What had looked like a processional fixture beforehand proved to be anything but as four goals, two dismissals and two injuries made for a scintillating evening on Teesside.
Two goals in quick succession from Danny Graham and Jacob Butterfield midway through the first half put the hosts in charge before 20-year-old Emyr Huws forced Birmingham back into the game with a glorious strike from 40 yards.
Boro's George Friend and Blues striker Lee Novak were forced off through injury before the break and high tensions led to early baths for home defenders Ben Gibson and Kenneth Omeruo in the closing stages, before Lee Tomlin's late strike sealed the win.
With sixth-placed Reading losing at Bournemouth on Tuesday night, Boro could feasibly gatecrash the play-off places with a strong finish over five games, but Karanka is urging serious caution.
"I don't want to speak about the play-offs because the moment we speak about the play-offs we will lose our next game," said the Spaniard.
"I prefer to think about the next game at Burnley as being one against a team near the top of the table and we have to go there to win the game.
"We have won the last three games and for this I am very happy."
Reflecting on the match, which could have easily swung back in Birmingham's favour after Omeruo and Gibson saw red cards for needless tussles at the death, Karanka said: "Tonight's was the kind of game that has everything: goals, chances, injuries and sendings-off.
"I think it was a crazy game but the most important thing was that we won the game.
"I thought the score might have even been 7-4 or 7-5 and the people here today enjoyed watching the game."
With Gibson and Omeruo confirmed absentees for the trip to Turf Moor, Karanka will hope left-back Friend's hamstring injury is not too serious.
The former Doncaster man never recovered from an early pull and was withdrawn for Jonathan Woodgate.
Lee Clark's main concern is keeping Birmingham in the Sky Bet Championship - they are only four points clear of the relegation zone - and may have lost eight-goal striker Novak for the run-in.
The ex-Huddersfield man was carried off on a stretcher in first-half stoppage time and Clark is fearing the worst.
"We're not sure yet, it's not good at the moment," he said. "He's in a lot of pain. It's a blow as people can see the importance of him to us with each game."
Blues now have 10 days to prepare for their next game - against Nottingham Forest - and Clark will spend the time drilling "the basics" into his players.
"Our decision-making in the second half was astounding," he added. "It's starting to scare us, to be honest, in terms of the young players coming through.
"They need the mentality of how to win games, I think they'd rather do a trick or a flick rather than put the ball in the net.
"We've got to use these 10 days right in terms of preparing the players. We've got to make them aware.
"It's the basics that have done us tonight.
"My boy, who's nine years of age and in the academy, I hear his coach telling the players the same things so we've got to start doing it better."