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Recorded Dementia Diagnoses - Methodology of Indicators

We collect and publish data about people with dementia at each GP practice so that the NHS (GP’s and commissioners) can make informed choices about how to plan their services around their patient needs. There are measures used to assess the number of patients with dementia, and those who have had a formal diagnosis. The recorded dementia diagnoses data also contains an indicator called Dementia 65+estimated diagnosis rate. The methodology can be found below

Indicator specification

Indicator title

Dementia: 65+ Estimated Diagnosis Rate

Changes from previous versions

From 2017/18 this indicator methodology replaces those used previously by the below domains and will not produce comparable results when applied to the same source data. Where the new indicator time series overlaps with previously published periods, values will differ.

Indicator family name

CCG Outcomes Indicator Set (OIS) Domain 2 – Enhancing the quality of life of people with long term conditions. 

Public Health Outcomes Framework - Healthcare and premature mortality domain; mental health, dementia and neurology: Dementia profile

CCG Improvement and Assessment Framework – Better Care

NHS England Operational Information for Commissioning - Delivering the Forward View

Condition/topic area

Long term conditions

Detailed description

Plain English description

Not everyone with dementia has a formal diagnosis. The indicator compares the number of people thought to have dementia with the number of people diagnosed with dementia, aged 65 and over. The target is for at least two thirds of people with dementia to be diagnosed.

Technical description

The rate of persons aged 65 and over with a recorded diagnosis of dementia per person estimated to have dementia given the characteristics of the population and the age and sex specific prevalence rates of the Cognitive Function and Ageing Study II, expressed as a percentage with 95% confidence intervals. Significance is determined by the non-overlapping of confidence intervals with the 66.7% benchmark.

Data sources

Denominator: registered patients

Patients aged 65+ registered for General Medical Services, counts by 5-year age and sex band from the National Health Application and Infrastructure Services (NHAIS / Exeter) system; extracted on the first day of each month following the reporting period end date of the numerator. Source: NHS Digital.

Plain English: Patients registered at a general practice with 5-year age band and sex. It is extracted on the first day of the month, the day after the end of the data period for the numerator.

Numerator: recorded dementia prevalence

Patients aged 65+ registered for General Medical Services with an unresolved diagnosis of dementia, counts by 5-year age and sex band from GP Clinical Systems via the General Practice Extraction Service (GPES); extracted on the reporting period end date (the last day of the month). Source: NHS Digital.

Plain English: Patients registered at a general practice with a diagnosis of dementia, including counts by 5-year age bands and sex. It is extracted on the last day of the month.

Reference rates: sampled dementia prevalence

Age 65+ age and sex-specific dementia prevalence rates, binomial proportions with 95% confidence limits by 5-year age and sex band from the Medical Research Council Cognitive Function and Ageing Study II (CFAS II). Reference rates remain static. Source: MRC CFAS II.

Plain English: For patients aged 65 years old and over, there are reference rates for dementia prevalence provided by the Medical Research Council Cognitive Function and Ageing Study II (CFAS II), including 5-year age bands and sex. The same rates are used for the methodology.

Organisational data

GP practices open and active on the reporting period end date from the NHS Business Services Authority Prescriptions Services (NHS BSA), with postcodes and Clinical Commissioning Group (CCG). Source: NHS Digital Organisational Data Service.

Office for National Statistics (ONS) mappings from CCG to Sustainability and Transformation Plan Footprint (STP); NHS England Local Office (DCO); and NHS England Region. Source: ONS Open Geography.

ONS mappings from postcode to local authority (LA). Source: ONS Open Geography.

Public Health England (PHE) mappings from LA to PHE Centre; County Council, PHE Region; ONS Group and Sub-Group; Average LA Deprivation Decile; Devolved Area. Source: Public Health England.

Plain English: To understand the Estimated Diagnosis Rate at different level of NHS geographies the relevant information is used to map general practice level data to higher geographical regions.

Construction

Introduction

This indicator reports the rate of persons aged 65 and over with a recorded diagnosis of dementia per person estimated to have dementia given the characteristics of the population and the age and sex-specific prevalence rates of the CFAS II study, expressed as a percentage with 95% confidence intervals.

Applying the age and sex-specific 65+ prevalence rates of the CFAS II population (the reference rates) to the age and sex structure of the registered patients in the subject population (the denominator), yields the number of people aged 65+ one would expect to have dementia within the subject population. Dividing the actual number of cases recorded in the subject population (the numerator) by the estimated number yields the estimated diagnosis rate.

95% confidence intervals are derived from the 12 individual measures of uncertainty given with the CFAS II reference rates and the uncertainty around the numerator. The indicator is calculated 100,000 times, resampling randomly each time from the distributions of the 13 variables, to produce an overall distribution of indicator values closely approximating the true distribution. The 2,500th smallest and the 2,500th largest values in the distribution give robust estimates of the 95% lower and upper confidence limits respectively to one decimal place.

This indicator is expressed as a percentage.

