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She sent him to the hospital for an arterial blood gas draw.

Mr Johnson then asked Ms Letby about a blood gas record chart relating to one of the babies in this case, which was also found at her home.

“What we were seeing anecdotally was exactly what we ended up showing in the final paper, that on the monitor in the patient’s room, the pulse oximeter would be reading ‘normal,’ but when we got an arterial blood gas, the saturation on the gas was low,” he said.

The study compared pulse oximeter measures with values obtained from a more invasive type of test, called an arterial blood gas test, carried out in the same patients at about the same time.

Arterial blood gas tests are used more rarely, because they require drawing blood from an artery, which is a more invasive procedure than drawing blood from a vein.