Det Con Stephen Benbow, 32, who had been drinking in the King's Arms, Hampton Court, said that he could not remember why he veered across the road because he suffered from concussion during the accident.
He claimed that he left the pub at closing time after drinking about three and a half pints, but a blood sample was not taken for more than three hours. Even then, the test showed that he was only under the legal drink-drive limit by 11mgs.
The other driver, Raymond Close, 34, of Tolworth, Surrey, died instantly in the crash on Upper Sunbury Road, Hampton, at 11.20pm on 1 December last year.
Mr Benbow, who has been suspended from the Metropolitan Police's Nine Regional Crime Squad since the accident, denies causing death by dangerous driving. He told the court: 'I was not affected in any way whatsoever by the drink, and I would not have got into the car if I thought I was over the limit, I would have got a taxi.
'I can't remember what happened once I hit my head. All I do know is that for some reason I lost control of the vehicle.'
The trial continues today.