sub-classification of many neoplastic lesions. Despite its extensive use for more than 40 years, lack of standardization is a major problem; many factors during the pre-analytical, analytical and post-analytical phases affect the final results. Nordic immunohistochemical Quality Control (NordiQC) was established in 2003 to evaluate the inter-laboratory consistency of IHC, focusing mainly on the analytical part. More than 26,000 IHC slides have been evaluated during the period 2003-2013; 15 - 300 laboratories have participated in each assessment. Overall, 71% of the staining results assessed have been evaluated as sufficient for diagnostic use, while 29% were judged insufficient. All IHC protocols used for the stained slides submitted to NordiQC have been evaluated by focusing on the technical calibration performed by the laboratories, and specific parameters that gave sufficient or insufficient results have been identified. The most common causes for insufficient results were: inadequate calibration of the primary antibody, use of an inadequate primary antibody, inappropriate choice of epitope retrieval method, insufficient heat induced epitope retrieval (HIER) and use of an inadequate detection kit. Approximately 90% of the insufficient results were characterized by either a signal that was too weak or false negative staining, whereas in the remaining 10%, a poor signal-to-noise ratio or false positive staining was seen. Identification of positive and negative tissue controls to ensure appropriate calibration of the IHC assay combined with individually tailored suggestions for protocol optimization have improved IHC staining for many markers and thus inter-laboratory consistency of the IHC results. The overall data generated by NordiQC during 11 years clearly indicates that external quality assessment is a valuable and necessary supplement to internal quality control.
Keywords: external quality control; immunohistochemistry; internal quality control; standardization; tissue controls.