A pensioner died after she was given the wrong type of blood following an operation at a private hospital in Leeds, the city's Crown Court was told.
Joan Jones died four days after her operation
Joan Jones, 76, whose blood group was O rhesus negative, was incorrectly given AB rhesus negative at a BUPA hospital and died of multiple organ failure.
David Pratt, who was deputy manager of the hospital's pathology laboratory, denies manslaughter.
Mrs Jones died four days after an orthopaedic operation in October, 2004.
Jeremy Richardson QC, prosecuting, said: "The cause of her death was multi-organ failure due to being given an incompatible blood transfusion.
"In simple terms, she was given the wrong blood and this caused her death."
The jury was told Mrs Jones underwent the operation on 1 October 2004 at the BUPA hospital in Roundhay but her condition began to deteriorate shortly afterwards and she was transferred to Leeds General Infirmary, where she died on 5 October.
"He made these mistakes and they were so serious as to amount to gross negligence and therefore the criminal offence of manslaughter," Mr Richardson said.
The court heard Mr Pratt , of Tealby Close, Leeds, had worked in the pathology laboratory at the hospital since 1989, had more than 25 years of experience, and was "more than fully qualified" to fulfil his role.
The jury was told that prior to a blood transfusion taking place, the patient is first grouped according to their blood type, and then this is "cross-matched" or compared with the donor blood before the procedure is carried out.
The jury also heard that there would be a "very obvious reaction" in the cross-matching stage if a patient was about to be given the wrong type of blood.
Mr Richardson said there were national standards and internal standard operating procedures (SOPs) in place in a bid to ensure nothing went wrong.
The trial is continuing.