Not otherwise specified (NOS)

Last revised by Assoc Prof Frank Gaillard on 21 Sep 2021

Not otherwise specified (NOS) is a term used in a variety of classification systems to denote an entity that has been only incompletely characterized; enough for a general diagnosis but not to the point of a complete diagnosis. 

The definition and use will vary between different classification systems. For example in the WHO Classification of CNS tumors, a tumor is deemed NOS if all the tests necessary to reach a specific diagnosis have not (for any reason) been carried out. 

As a specific example, the diagnosis of an oligodendroglioma requires both IDH-mutant and 1p19q co-deletion status to be established. If for some reason these are not available then the tumor can only be classified based on histological features and a diagnosis of oligodendroglioma NOS give. 

Not otherwise specified (NOS) should not, however, be confused with not elsewhere classified (NEC) which denotes a fully characterized entity that does not fit into established diagnostic classifications. 

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