Under U.S. accounting standards, impairment losses cannot be reversed even if the asset's value later increases. Companies must also regularly assess intangible assets like goodwill, patents ...

Ramanna, Karthik, and Ross L. Watts. "Evidence on the Use of Unverifiable Estimates in Required Goodwill Impairment." Review of Accounting Studies 17, no. 4 (December 2012): 749–780.