What is the most reliable finding to confirm a necrotic pulp?

The presence of an area of radiolucency surrounding the apex of a tooth is considered the most reliable finding to confirm a necrotic pulp. When the pulp becomes necrotic, it often leads to the death of the pulp tissue, allowing for bacterial infiltration and subsequent periapical pathology. This condition is typically visible on radiographs as a radiolucent area at the apex of the tooth's root, indicating bone loss and inflammation in the surrounding area.

In contrast, tooth sensitivity to cold may indicate a vital pulp or reversible pulpitis rather than necrosis, as it suggests that some nerve function remains intact. The presence of a fistula can indicate that there is an infection or drainage path, but it is not specific enough to definitively confirm necrosis since it could be associated with other dental conditions. Pain during percussion can suggest inflammation but does not specify the status of the pulp itself and can be present in conditions of both vital and non-vital pulp.