Admitted patients are people who are formally admitted to a hospital bed for day case procedures, and those who are admitted for a longer period as inpatients.

Each outpatient visit is termed an attendance and is roughly equivalent to an admitted patient episode except that a patient can have several episodes between admission and discharge, whereas each outpatient attendance will be separated in time from the next. However, it is quite possible for a single patient to have many outpatient attendances over the course of a year, often at several different clinics.

All treatment provided to NHS patients in hospitals, treatment centres and diagnostic centres is required for HES even where the care is taking place in the independent sector. Where the independent sector is caring for patients on behalf of the NHS they are obliged to ensure that the same information is collected and sent for HES and that it uses the same national data standards. It is unlikely that the independent sector treatment and diagnostic centres are in a position to transmit data directly to the Seconday Uses Service, but they may have an agreement with an NHS organisation to transmit the information on their behalf. However, it is likely that at the moment the information is less complete than it is for most NHS organisations.

Yes and no. We believe that some of the care provided in mobile treatment centres is being sent and captured by HES. However, at the moment we do not have a means of differentiating the activity from that provided by the rest of the organisation.

Yes. It became mandatory for trusts to submit these data from April 2003. However, there may still be low coverage. Regular day and night attenders are not generally included in the main publication tables on the web site and they are not included in the grossed and ungrossed statistics measures in the online HES system, otherwise they will distort annual comparisons.

Each of the other parts of the UK - Scotland, Wales and Northern Ireland - have their own, similar systems. Similarly, most European countries collect information on hospital activity.

From 2004-05, HES has information about the registration and qualification specialty of the consultant leading the team as well as information about the specialist service in which the patient was treated. Prior to this date, all specialty information referred to the consultant registration.

Yes, but only the postcode and not the rest of the address. The patient's postcode is used to automatically derive information about their place of residence - e.g. which local authority area they lived in. However, access to the postcode itself is very strictly controlled. HES can therefore be used to investigate possible geographic factors in the prevalence of diseases, and to give information about the distances patients travel in order to be treated in particular hospitals.

The HES publication tables provide both mean (average) and median lengths of stay. The length of stay is the number of days between admission and discharge, and varies depending on diagnosis, the nature of any operation performed, and the patient's age and general condition. While in hospital, a patient may be transferred from one consultant to another (this happens in about 10% of cases). HES therefore also records the duration of each consultant episode - the number of days the patient was under the care of a particular consultant. A closely related figure provided by HES is the number of occupied bed days during the financial year, which is a useful indication of the resources expended in delivering admitted patient care.

Note that day cases - normally minor surgery and not involving an overnight stay - are excluded from all length of stay calculations. This is because the date of admission and the date of discharge will be the same. HES does not record the times of admission and discharge, so is unable to give information about the relative duration of different day case treatments.

The HES calculated waiting time for admitted patients has always been slightly different from the NHS supplied waiting time. This was largely because the HES waiting time was the maximum waiting time at a given provider, with no allowance or deduction made for any days when the patient wasn't available for treatment, for example due to illness or holidays. These deductible days are termed suspensions or deferrals from the waiting list.

In 2003, changes were made to the official waiting time calculations to reflect more closely the patient's perception of how long they had waited (DSCN 37/2003). The new official waiting time is calculated from the time at which the patient is placed on the waiting list at any provider until the time at which the patient is admitted for the treatment required, regardless of whether the admission is to the same, original provider or to a subsequent provider who has taken on responsibility for the patient. This can often be a result of the Patient Choice initiative. This means that there will still be discrepancy with the HES calculated waiting time, which only measures the time waited on the list of the provider who actually treated the patient. However, detailed analysis has been done which demonstrates that HES can be a useful source of information on waiting times as long as the issues are understood.

Operations can be relatively simple, such as the removal of a skin lesion, or they can be complex, such as when a patient has suffered a serious road traffic accident and has multiple injuries that need to be treated.

What a patient thinks of as an 'operation' can actually be made up of several separate procedures and interventions. These different aspects of each operation (the actual procedures and interventions that are used) are individually recorded in HES using OPCS-4.3 codes. For example, a patient who undergoes surgery to replace their knee joint could have three OPCS-4.3 codes recorded in one episode:

• Main procedure/intervention = W40.1 Primary total prosthetic replacement of knee joint using cement (surgical procedure)
• 1st secondary procedure/intervention = U13.4 Plain x-ray of joint (intervention)
• 2nd secondary procedure/intervention = Z94.2 Right sided operation (laterality of operation)

Historically within HES, the words 'operation' and 'procedure' have been used interchangeably. However, when OPCS-4.3 codes were introduced in 2006-07, the classification was also renamed the Classification of Interventions and Procedures (from the Classification of Surgical Operations and Procedures). In line with these changes, HES changed how it refers to operations, procedures and interventions.

The two main clinical classifications currently used in HES are ICD-10 (International Classification of Diseases, 10th revision) and OPCS-4 (Office of Population, Censuses and Surveys: Classification of Interventions and Procedures, 4th revision). Both classifications have changed over time:

• Diagnoses are recorded using ICD, which describes the conditions treated or investigated. ICD-10 was first used for coding diagnoses in 1995-96 (ICD-9 was used from 1989-90 to 1994-95).
• Procedures and interventions are recorded using OPCS-4, which describes the procedures and interventions used during treatment. OPCS-4.3 was first used for coding procedures and interventions in 2006-07 (OPCS-4.2 was used from 1989-90 to 2005-06 to record operations).

You can read more information about the changes on the Connecting for Health website.