Blues chief Alex McLeish is hoing to conclude a deal for the Northern Irishman some time next week after being told he does not figure in Steve Bruce's plans.
Birmingham were interested in Fulham's Paul Konchesky, but with Mark Hughes having just taken over at Craven Cottage it seems unlikely that anyone will be leaving in the near future, and McLeish has now switched his attention to McCartney.
Sources in the Midlands say the only thing holding Birmingham back from making a formal bid is a concern over the ankle injury McCartney suffered in Sunderland's friendly against Brighton in Portugal last week.
With that in mind, Birmingham are also understood to be exploring alternative options, with Aston Villa's Nicky Shorey and South Africa international Tsepo Masilela also on their radar.
McCartney is in his second spell at Sunderland, having come through the ranks before leaving for West Ham in 2006 then rejoining in a £4.5million deal two years later.
However, the Northern Ireland defender has fallen out of favour on Wearside and found himself out in the cold for the final three months of the season.