Four-month-old Paul Crampton suffered three hypoglycaemic attacks in six days, when low blood sugar levels caused him to become cold, clammy and lethargic, Nottingham Crown Court was told on the eighth day of the trial of Beverley Allitt.
Dr Charith Nanayakkara, consultant paediatrician at the Grantham and Kesteven Hospital in Lincolnshire, said that tests on a sample of Paul's blood taken after the third attack found an insulin level of more than 500 milliunits per litre.
'In this child, who was not a diabetic, normal levels should be anything up to 15 or 16 . . . that's very high indeed,' he said.
Dr Fredrick Porter, another consultant paediatrician at the hospital, said: 'The cause in this case, we believe, was that the child had been injected with a large amount of insulin by the accused.'
The attack could have had 'very serious consequences', Dr Porter said.
Ms Allitt, a former nurse at the hospital, denies murdering four children and attempting to murder and causing grievous bodily harm to nine others, all patients on the children's ward.
Ms Allitt, 24, from Corby Glen, near Grantham, also denies attempting to murder and to cause grievous bodily harm to her flatmate's 15-year-old brother and to a 73-year-old woman.
Earlier, the court was told that a ward allocation book that showed which nurse had been assigned to which patients had been taken by Ms Allitt without permission. It was found when the police called at her in April 1991.
The case continues today.