The 1-0 win at Brentford, courtesy of a howler by Bees goalkeeper David Button, equalled a record for the second tier of English football by going nine games without conceding a goal.
Button’s high punch into the swirling wind looped high above the bar and dropped over the line, though Daniel Ayala made sure of things and tried to claim the goal.
Boro boss Karanka admitted his side rode their luck to take the three points back to Teesside and only their defensive resilience ensured yet another clean sheet.
He said: “When you are top this kind of thing happens in your favour. We have been really lucky today because to be fair in the first half Brentford had three chances and we have made two or three mistakes.
“Sometimes you pay for those mistakes but when you are top of table the luck can stay with you. We were lucky because in the first half I thought Brentford played really well.”
But Karanka believes the secret of his side’s mean streak is their willingness to work for each other.
He added: “They are confident on the pitch and that confidence means you can make mistakes but there are another 10 players who know you and are always there just in case.”
Karanka had keeper Dimi Konstantopoulos to thank for a handful of saves that kept his side in the hunt, denying Alan Judge, Lasse Vibe and Maxime Colin in an end-to-end affair.
Brentford head coach Dean Smith was pleased with the performance after Saturday’s FA Cup exit at the hands of his former club Walsall.
“I’m disappointed with the result but as I’ve said before we can only control performances and the performance was very good,” he said.
“If we continue to look after our performances the results will follow. The performance tonight would have been enough to beat many sides in this division.
“Boro are a good team with good players and we more than matched and at times looked marginally better than them.”
Smith absolved keeper Button of any blame for the defeat, insisting: “Everyone makes mistakes, but it’s how you respond to them that matters. As it was David made several decent stops during the game.
“We also had two or three good chances and really set about them in the first 20 minutes so if we’d taken one of them it would have been a very different game.
“At the end they had to hang on and resort to some cynical fouls which was a credit to the way we went about our job with good intensity.”
Brentford: Button, Colin, Dean, Tarkowski, Bidwell, Woods,Diagouraga, Saunders, Judge, Swift, Vibe.
Subs: Djuricin, McEachran, Hofmann, Bonham, Yennaris, Barbet, Canos.
Middlesbrough: Konstantopoulos, Nsue, Ayala, Amorebieta,Friend, Clayton, Leadbitter, Stuani, Downing, Adomah, Nugent.
Subs: Garcia, De Pena, Mejias, Fry, Kalas, Fabbrini, Forshaw.
Referee: Gavin Ward (Surrey)