Background and approach

How we estimate population health

Often-cited measures of population health are ‘healthy’ and ‘disability-free’ life expectancy – the number of years a person born can expect to remain in good health or without disability. Healthy and disability-free life expectancy are based on people’s self-reported experiences of illness. This is dependent on several interrelated factors such as: the presence of illness (and its effect on the ability to undertake daily activities); condition management; availability and effect of treatment and finally the context and expectations of the person. Diagnosed illness has been shown to have a more direct relationship with health care demand. Moreover, studies have shown that the relationship between self-reported health and diagnosed health can vary for different population groups.,,,, This suggests that diagnosed illness may be a more accurate predictor of demand for health care than healthy and disability-free life expectancy.

In this report we therefore focus on levels of diagnosed illness using primary care administrative data from the Clinical Practice Research Datalink (CPRD), linked to secondary care records (from the Hospital Episode Statistics (HES)) and mortality records. Although no measure is perfect and some illness remains undiagnosed in health records (see Box 1), patient-level primary care records have several advantages. First, they capture instances of well-managed illness that might not be reflected in hospital records. Second, they also enable estimation of illness for more granular population groups than more aggregated health data. Access to patient-level primary care records is a rarity in UK health policy research – to our knowledge this is the first time they have been used in a model projecting health needs in England.

How we use the Cambridge Multimorbidity Score (CMS)

In order to quantify illness and multimorbidity more precisely, we use the Cambridge Multimorbidity Score (CMS). The score assigns a weight or score to 20 conditions based on how the illness is likely to affect patients’ use of primary care, emergency health services and likelihood of death. For instance, cancer and heart failure are given higher scores than hypertension (high blood pressure) or hearing loss, because they are more likely to lead to death, unplanned hospital admissions or greater primary care needs. For those with multimorbidity, scores are added together, meaning individuals with the same score can have a different number and combination of conditions. This provides a common metric across illnesses and allows us to aggregate multimorbidity. We then use the average score for different population groups to define average illness levels. The assigned scores for the CMS are reproduced in the modelling working paper, section 8.1.

In addition, we split the population into three groups by CMS, as summarised in Table 1. We did this in order to summarise the complexity of multimorbidity in the simplest way possible: these categories are particularly useful when projecting the total number of people with similar levels of illness that lead to high health care needs.

Table 1: Using the Cambridge Multimorbidity Score (CMS) to estimate average levels of illness within age groups, England, 2019

Illness measure

CMS range

Percentage of population aged 30–69 years

Percentage of population aged 70 years and older

Percentage of total population aged 30 and older

No illness

0

53%

12%

45%

Some illness

CMS greater than zero but up to 1.5

37%

41%

38%

Major illness

CMS greater than 1.5*

10%

47%

17%

Note: Each column adds up to 100%.

Nearly half (45%) of the population aged 30 years and older in England have none of the 20 chronic conditions. Nearly a fifth (17%) have major illness as indicated by a CMS greater than 1.5. There is no officially recognised threshold for an individual’s overall CMS that indicates high health care needs or a high risk of mortality. We chose a threshold of 1.5 because it can indicate the presence of multimorbidity for conditions other than single diagnoses of cancer or dementia.

As illnesses tend to accumulate, many people will pass through the three groups as they age and are diagnosed with more conditions, causing their CMS to rise. The relationship between CMS and age is highlighted by the contrast between the third and fourth columns in Table 1. Almost half of those aged 70 years and older in our data fall into the major illness category in 2019, compared with just 10% of the 30–69-year-old population.

How we project future levels of illness

Modelling

The REAL Centre’s demand model projects future levels of illness by combining the CMS with a microsimulation model, the IMPACTNCD framework, developed by the University of Liverpool. Our model uses epidemiological evidence on the relationships between risk factors such as smoking and diet, the 20 health conditions we measure and mortality to project levels of diagnosed ill health in England’s population aged 30 years and older over the next two decades.

Broadly, the model works by creating a population that is representative of England from 2013 to 2019 using primary care administrative records. The model is then used to project longevity and health by simulating the onset of 20 chronic conditions and deaths for each subsequent year. This means that for each year, the model randomly assigns conditions to each individual with probabilities based on their characteristics including age, sex, geographic region, ethnicity and socioeconomic status. The probability of each individual dying each year is then ascribed on the same basis. The probability of onset of illness and death also depend on their assigned exposure to biological and behavioural risk factors and epidemiological evidence on risk factors. The risk factors we include are smoking, alcohol, diet, exercise, BMI, blood pressure and cholesterol. Future levels of risk factors are based on an assumed log-linear continuation of current trends, ie trends continue but the rate of change decreases over time.

Individual-level health changes produced by the model are then summed up to form the basis for our projections. Further details can be found in our modelling working paper, section 8.3. To isolate the effect of changes to the average level of illness from changes in population structure over time, we show trends in illness standardised for age, sex and deprivation alongside overall trends throughout the report.

Box 1: What are the limitations of our approach?

Projections are designed to make an informed assessment of what could happen in the future based on current data and trends. There is inherent uncertainty in projections analysis as it requires many assumptions about the factors that determine the number of people living with health care needs. However, given the extent of recent improvements in methods and data linkage, this report gives a uniquely detailed presentation of ageing and multimorbidity. The model itself is shared in greater detail in our modelling working paper and technical appendix. Aside from the model, we highlight four key limitations of the data and methods we use.

