Table 2.

Summary of the 840 IMD cases diagnosed in England during 2007–2011 that were confirmed by PHE MRU but could not be linked to a Hospital Episode Statistic (HES) admission matched to an A39* (meningococcal disease) or G00* (bacterial meningo-encephalitis) diagnosis (MRU+/HESA39- cases)

Outcome of linkage	Comment
Linked with a non-specific Infection-related code (n = 248)	These MRU-confirmed cases did not link with a HES A39* (meningococcal disease) or G00* (bacterial meningo-encephalitis) diagnosis but had non-specific infection-related codes, such as rash (n = 75), unspecified fever (n = 73), nausea and vomiting (n = 45), unspecified septicaemia (n = 31), unspecified viral infection (n = 23), lobar pneumonia (n = 17), unspecified pneumonia (n = 13) and unspecified viral meningitis (n = 12). Compared to children and adults, those aged ≥65 years were over-represented among the unlinked cases, possibly because IMD was less likely to be considered in the differential diagnosis for this age group. The ≥65 year-olds were also more likely to have a non-meningococcal cause (e.g. pneumococcal pneumonia) recorded in their discharge diagnosis
NHS numbers available but did not link to HES admission (n = 210)	These MRU-confirmed cases had NHS numbers but did not link to a HES admission within 30 days of sample receipt. They were more likely to be infants, toddlers or young adults (15–24 year-olds) and had the highest case fatality across the age groups (35 %) (Table 3). The most likely explanation for non-linkage is that these patients died before they could be hospitalised
Another pathogen was recorded in HES (n = 71)	These MRU-confirmed cases were coded in HES as having another infection, such as group B streptococcal (n = 14), S. pneumoniae or E. coli infection, or simply Gram-negative septicaemia (n = 12).
Cases without an infection-related code (n = 26)*	This was the smallest group where the MRU-confirmed case linked to a HES admission that did not have an infection-related code. Two cases had a HES discharge diagnosis of “diagnosis not known”. This could be coding error in HES or the patient may have been hospitalised with another illness within 30 days of MRU-confirmation of IMD
No NHS Number and not linked to HES admission (n = 285)	These cases followed a similar age-distribution as MRU-confirmed IMD cases, which may suggest that they are genuine IMD cases but could not be linked to HES because of lack of sufficient identifiers.

IMD invasive meningococcal disease, MRU Meningococcal Reference Unit, HES Hospital Episode Statistic, NHS National Health Service