joint strategic needs assessment foundation profile - JSNA

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The PHO then applied two separate adjustments to type 2 prevalence rates to reflect
differences in „time‟ and „place‟ between the 1986-1989 Coventry study and England
2001.
For the „time‟ adjustment, type 2 prevalence rates were upwardly adjusted to reflect
presumed increases in the national type 2 diabetes rate since the Coventry study was
conducted, using prevalence of obesity and overweight as a marker of increasing
diabetes prevalence. A Diabetes Index was calculated for each gender for each year
using the estimated percentage of population within different BMI classification
categories (normal weight, overweight and obese) multiplied by the relative risk of type 2
diabetes (Must, Spadano et al. 1999) for the different BMI categories and then summed.
For each gender separately, the Diabetes Indices were then plotted for each year 1991
to 2002 and extrapolated backwards using linear regression to estimate the value of the
index from 1986 to 1990. The adjustment factor was then the ratio of the Diabetes
Index for 2001 and the average of the Diabetes Index for 1986-1989. The adjustment
ratio for „time‟ was +29.9% for men and +20.6% for women.
For the „place‟ adjustment, type 2 diabetes prevalence rates were downwardly adjusted
to reflect the fact that there were higher levels of obesity and overweight and therefore
presumably of diabetes in Coventry (Foleshill electoral ward) than in England in 1991. A
similar method to the one used to adjust for „time‟ was used to adjust for „place‟. The
adjustment ratio for „place‟ was –7.0% for men and –13.4% for women.
Phase 2 of the modelling involved an adjustment for deprivation. Numerous UK studies
(Unwin, Watson et al. 1995; Eachus, Williams et al. 1996; Ismail and al 1999; Connolly,
Unwin et al. 2000; Evans, Newton et al. 2000; Office for National Statistics 2000; Riste
and al 2001; Abbas 2003) have demonstrated a strong positive association between
increasing socio-economic deprivation and the prevalence of type 2 diabetes. The type
2 registered diabetes prevalence from a study undertaken by the National Clinical Audit
Support Programme (NCASP) team (now the National Diabetes Audit (NDA) team) was
examined by deprivation quintiles (using Townsend‟s deprivation). The NCASP/NDA
study was selected for a number of reasons: (i) the diabetes prevalence rates are
adjusted for age, sex and ethnicity to be representative of the whole of England
population structure for 2001; (ii) the sample size of 57,800 persons with registered
diabetes is very large compared to most other studies; (iii) it was conducted in the same
year (2001) for which the PBS model estimates diabetes prevalence; and (iv) the
registers on which the study is based cover areas with diverse demography. The type 2
registered diabetes prevalence from the NCASP/NDA study were 1.67%, 1.76%, 2.12%,
2.43% and 2.95% in the five (Townsend) deprivation quintiles with the prevalence
increasing as deprivation increased. These figures were weighted by the population
distribution within the deprivation quintiles (based on IMD 2004 deprivation 76 ) giving a
76 It was assumed that the Townsend deprivation quintiles and the Index of Multiple Deprivation 2004
quintiles had the same relative risks of Type 2 diabetes. The IMD 2004 is a better measure of deprivation
within London as the Townsend scores were originally derived for northern England and in particular used
car ownership as a measure of affluence. This resulted in some very affluent areas of central London
being recorded as deprived using the Townsend scores.
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