I am interested in the article usage in this specific expression. I see that sometimes 'a' and sometimes the zero article is used. The zero article seems to prevail, though.
Is the article simply optional or are there some kind of restrictions on when you can use it?
The Merriam-Webster's Learner's Dictionary:
The driver was taken to the hospital where he was reported to be in (a) good/stable/critical condition.
The Oxford Advanced Learner's Dictionary:
a used car in perfect condition
The house is in a generally poor condition.