First, throughout this report we project trends in diagnosed morbidity. We use novel data that link primary and secondary care health records to measure morbidity rates for different population groups. However, these will differ from actual population morbidity rates because they do not account for unmet need. Over time the quality of electronic health records has improved. Whether changes in diagnosis over time were the result of true changes in people’s underlying health or better data recording cannot be deduced from these data. However, the impact of diagnosis and coding practices is expected to have a far more muted effect on major illness.

Second, our projections assume that the impact of ill health on care use remains the same over time. This is because our definition of morbidity is based on the CMS, which encapsulates the static relationship between individual conditions and their impact on health care use and mortality at a given point in time. But in reality, this impact can change over time due to changes in the severity of these illnesses, the stage of illness at which patients are diagnosed, or how they are treated and managed by the NHS and other care services. Medical science and technology has changed a lot, with innovations in pharmaceuticals, surgery and other treatments, as well as improvements in care delivery or screening policies leading to earlier diagnosis. All these can make a substantial difference to patients’ quality of life and outcomes. However, given the high uncertainty around the scale of these possible future improvements, in this report we project forward the status quo of the current state of care. Third, our projections make use of the Office for National Statistics (ONS) principal population projections. Additional uncertainty from the potential variation from those projections is not included in this analysis.

Fourth, the CMS does not include every single chronic condition. It is made up of 20 conditions that contributed to 73% of disability-adjusted life years due to chronic illness in England in 2019. This means the CMS cannot capture the full burden of chronic illness, but does capture a large majority of it.

Potential pathways for overall changes in the population’s health

The model can be used to analyse several aspects of projected future levels of illness. In this report we focus on two. The first is how the time spent in ill health by the average person is projected to change over time. The second is how the overall level of ill health is projected to change across the population, including the projected growth in the size and the ageing of the population in England (henceforth ‘population change’).

Several paths for population health are possible, depending on the relative changes in onset of illness and life expectancy:

  • More illness (‘expansion of morbidity’): as a result of longer life expectancy, earlier onset of illness or both, people spend more time in ill health.
  • Less illness (‘compression of morbidity’): population health improves faster than life expectancy, so people become ill later and spend less time in ill health.
  • No change: the time people spend in ill health remains the same. The population might live longer, but the additional years are spent partly in poor health, partly in good health.
  • More illness, but less severity (‘dynamic equilibrium’): people spend a greater share of their lives with diagnosed conditions, but the care needs associated with dealing with those conditions are lower.

The path taken has direct implications for health care demand, as longer periods of morbidity imply a greater need for health services. The consequences for health and wellbeing are more ambiguous as it depends not just on how the length of time with illness is changing but why. This is because if an individual’s ill health is well managed they can have an independent, enjoyable and active life. In this report, when we refer to health care demand, we focus only on the implications for the NHS given limited data on care needs and the complex relationship between health and social care that has been investigated elsewhere.

To adequately inform resource planning, we must also take into account population change. To do this, we combine estimates of the time spent in ill health with ONS projections of the size and demographic structure of the population. It is this overall burden of illness that has direct implications for health care demand. Previous attempts to project health spending, such as by the OBR in the UK and the OECD, have relied on different assumptions about future population health trajectories.,, Our results provide projections of population health using current trends in risk factors and life expectancy, and therefore may help provide some consistency in these exercises.

Due to our focus on long-term trends in ageing and health and our data sample covering up until 2019/20, we do not include COVID-19 and its impacts in our analysis. The pandemic has affected the lives of many, caused disruption to health, diagnostic and community services, and the impacts are not yet fully realised. Our focus on the decades to come means it is not possible to include sensible estimates of the long-term impacts of COVID-19 on ageing and multimorbidity.


** The analysis was conducted using the Clinical Practice Research Datalink (CPRD). The data are provided by patients and collected by the NHS as part of its care and support. Regulatory approvals to use CPRD data for this analysis were granted by the CPRD Independent Scientific Advisory Committee (ISAC protocol number 20-000096). (https://cprd.com/protocol/econometric-analysis-distribution-primary-and-secondary-costs-and-activity-patients-non).

†† These conditions are: dementia, cancer (all types), chronic obstructive pulmonary disease (COPD), atrial fibrillation, heart failure, constipation, epilepsy, chronic pain, stroke/transient ischaemic attack (TIA), diabetes (type 1 or 2), alcohol problems, psychosis/bipolar disorder, chronic kidney disease, anxiety/depression, coronary heart disease, connective tissue disorders, irritable bowel syndrome, asthma, hearing loss and hypertension.

‡‡ In 2019 for those aged 30 years and older in England with any illness, nearly a third of all individuals had a score greater than 1.5 (70th percentile). The mean score was 1.2 and the median score was 0.8.

§§ The model begins in 2013 to allow for quality assurance checks between the modelled and observed data but our analysis uses 2019 as the starting year.

¶¶ People living in more deprived areas tend to be younger compared with those living in more affluent areas. This is because, among other things, individuals from more affluent areas tend to live for longer. Population growth is also different among deprived and affluent areas. Therefore, when we present outcomes by year, we use this standardisation to account for the effect of different age distributions and different growth among the age – sex – decile groups of Index of Multiple Deprivation areas.

*** In its central projection of health spending, the Office for Budget Responsibility (OBR), the independent government spending watchdog for the UK, assumes constant levels of health by age and sex that, given the projected increases in life expectancy, means an expansion of morbidity.

††† The OECD (2012) assumes, in its cost pressure and cost containment scenarios, various degrees of compression of morbidity (or ‘healthy ageing’) by modelling projected additional years of life to be spent in good health.

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