Accredited official statistics

Glossary, data sources, and further information

Published 24 September 2026

Applies to England

Glossary and terms, data sources and methodology, and background information for the annual epidemiological commentary is detailed here.

Further information for Gram-negative, methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-susceptible Staphylococcus aureus (MSSA) bacteraemia is available at Epidemiological analysis of Gram-negative, MRSA, and MSSA bacteraemia.

Additional information for C. difficile infection is available at Epidemiological analysis of C. difficile infections.

Glossary

Age-sex standardised incidence rate

Populations differ in their age and sex composition, so sub-populations (for example, integrated care boards (ICBs), ethnic groups, or deprivation quintiles) can have different age and sex distributions. For diseases such as bacteraemia, susceptibility may be higher among neonates and older adults, or by sex. Age-sex standardisation adjusts for this by incorporating the age and sex distribution of a standard population. Each sub-population’s crude age-sex-specific rates are re-weighted using this standard age profile. The result is the incidence rate that would have occurred in the sub-population if it had the same age and sex distribution as the standard population. This makes fairer comparisons between groups possible. Further information on directly standardised rates which are used in this report.

Average

Scientifically speaking, this is a measure of location. It is a way of summarising the data while reducing the influence of extreme values. There are 3 main statistics to estimate this ‘average’ value – the mean, mode and median (further described in this glossary). Each of these methods has their own strengths and weaknesses.

Bacteraemia

The presence of bacteria in blood.

Bias

Bias is the systematic deviation of either results or inferences from the real situation.

Case-fatality rate (CFR)

CFR is a measure for comparing survivability of different infections and is expressed as the number of deaths out of all reported cases for that infection, as a percentage.

Confidence interval (CI)

CIs indicate the likely range in which an estimated parameter (such as a mean or rate) is likely to fall. For most scientific studies, it is impractical or impossible to measure every single member of a population and therefore the true population mean cannot be determined. Instead, a representative sample is taken, and the sample mean is used as an estimate of the population mean. Although the sample is intended to be representative, a different sample from the same population may provide a different result simply by chance. A CI, over unlimited repetitions of the sample, should contain the true value of a parameter (such as the true population mean) no less than its CI. It is usual to calculate the 95% CIs. That means that if we were to draw several independent, random samples from the same population and calculate 95% CIs from each of them, then 95% of such CIs would contain the true population mean. If we took 20 samples from the same population and calculated 95% CIs, then 19 of 20 (95%) of these 95% CIs would contain the true population meanwhile 1 of 20 (5%) will not.

Denominator

The lower portion of a rate or ratio quotient. This should reflect the population at risk of developing a disease for a given time (which is one year for this report).

Epidemiology

Study of the occurrence and distribution of events (mostly health-related) in a population.

Gram-negative bacteria

Class of bacteria that do not retain crystal violet stain as used as part of a differential staining technique (called the Gram stain). The Gram stain is used as a way of identifying bacteria and the difference in staining results are due to differences in the bacterial cell wall, which has important implications for antimicrobial usage.

Hepatobiliary

Term referring to the hepatobiliary system of the body, which includes the liver, gallbladder, bile ducts, biliary tract, and pancreas. Within the report, hepatobiliary is used as a category for the source of bacteraemia, referring to bacteraemia arising from infection of the hepatobiliary system.

Incidence and incidence rate

New cases of a disease occurring in a study population. An incidence rate is then the number of new cases that occur in a defined population in a defined period of time.

Gastrointestinal (excluding hepatobiliary)

Term referring to the gastrointestinal system of the body, excluding the hepatobiliary section (see Hepatobiliary). The gastrointestinal system includes the oesophagus, stomach, small intestine, large intestine, and appendix. Within the report, gastrointestinal (excluding hepatobiliary) is used as a category for the source of bacteraemia, referring to bacteraemia arising from infection of the gastrointestinal system.

NHS integrated care board (ICB)

An administrative unit of the NHS. NHS England has 4 administrative regions: North of England, Midlands and East of England, London, and South of England. Below these regions are 36 administrative geographies referred to as ICBs. As of 1 April 2026, there were 6 new ICBs established across England and the abolition of 12 existing ICBs. There was also a change in the boundary of an existing ICB. This report uses the new ICB structure as of 1 April 2026. Case counts and rates for the previous ICB structure are available in the accompanying data.

