Limitations in software being used by some ambulance call handlers means there is a “significant risk” signs of sepsis are being missed.
The East of England Ambulance Service (EEAS) said it was working with suppliers so extra questions could identify a patient with sepsis.
A report found one patient who called 999 should have been flagged as a risk.
Instead of sending a ambulance, the patient was told to go to their GP or make their own way to hospital.
The patient was later found to have an immune condition and bacterial infection and had to spend seven days in hospital.
The EEAS looked at the case, which happened in December 2018, and established there was “no specific protocol” in 999 software for identifying sepsis, also referred to as blood