Comparability of drug-related death statistics across the United Kingdom
Statistics on deaths related to drug poisoning in the United Kingdom are compiled separately by:
- the National Records of Scotland (NRS)
- the Northern Ireland Statistical Research Agency (NISRA)
- the Office for National Statistics (ONS), who are responsible for England and Wales
The definitions used for drug-related death statistics are consistent across the UK, but there are important differences in data collection methods and in the death registration systems that affect these statistics. In this article we discuss these differences and their consequences for the comparability of the figures.
There are two headline figures used in drug-related death statistics: drug poisoning and drug misuse.
Drug poisoning
Deaths related to drug poisoning is the measure that should be used for comparing figures across the UK constituent countries. This is the “wide” definition. These deaths are currently referred to as “drug-related” deaths or “drug poisoning” deaths. In past reports, NRS had previously used the term “drug-related” to refer to “drug misuse”. The terminology was changed for the 2021 edition of the report to align with the rest of the UK. The underlying definitions have not changed.
The definition uses information on the cause of death supplied when deaths are registered and gives complete population coverage.
For a death to be classified as a drug poisoning it must have an applicable International Classification of Diseases (ICD) code assigned as the underlying cause of death. This is determined by World Health Organisation coding rules from the condition or conditions reported by the certifier, as recorded on the death certificate.
International Classification of Diseases, Tenth Revision (ICD-10) codes used to define deaths related to drug poisoning
The codes used to define deaths related to drug poisoning are as follows:
- codes F11 to F16 and F18 to F19 are used to refer to deaths relating to mental and behavioural disorders due to drug use (excluding alcohol and tobacco)
- codes X40 to X44 are used to refer to accidental poisoning by drugs, medicaments, and biological substances
- codes X60 to X64 are used to refer to intentional self-poisoning by drugs, medicaments, and biological substances
- code X85 is used to refer to assault by drugs, medicaments, and biological substances
- codes Y10 to Y14 are used to refer to poisoning by drugs, medicaments and biological substances, undetermined intent
Age-standardised mortality rates (ASMRs) allow for differences in the size and age structure of populations. This means they allow valid comparisons to be made between geographical areas, the sexes, and over time. Figure 1 shows ASMRs for drug poisoning deaths in the UK, and each of its constituent countries.
Figure 1: Age-standardised mortality rates for deaths related to drug poisoning, United Kingdom and constituent countries, registered between 2011 and 2021
A line chart showing the rate of drug-related deaths in the United Kingdom and its constituent countries from 2011 to 2021. The rate in Scotland has been consistently higher than the rest of the UK. In 2021 the rate in Scotland was more than double the rate in any other UK country.
Sources: Office for National Statistics, National Records of Scotland, and Northern Ireland Statistical Research Agency
Notes
- Age-standardised mortality rates per 100,000 people, standardised to the 2013 European Standard Population.
- Cause of death was defined using the International Classification of Diseases, Tenth Revision (ICD-10).
- Figures are for deaths registered, rather than deaths occurring in each calendar year.
- Figures include deaths of non-residents.
Drug Misuse
Deaths related to drug misuse is a subset of deaths related to drug poisoning. Figures for drug misuse should not be used for comparisons between UK countries. For a death to be classified as drug misuse the underlying cause of death must:
- be assigned an ICD-10 code of F11 to F16, or F18 to F19, regardless of the substances mentioned
- be assigned an ICD-10 code of X40–X44, X60–X64, X85, or Y10–Y14, and at least one of the substances mentioned is controlled under the Misuse of Drugs Act (1971)
This definition relies both on the underlying cause of death, and on additional information about the specific substances involved in the death. It is here that the comparability issues arise, as the extent to which this information is collected and recorded varies across the UK.
Differences in the death registration systems
In England and Wales, suspected drug-related deaths are referred to the local coroner’s service, who determine the cause of death. The ONS does not have access to post-mortem reports or toxicology results, so the accuracy of figures depends on the information provided by the coroner on the death certificate. The information on these certificates can often be incomplete, which means that figures for drug misuse are underestimates.
The process is similar for Northern Ireland. Deaths that were unexpected or suspicious are referred to the Coroner Service for Northern Ireland (CSNI) for further investigation. NISRA receives the coroner’s final decision which categorises the:
- type of death – for example, if it is drug-related
- intent – for example, if it is accidental, undetermined, or intentional
In most cases the decision includes details of the specific drugs that were above the threshold for therapeutic use, those that contributed to the death, or drugs that fit both categories. However, like the ONS, NISRA do not currently have access to toxicology reports or any supplementary information on all drugs found in the system of the person who has died.
