Proficiency testing in immunohistochemistry—experiences from Nordic Immunohistochemical Quality Control (NordiQC)

Despite extensive use of immunohistochemistry (IHC) for decades, lack of standardization remains a major problem, even aggravated in the era of targeted therapy. Nordic Immunohistochemical Quality Control (NordiQC) is an international academic proficiency testing (PT) program established in 2003 primarily aimed at assessing the analytical phases of the laboratory IHC quality. About 700 laboratories from 80 countries are currently participating. More than 30,000 IHC slides have been evaluated during 2003–2015. Overall, about 20 % of the staining results in the breast cancer IHC module and about 30 % in the general module have been assessed as insufficient for diagnostic use. The most common causes for insufficient results are less successful antibodies (poor and less robust antibodies, poorly calibrated ready-to-use (RTU) products, and stainer platform-dependent antibodies; 17 %), insufficiently calibrated antibody dilutions (20 %), insufficient or erroneous epitope retrieval (27 %), less sensitive visualization systems (19 %), and other (heat- and proteolysis-induced impaired morphology, endogenous biotin reaction, drying out phenomena, stainer platform-dependant protocol issues; 17 %). Approximately, 90 % of the insufficient results are characterized by either a too weak or false negative staining, whereas in the remaining 10 %, a poor signal-to-noise ratio or false positive staining is seen. Individually tailored recommendations for protocol optimization and identification of best tissue controls to ensure appropriate calibration of the IHC assay have for many markers improved IHC staining as well as inter-laboratory consistency of the IHC results. RTUs will not always provide an optimal result and data sheets frequently misguide the laboratories hampering the improvement in IHC quality. The overall data generated by NordiQC during 12 years indicates that continuous PT is valuable and necessary. Detailed description of the results of the NordiQC programme is available on www.nordiqc.org and summarized in this paper.