Data Fields

NHAIS registered patients

PRACTICE_CODE
AGE
SEX
VALUE
EXTRACT_DATE

GPES recorded dementia prevalence

PRACTICE_CODE
AGE
SEX
VALUE
ACH_DATE

 

  

CFAS II reference rates 

Sex

Age

Rate

Lower

Upper

M

65–69 years

0.012

0.006

0.023

M

70–74 years

0.030

0.020

0.044

M

75–79 years

0.052

0.038

0.070

M

80–84 years

0.106

0.082

0.137

M

85–89 years

0.128

0.090

0.180

M

≥90 years

0.171

0.106

0.264

F

65–69 years

0.018

0.009

0.036

F

70–74 years

0.025

0.016

0.039

F

75–79 years

0.062

0.045

0.084

F

80–84 years

0.095

0.073

0.123

F

85–89 years

0.181

0.145

0.222

F

90 years +

0.350

0.284

0.423

 

NHS BSA organisational data

PRACTICE_CODE
STATUS
OPEN_DATE
CLOSED_DATE
PRESCRIBING_SETTING
POSTCODE
COMMISSIONING_ORGANISATION

Data filter

NHAIS registered patients

  1. Field Name    EXTRACT_DATE
    Conditions    = reporting period end date +1
    Rationale:    Returns data as close to the reporting period end date as possible
  2. Field Name    VALUE
    Conditions    sum(VALUE) > 0
    Rationale    Returns data for practices with at least one registered patient of any sex or age
  3. Field Name    AGE
    Conditions    > 64
    Rationale    Returns data for patients aged 65 and over

GPES recorded dementia prevalence

  1. Field Name    ACH_DATE, PRACTICE_CODE
    Conditions    = max(ACH_DATE) per PRACTICE_CODE where ACH_DATE >= reporting period end date -182
    Rationale:    Returns the most recent data available for each practice to a maximum of 6 months prior to the reporting period end date
  2. Field Name    AGE
    Conditions    > 64
    Rationale    Returns data for patients aged 65 and over

NHS BSA organisational data

  1. Field Name    STATUS
    Conditions    = A
    Rationale:    Returns data for active practices
  2. Field Name    OPEN_DATE
    Conditions    <= reporting period end date
    Rationale    Returns data for practices open as at the reporting period end date
  3. Field Name    CLOSED_DATE
    Conditions    >= reporting period end date; or NULL
    Rationale:    Returns data for practices not closed as at the reporting period end date
  4. Field Name    PRESCRIBING_SETTING
    Conditions    = 4
    Rationale    Returns data for practices with GP prescribing cost centres

NHAIS registered patients, GPES recorded dementia prevalence, NHS BSA organisational data

  1. Field Name    PRACTICE_CODE
    Conditions    Inner join
    Rationale:    Return data only for practices existing in all three sources as queried above - for example, open practices, with one or more registered patient, with dementia data available within the last 6 months.

Calculation formulae

Calculate the estimated number of cases of dementia for each organisation (denominator) by applying the age and sex-specific reference rates to the age and sex structure of its population:

Ek=∑ijNijk×pijEk=∑ijNijk×pij

Where:

Ek is the estimated value for the subject organisation k

Nijk is the population (65+ patient list size) for each combination of age band i and sex j in subject organisation k

pij is the binomial proportion for each combination of age band i and sex j in the reference population (CFAS II)

 

Calculate the estimated diagnosis rate for each organisation (indicator value) by dividing its observed dementia diagnoses by its estimated value and express this as a percentage:

λk=OkEk×100λk=OkEk×100

Where:

λkλk is the estimated diagnosis rate for the subject organisation k

Ok is the recorded 65+ dementia diagnoses in the subject organisation k

Ek is the estimated value for the subject organisation k

 

Calculate the upper and lower 95% confidence limits for each organisation’s indicator value by simulation. Repeat the indicator calculation 100,000 times, randomly resampling each time from the age and sex-specific expected distributions, and the recorded diagnoses count distribution, to create a distribution of 100,000 random samples from the overall indicator distribution. Take the 2500th smallest and the 2500th largest values from this distribution as estimates of the 95% lower and upper confidence limits respectively:

λLLk=λsimk(n)=n(λsimk1,...,λsimk100,000)λkLL=λsimk(n)=n(λsimk1,...,λsimk100,000)

λULk=λsimk(100,000−n)=100,000−n(λsimk1,...,λsimk100,000)λkUL=λsimk(100,000−n)=100,000−n(λsimk1,...,λsimk100,000)

Where:

λLLkλkLL  is the lower 95% confidence interval for subject organisation k

λULkλkUL  is the upper 95% confidence interval for subject organisation k

n defines the threshold of the indicator distribution based on the number of repetitions, 100,000, and level of confidence, 95%: 100,000 * (1-0.95) / 2

λsimk1,...,k100,000λsimk1,...,k100,000 is the order of randomly sampled indicator values for subject organisation k produced by repetition of the following:

 

(λsimk=OrandkErandk×100)1,...,100,000(λsimk=OrandkErandk×100)1,...,100,000

Where:

