Isocitrate dehydrogenase 1 (IDH1) gene mutations occur in ~ 60 - 90% of diffuse and anaplastic gliomas and secondary glioblastomas. IDH status is strongly associated with patient survival times and IDH testing is relevant for clinical patient management and for stratification in clinical trials. A recent interlaboratory ring trial shows that immunohistochemistry is a highly reliable method to detect the most common IDH mutation (R132H), while IDH gene sequencing is less robust. These results support initial immunohistochemistry and subsequent gene sequencing in cases with negative or inconclusive immunostaining result as valid algorithm for IDH testing. Furthermore, they highlight the need for strict quality control of DNA-based biomarker analyses on formalinfixed and paraffin-embedded tumor samples.