Mean

The arithmetic mean is the common statistic people regard as ‘average value’. It is calculated by summing all the values in a series and then dividing it by the number of included in the series. Mathematically, this is described by the following formula:

mean = (a1 + a1 + … + an) / n

A real-world example would be if you wanted to calculate the mean amount spent on food shopping over a 4-week period (that is, the average amount per week) having spent £51 in week one, £59 in week 2, £67 in week 3 and £52 in week 4:

mean cost of food per week = (£51 + £59 + £67 + £52) / 4 = £57.25

Median

The median of a series of numbers is the mid-point of that series. This provides a measure of an average value that is less affected by extreme values. The median of the following set of numbers [1, 2, 3] is 2, while the median of the set of numbers [1, 1, 1, 2, 10, 15, 16, 20, 100, 105, 110] is 15. To calculate it, the set of numbers needs to be arranged in order of magnitude and then the median is the number exactly in the middle. If there is an even number of values in a set, then the median value is the arithmetic mean of the 2 central values.

Mode

The mode is the most frequent value in a set of data (numbers or text values). In the following set of numbers [1, 1, 1, 2, 10, 15, 16, 20, 100, 105, 110] the mode is 1 as it was included in the set 3 times, while the other numbers were only included once.

Rate ratio

The rate ratio is the quotient of 2 rates. For example, if the rate of MRSA bacteraemia were 2 per 100,000 population in a year amongst male cases, and 4 per 100,000 population in a year amongst female cases, the female-male rate ratio would be 2.0 as the rate would be 2 times higher amongst female than male cases.

Respiratory tract

Term referring to the respiratory tract of the body, which includes the upper and lower airways and the lungs. Within the report, respiratory tract is used as a category for the source of bacteraemia, referring to bacteraemia arising from infection of the respiratory tract.

Urinary tract

Term referring to the urinary tract of the body, which includes the kidneys, ureters, bladder, and urethra. Within the report, urinary tract is used as a category for the source of bacteraemia, referring to bacteraemia arising from infection of the urinary tract.

Data sources and methodology

For further information on the methodology used to analyse data in this report, please refer to our quality and methodology information report.

The Healthcare-Associated Infections (HCAI) Data Capture System (DCS) is a web portal designed by the UK Health Security Agency (UKHSA) to collect an enhanced data set.

Trusts using the website have access to all the data they have entered. This can be compared to a regional and national aggregate total also available to trusts from the website. Sub-integrated care boards (sub-ICBs), ICBs, local authorities, and directors of public health (DPH) are also able to register as users, allowing them to access data specific to their patients.

The data set to be collected is described in the mandatory HCAI surveillance protocol available on the same site. Case unlocks can be requested by reporting organisations using the process described in the unlock requests user guide. Revisions to data is covered by a data-specific revisions and correction policy.

An R package for working with data downloaded from the DCS can be found on GitHub.

Inclusion criteria for reporting to the surveillance system

E. coli, Klebsiella spp. and P. aeruginosa bacteraemia

The following E. coli, Klebsiella spp. and P. aeruginosa positive blood cultures must be reported to UKHSA: all laboratory-confirmed cases of bacteraemia (including Klebsiella aerogenes), except cases identified post-mortem.

MRSA bacteraemia

The following positive blood cultures must be reported to UKHSA, for the mandatory MRSA surveillance: all cases of bacteraemia caused by S. aureus resistant to meticillin, oxacillin, cefoxitin or flucloxacillin.

MSSA bacteraemia

The following positive blood cultures must be reported to UKHSA, for the mandatory MSSA surveillance: all cases of bacteraemia caused by S. aureus which are susceptible to meticillin, oxacillin, cefoxitin, or flucloxacillin, that is, not subject to MRSA reporting.

C. difficile infection (CDI)

Any of the following defines a CDI in patients aged 2 years and over and must be reported to UKHSA:

• diarrhoea stools (Bristol Stool types 5 to 7) where the specimen is CDI toxin positive
• toxic megacolon or ileostomy where the specimen is C. difficile toxin positive
• pseudomembranous colitis revealed by lower gastro-intestinal endoscopy or Computed Tomography
• colonic histopathology characteristic of CDI (with or without diarrhoea or toxin detection) on a specimen obtained during endoscopy or colectomy
• faecal specimens collected post-mortem where the specimen is C. difficile toxin positive or tissue specimens collected post-mortem where pseudomembranous colitis is revealed or colonic histopathology is characteristic of CDI

Background information

This report outlines the latest trends and developments in Gram-negative bacteraemia and CDIs. It aims to provide valuable insights into the incidence, prevalence and future work related to these infections, contributing to the ongoing efforts to enhance patient safety in healthcare settings.