Orandk is the randomly sampled diagnoses count value for organisation k produced by the inverse cumulative probability function with:

probability: Rϵ(0,...,1)Rϵ(0,...,1)

mean: OkOk

standard deviation: O−−√kOk

Erandk is the randomly sampled expected value for organisation k produced as follows:

Erandk=∑iiNijk×prandijErandk=∑iiNijk×prandij

Where:

NijkNijk is the population (65+ patient list size) for each combination of age band i and sex j in subject organisation k

prandijprandij is the randomly sampled binomial proportion for each combination of age band i and sex j in the reference population (CFAS II) produced as follows:

prandij=exp(picfij)1+exp(picfij)prandij=exp⁡(pijicf)1+exp⁡(pijicf)

Where:

picfijpijicf is the inverse cumulative probability function for each for each combination of age band i and sex j in the reference population (CFAS II) with:

probability: Rϵ(0,...,1)Rϵ(0,...,1)

mean: loge(pij100−pij)loge⁡(pij100−pij)

Standard deviation: loge(pULij100−pULij)−loge(pLLij100−pLLij)2/1.96loge⁡(pijUL100−pijUL)−loge⁡(pijLL100−pijLL)2/1.96

Where:

pULijpijUL is the lower 95% confidence limit for each combination of age band i and sex j in the reference population (CFAS II)

pLLijpijLL is the lower 95% confidence limit for each combination of age band i and sex j in the reference population (CFAS II)

Technical guide

The estimated dementia diagnosis rate (EDDR) is calculated for each area (local authority or CCG for example) in two stages:

  1. The expected number of people with dementia in the area is estimated by applying prevalence estimates obtained from survey data at national level for each age/sex group to the estimated population in each age/sex group in the area and summing across all age/sex groups.
  2. The total observed number of people diagnosed with dementia in the area is divided by the expected number obtained from stage 1 to give the estimated diagnosis rate, that is - the proportion of expected cases that have been diagnosed by GPs.

To obtain approximate 95% confidence intervals for the EDDR, the uncertainty (confidence intervals) around the original survey estimates at age/sex group level must be taken into account, together with the random variation element of the observed total of diagnosed patients. This is done by simulation using the following steps:

  1. For each age/sex group 𝑖, the prevalence estimates (𝑝𝑖) were published with 95% confidence limits (𝑝𝐿𝑖 and 𝑝𝑈𝑖). All these are transformed by taking the logits3: logit(pi)=ln(pi/(1−pi))logit(pi)=ln(pi/(1−pi)), logit(pLi)=ln(pLi/(1−pLi))logit(pLi)=ln(pLi/(1−pLi)), logit(pUi)=ln(pUi/(1−pUi))logit(pUi)=ln(pUi/(1−pUi))
  2. The standard error of logit(pi)logit(pi) is estimated using the published confidence intervals: se(logit(pi))=(logit(pUi))−(logit(pLi))2×1.96se(logit(pi))=(logit(pUi))−(logit(pLi))2×1.96
  3. The prevalence estimates themselves are assumed to be binomially distributed, and the logit-transformed prevalence estimates are assumed to be normally distributed: N(logit(pi)),se(logit(pi))N(logit(pi)),se(logit(pi))
  4. The observed total number of diagnosed patients (𝑂) is assumed to be Poisson distributed, but since the counts are all large4 the normal approximation to the Poisson is extremely accurate and hence they are assumed to be normally distributed, N(0,0–√)N(0,0)
  5. Randomised expected values are calculated for each age/sex group along with a randomised observed value. This is done by generating random numbers (𝑟𝑖 and 𝑟𝑂) from a uniform (0,1) distribution using the Mersenne Twister algorithmi and transforming them to obtain random numbers from the appropriate normal distributions.
  6. For the randomised expected values, the inverse of the normal cumulative distribution is calculated for probability riri , mean logit(pi)logit(pi) and standard deviation se(logit(pi))se(logit(pi)) to give logit(pisim)logit(pisim) 
  7. For the randomised observed value the inverse of the cumulative normal distribution is calculated for probability r0r0 , mean 0 and standard deviation √𝑂 to give 0sim0sim
  8. Each pisimpisim is calculated by reversing the logit transformation: pisim=elogit(pisim)1+elogit(pisim)pisim=elogit(pisim)1+elogit(pisim)
  9. Each pisimpisim is multiplied by the relevant population to obtain the simulated expected count for the age/sex group, EisimEisim 
  10. The total expected count is generated by summing across all age/sex groups: Eisim=∑EisimEisim=∑Eisim
  11. The simulated EDDR is calculated: EDDRsim=OsimEsimEDDRsim=OsimEsim
  12. Steps 3 to 7 are repeated 100,000 times
  13. From the randomly generated sample of 100,000 EDDRsimEDDRsim values, the 2,500th smallest and largest are taken as the values for the 95% lower and upper confidence limits respectively.

 

Last edited: 9 June 2020 10:46 am