In Scotland, if the death was sudden, unexplained or suspicious it is reported to the Crown Office and Procurator Fiscal Service (COPFS) for investigation. For these deaths a post-mortem investigation is often carried out and toxicology tests are done to see which substances are in the body. If the death involved drugs or the person was a drug user, the pathologist sends this additional information to the NRS. This records which substances were found in the body and which of them contributed to the death.
In summary, for England, Wales and Northern Ireland, the only information received by the ONS and NISRA is what is included on the death certificate. The amount of information varies and can be very limited. For drug-related deaths in Scotland, NRS receives additional information on the drugs involved.
The effect of death registration systems on drug-related death statistics
The differences mean that the amount of information held on drug-related deaths varies across the UK. In 2021, 25.1% of drug-related deaths registered in England and Wales had no information on the specific substances involved. The equivalent figure for Scotland was 1.9%, and for Northern Ireland the figure was 6.6%.
The proportion of drug-related deaths where no information about specific substances is known has remained consistent over time in Scotland and Northern Ireland. Meanwhile, the proportion of these deaths has been increasing over time in England and Wales, with important consequences for drug-related death statistics.
Figure 2: Percentage of drug-related deaths in England and Wales, Scotland, and Northern Ireland, where no information is held on the specific substances involved
A line chart showing the proportion of drug-related deaths, about which the drug types are not known, has increased in England and Wales relative to the rest of the UK.
Sources: Office for National Statistics, National Records of Scotland, and Northern Ireland Statistical Research Agency
Notes
- ‘No information’ refers to deaths where the information on the substance is limited to ‘drug’, ‘multi-drug’ or equivalent. Broad drug categories such as ‘painkiller’ are not included.
- Cause of death was defined using the International Classification of Diseases, Ninth Revision (ICD-9) and Tenth Revision (ICD-10).
- Figures are for deaths registered, rather than deaths occurring in each calendar year.
In most cases a death cannot be called a drug misuse death when no information on the specific substances is known or provided. Because of this, figures for drug misuse are underestimates. But the figure for England and Wales underestimates the number by a far greater extent. Approximately a thousand death certificates were missing information about specific substances in 2021 alone.
To demonstrate the potential effect of this missing information in England and Wales, we can create an adjusted rate of drug misuse deaths. This assumes that the deaths without information on drug type have the same distributions as those about which the information has been provided. Please note that this is just an example and may not reflect reality.
Figure 3: Published and adjusted drug misuse age-standardised rates per 100,000, England and Wales, 1993 to 2021
A line chart showing that the published rate of drug misuse is likely to have increasingly underestimated the true rate over time.
Source: Office for National Statistics
Notes
- The adjusted misuse rate is for illustrative purposes only and should not be used for comparisons with published rates of drug misuse.
- Cause of death was defined using the International Classification of Diseases, Ninth Revision (ICD-9) and Tenth Revision (ICD-10).
- Figures are for deaths registered, rather than deaths occurring in each calendar year.
The effect of the missing data can be seen in the way the adjusted misuse rate differs from the published misuse rate. As the amount of missing data has increased, the published drug misuse rate is likely to have underestimated the true rate of drug misuse deaths by an increasingly large amount.
The different levels of missing data mean that the respective figures for drug misuse published by the ONS for England and Wales, NRS for Scotland, and NISRA for Northern Ireland, are not directly comparable. It also means that you should take care when comparing different years within the England and Wales time-series. The same problems for comparability found with drug misuse also apply to figures for all individual substances and drug categories.
Future improvements to UK drug-related death statistics
The ONS will work to improve the quality of drug-related death statistics for England and Wales. This will include more regular processing of drug misuse data as well as working with the Office of the Chief Coroner to improve the quality of information around drug-related deaths. The ONS will also explore new data sources and provide new analyses to support the UK government drug strategy.
The respective producers of UK drug-related statistics will work together to ensure there is:
- transparency around differences in the quality of data
- coherence in definitions and terminology
- clarity on which measures should be used for comparisons, and which should not
UK drug-related deaths annual bulletins
The following bulletins about UK drug-related deaths are produced each year:
- Deaths related to drug poisoning in England and Wales – Office for National Statistics (ons.gov.uk) — this bulletin is prepared by the Office for National Statistics (ONS)
- Drug-related Deaths in Scotland — this bulletin is prepared by National Records of Scotland (NRS)
- Drug-Related Deaths — this bulletin is prepared by the Northern Ireland Statistics and Research Agency (NISRA)
Paul Breen