Single-Word Requests – Word for a Person Starting a Process or Activity

For a status timestamp you might use Updated.

The reason for asking about context is because if the column contained details of what the last activity actually was, it may have been appropriate to use something like Progress.

'Extraction' might be a good fit for a direct opposite of injection, though it's most commonly used to refer to tooth removal. From https://www.merriam-webster.com/dictionary/extraction:

1:  the act or process of extracting something

And,

3:  something extracted

On the other hand, perhaps 'draw' is the word you seek. I would use it before I'd use extraction; though a cursory dig yielded no formal support for the use of 'draw' as a noun in this specific way.

The nurse gave an injection.

The nurse took a draw.