Age standardised detection ratios
Updated 27 September 2024
Applies to England
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The NHS BSP has used invasive cancer detection rate targets since breast screening started in 1988. The range of targets were increased in 1993 and they were based on performance achieved in the Swedish two-county trial which had shown a reduction in mortality from breast cancer after 7 years of follow-up. Expected cancer detection rates have been updated periodically since that time to account for changes in background rates of incidence.
As the age of women screened is the major determinant of cancer detection rates, it is difficult to compare breast screening services with differing age demographics. This is why indirect age standardisation is used to compare performance more effectively.
The standardised detection ratio (SDR) for any given breast screening service is the ratio of the observed number of invasive cancers to the expected number. The expected number of cancers were taken from rates detected in the Swedish two-county trial in specific age cohorts and applied to estimated underlying background rates in England and Wales (from 1995). The estimated rates were taken from incidence rates in 1987 (prior to the English screening programme) and estimated background rates based on extrapolated rates to the year 1995. Due to uncertainty over the modelling, the midpoint of the 2 values was chosen as the estimated rate.
More recent extrapolation of data by Waller M and others (2007) suggests the current rates to be about 25% higher than in 1995. A conservative increase of 20% was made to the expected rates in 2021. The rates which are used to calculate the expected rates from 01 April 2021 onwards are shown in Table 1 below.
Table 1:
| Age band | Prevalent screen SDR | Incident screen SDR |
|---|---|---|
| 44 or younger | 2.12 | 2.02 |
| 45 to 49 | 3.56 | 2.33 |
| 50 to 52 | 4.37 | 3.84 |
| 53 to 54 | 5.33 | 4.15 |
| 55 to 59 | 7.03 | 4.55 |
| 60 to 64 | 10.46 | 5.30 |
| 65 to 69 | 12.91 | 5.45 |
| 70 | 14.76 | 5.65 |
| 71 to 74 | 16.67 | 5.92 |
| 75 or older | 18.89 | 6.25 |
Advantages of using SDRs are that:
- they are superior to using crude detection rates and can be calculated from the KC62 national returns
- the assumption remains that achieving the standard should allow mortality reduction to be similar to the original Swedish two-county trial, which demonstrated a relative risk reduction from breast cancer mortality of 0.69% (CI 0.56% to 0.84% P less than 0.0001)
- SDRs can be measured, and performance over a longer period (usually 3 years) can be easily calculated
- expected numbers of invasive cancers detected for individual screening programmes can be predicted by knowing the numbers and age profiles of those attending and applying them
For the calculation of SDRs, microinvasive disease is included with non-invasive cancer and is not categorised as invasive cancer.