A spinal tap was performed on the wrong baby at Tunbridge Wells Hospital, it has emerged.
Health chiefs are carrying out an investigation after the procedure - known formally as a lumbar puncture - was carried out on the child on February 12.
It is performed by inserting a needle between two vertebrae to remove a sample of fluid and works to diagnose diseases of the central nervous system, including the brain and spine.
Maidstone and Tunbridge Wells NHS Trust, which runs the hospital, said two babies met the criteria for sepsis screening due to a history of maternal sepsis and the mother's water breaking.
Both youngsters were taken to the neonatal unit from the postnatal ward to have bloods taken and checked for inflammation.
Their parents were each told there may be a need for a spinal tap but results showed only one of them - Baby 1 - had raised inflammation markers, known as C-Reactive Protein (CRP), and would need the procedure.
Generally, lumbar punctures are only considered when patients have CRP of 20, but Baby 2 was taken back to the neonatal unit and underwent the procedure, despite having CRP of five.
This was raised significantly to 72 after it was performed, though hospital bosses say the infant remained well throughout and began taking antibiotics.
Baby 1 then subsequently had the spinal tap the following day and was not negatively affected by the delay.
Ultimately, no harm occurred to either baby, the hospital insists, but it has been documented as a 'never event' - patient safety incidents that are wholly preventable.
To prevent a repeat in the future, an email was sent to all staff insisting that all babies needing a lumbar puncture must have their name band checked with the results on the neonatal unit, and that this should be documented in the medical notes.
Furthermore, a guideline for carrying out the procedure has been drafted and will be more widely distributed once signed off, while a draft parent information leaflet has also been written and will be taken to the neonatal guideline group this month.
A trust spokesman said: “Patient safety is our absolute priority and we are currently carrying out a detailed investigation into the matter.
"While we investigate we have taken a number of immediate actions to further improve the care we give our patients.
“Never events are rare in our hospitals and it is our aim to ensure we consistently deliver the very highest standards of clinical care for our patients.”