Cookies on the
NELA website

Cookies are small text files held on your computer. They allow us to give you the best browsing experience possible and mean we can understand how you use our site. Some cookies may have already been set. You can delete and block the cookies but this may impair the functionality of the website. To find out more about the cookies, please see our privacy policy.

Case Ascertainment Queries

NELA follows a standard approach for dealing with queries about case ascertainment. The sections below provide information on how case ascertainment rates are calculated and queries processed.

Please check back regularly for new updates. If you have additional queries, please contact us.


How are case ascertainment rates calculated?

The NELA project team have developed an algorithm that extracts cases that meet the NELA inclusion criteria from NHS England's Hospital Episode Statistics (HES). HES is comprised of patient data coded by the hospital's own coding team. Case ascertainment is calculated by applying this case inclusion algorithm to HES data.

How are case ascertainment rates fed back to hospitals?

During each data collection year, hospitals are provided with an estimate of case ascertainment on a monthly basis. This is calculated from historical HES figures. As such, it may not match current activity due to natural variation in caseload. It is also potentially inaccurate if there have been service reconfigurations such that case mix has changed significantly over time.

Can you provide case ascertainment figures for Wales?

At present, we are unable to provide case ascertainment figures for Welsh hospitals due to availability of PEDW data. However, we expect this to change in the near future.

How does HES data feed into the annual NELA reports?

For each annual report, contemporaneous HES data is used to calculate the actual case ascertainment rate, so that the information in the annual report is based on HES data that matches the data collection period (the overlap is not exact however, due to slight differences in annual reporting cycles).

I have a discrepancy between my expected and actual case numbers. Why is this?

We do receive queries from hospitals where there appears to be discrepancy between expected and actual cases, i.e the number of cases entered into NELA is less or more than that contained within HES. This has the potential to show a hospital's case ascertainment rating as e.g. amber, when it should be green, or vice versa. If there is discrepancy between a hospital's own figures, and HES, it will either be due to:

  • Accurate NELA data, but inaccurate coding by the hospital, such that HES indicates a higher caseload.
  • Accurate NELA data, but inaccurate coding by the hospital, such that HES indicates a lower caseload. This results in case ascertainment rates consistently >100%.
  • Inaccurate NELA data, but accurate coding by the hospital.

How can I submit a case ascertainment query?

In the event that we receive queries about case ascertainment, the following process is followed:

  • Please provide details of any recent service reconfiguration that may have affected your hospital's expected caseload.
  • We require confirmation that there has been a robust and systematic search for NELA cases by the local NELA leads, for example using theatre booking systems or theatre logbooks.
  • NELA will analyse the cases found within the NELA data and the HES data for that hospital. This will reveal if the hospital has an unusual spread of activity indicated by HES codes. It may also reveal that a particular group of patients has been entered into NELA erroneously (eg excluded cases). The results of this will be communicated to the local NELA leads.
  • NELA will provide a list of OPCS codes to the hospital. This list is available in our Audit Info Documents section. We would suggest that local NELA leads discuss this with their own coding department. Local NELA leads should consider asking their hospital coding department to provide a list of patients coded with the NELA OPCS codes, so they can compare their NELA submission to that produced by their hospital's coding team, which is the basis of the HES coded data. We are able to provide further information on the use of the OPCS codes if required.

Once these stages have been completed, NELA will be able to offer further advice and guidance on whether changes to the case ascertainment figures are warranted.

What can inaccurate OPCS coding mean for my hospital?

Inaccurate coding means that the hospital is not getting paid appropriately for the clinical activity it performs. Improving this has the potential to increase hospital income.

Can you send me the list of HES cases for my hospital?

Unfortunately, we are currently constrained by the legalities of data sharing agreements, such that we cannot provide hospitals with a list of cases found within HES. We are hopeful that this can be rectified in the near future.