Throughout this report, ‘financial year’ is abbreviated to ‘FY’ and refers to the period between 1 April to 31 March in the years stated.

Throughout this report, Office for National Statistics (ONS) mid-year population estimates for calendar year 2024 were used as a proxy to calculate rates of infection. This is because population estimates for calendar year 2025 were unavailable at the time of analysis.

Prior exposure

In April 2017, the mandatory surveillance programme began capturing information on whether a patient with CDI had previously been admitted to the same reporting trust within the past 84 days (12 weeks). With the prior trust exposure, cases are split into specific groups, a full definition of these groups can be found in the glossary.

Unlike previous reports, CDI now matches the bacteraemia in its definition of a hospital-onset or hospital-onset healthcare-associated (HOHA) case being a case detected within a hospital stay that is at least 2 days long, with the day of admission being counted as day one and specimen date occurring on day 3 or after.

In April 2019, the mandatory surveillance programme began capturing mandatory information data on whether a patient with Gram-negative and Staphylococcus aureus bacteraemia had been admitted to the same reporting trust in the previous 28 days. Cases are classified into specific ‘prior trust exposure’ groups, whose definitions can be found in the glossary.

Prior trust exposure reporting became mandatory at different times for CDI (during FY 2017 to 2018), for MRSA and MSSA bacteraemia (during FY 2018 to 2019), and for E. coli, Klebsiella spp., and P. aeruginosa bacteraemia (during FY 2019 to 2020). The report describes data from the first full financial year after prior trust exposure reporting became mandatory for each data collection. Caution is advised when interpreting prior trust exposure data before it became mandatory, when missing data was common. Since the relevant fields became mandatory, there was a sharp decline in missing data.

Seasonality

Seasonality was assessed as the number of cases each quarter as a percentage of total cases for the financial year. This analysis was performed separately for hospital-onset and community-onset cases.

Age-sex analyses

Missingness for data on age or sex was rare for cases in the surveillance period. For all age and sex analyses, cases in which the age and/or sex was missing or recorded as unknown were excluded.

Source of bacteraemia

The HCAI DCS provides NHS trust users the opportunity to add information regarding the likely source of bacteraemia. This term refers to the likely underlying cause of the bacteraemia, such as an intravenous catheter or another infection at a different site, such as a skin or soft tissue infection which then led to the bacteraemia.

Further information and contact details

This publication forms part of the range of accredited official statistics outputs routinely published by UKHSA which also include monthly and quarterly reports on the mandatory surveillance of MRSA, MSSA, E. coli, Klebsiella spp. and P. aeruginosa bacteraemia, and CDI.

Quarterly report output

Further epidemiological analyses by quarter can be found in UKHSA’s quarterly epidemiological commentary.

Monthly report outputs

The following monthly reports are produced by UKHSA:

• Gram-negative, MRSA and MSSA bacteraemia and CDI: monthly data trends
• E. coli, Klebsiella spp., P. aeruginosa, MRSA and MSSA bacteraemia – counts of total reported, hospital-onset healthcare-associated, community-onset healthcare-associated, community-onset community-associated for each bacteraemia by organisation
• CDI – counts of total reported, hospital-onset healthcare-associated, community-onset healthcare-associated, community-onset of indeterminate association, community-onset community-associated CDI by organisation

Feedback and contact information

To provide feedback or for any queries, contact mandatory.surveillance@ukhsa.gov.uk

Accredited official statistics

These official statistics were independently reviewed by the Office for Statistics Regulation in May 2022. They comply with the standards of trustworthiness, quality and value in the Code of Practice for Statistics and should be labelled ‘accredited official statistics’. Accredited official statistics are called National Statistics in the Statistics and Registration Service Act 2007. Further explanation of accredited official statistics can be found on the Office for Statistics Regulation website.

Citation

Please cite this document as follows:

UK Health Security Agency. Annual epidemiological commentary: Gram-negative, MRSA, MSSA bacteraemia and CDIs, up to and including financial year 2025 to 2026. London: UK Health Security Agency, September 2026.