Gravatt died of meningococcal septicaemia, in which the bacteria enter and multiply in the blood stream.
He was at first thought to have influenza, having arrived at hospital during the height of the 2009 influenza pandemic and because the two diseases' early symptoms can be similar.
At around 4.40pm he was given a 750mg dose of the antibiotic cefuroxime to treat pneumonia, which can result from the flu virus and was a working diagnosis at the time. At 6.40pm, after meningococcal septicaemia was diagnosed, he was given 2000mg of ceftriaxone, another antibiotic.
Thomas said treatment earlier than 4.40pm with an anti-meningococcal antibiotic "would have improved his chances of survival, but it is difficult to say by how much, given the range of variables that impact on mortality rates".
He said ceftriaxone is recommended for meningococcal disease because of its superior penetration into the cerebrospinal fluid, which is critical in meningitis caused by meningococcal bacteria. Gravatt did not have meningitis; he had the more dangerous variant of meningococcal disease, a severe infection of the blood.
"... the cefuroxime dose given to Zachary at [4.40pm] was sufficient to treat his meningococcal disease for at least the two hours prior to his receipt of the ceftriaxone at [6.40pm]."
Another expert witness, Dr David Knight, a Christchurch Hospital intensive care specialist, said the severity of Gravatt's meningococcal disease might have been detected sooner if he was seen earlier by an experienced senior intensive care doctor.
But Knight said he didn't feel comfortable commenting on the "likelihood of an improved outcome", had Gravatt been treated differently.
He said the diagnosis of meningococcal septicaemia should have been made at 5.30pm when the classic rash appeared on Gravatt's body.
Knight said the choice of cefuroxime antibiotic was consistent with many hospitals' in-patient pneumonia guidelines, but in serious infections the normal recommended dose was 1500mg, twice the amount Gravatt was given.
Cefuroxime was not a recommended first-line antibiotic for meningococcal septicaemia, but, at 3000mg, was appropriate for meningococcal meningitis.
"However, for the particular strain of [meningococcal bacteria] that Zachary was later found to be suffering from, 750mg is likely to have exceeded the minimum inhibitory concentration …"
"Cefuroxime was probably adequate to treat an intermediate resistant strain of [meningococcal bacteria]. However, the choice and dose of antibiotic were below standard guidelines for treatment of [meningococcal septicaemia].
"Guideline-appropriate antibiotics (ceftriaxone) were prescribed late and then only received after further delay."