It is important to note that this is a substantial simplification and depending on the combination of histological appearance, patient demographics and prior history a diagnosis can be reached without going through each of these steps. 

For example, 1p19q testing does not need necessarily need to be performed on every IDH positive tumor, especially if they have uniformly astrocytic morphology and clear cut ATRX loss, although increasingly this is occurring particularly if you have access to affordable routine glioma panel next-generation sequencing. 

There are also numerous "other tumors" that may be diagnosed without undergoing IDH sequencing or TERT/EGFR etc... For example, an IDH1 R132H negative glioma that is midline and has positive H3K27M on immunohistochemistry can be diagnosed as a diffuse midline glioma without sequencing.

Nonetheless, this diagram provides an overview of the main pathways for the classification of adult-type diffuse gliomas as described in the 5th edition (2021) WHO classification of CNS tumors ¹.