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Biomarker analysis for colorectal cancer has been shown to be reliable in Europe with 97% of samples tested by EQA participants to be correctly classified. This study focuses on errors during the annual EQA assessment. The aim was to explore the causes and actions related to the observed errors and to provide feedback and assess any improvement between 2016 and 2017. An electronic survey was sent to all laboratories with minimum one genotyping error or technical failure on ten tumor samples. A workshop was organized based on 2016 survey responses. Improvement of performance in 2017 was assessed for returning participants (n = 76), survey respondents (n = 13) and workshop participants (n = 4). Survey respondents and workshop participants improved in terms of (maximum) analysis score, successful participation, and genotyping errors compared to all returning participants. In 2016, mostly pre- and post-analytical errors (both 25%) were observed caused by unsuitability of the tumor tissue for molecular analysis. In 2017, most errors were due to analytical problems (50.0%) caused by methodological problems. The most common actions taken (n = 58) were protocol revisions (34.5%) and staff training (15.5%). In 24.1% of issues identified no action was performed. Corrective actions were linked to an improved performance, especially if performed by the pathologist. Although biomarker testing has improved over time, error occurrence at different phases stresses the need for quality improvement throughout the test process. Participation to quality improvement projects and a close collaboration with the pathologist can have a positive influence on performance.

Molecular testing of tumor samples to guide treatment decisions is of increasing importance. Several drugs have been approved for treatment of molecularly defined subgroups of patients, and the number of agents requiring companion diagnostics for their prescription is expected to rapidly increase. The results of such testing directly influence the management of individual patients, with both false-negative and false-positive results being harmful for patients. In this respect, external quality assurance (EQA) programs are essential to guarantee optimal quality of testing. There are several EQA schemes available in Europe, but they vary in scope, size and execution. During a conference held in early 2012, medical oncologists, pathologists, geneticists, molecular biologists, EQA providers and representatives from pharmaceutical industries developed a guideline to harmonize the standards applied by EQA schemes in molecular pathology. The guideline comprises recommendations on the organization of an EQA scheme, defining the criteria for reference laboratories, requirements for EQA test samples and the number of samples that are needed for an EQA scheme. Furthermore, a scoring system is proposed and consequences of poor performance are formulated. Lastly, the contents of an EQA report, communication of the EQA results, EQA databases and participant manual are given.