Editor—The Australian randomised trial of aortic aneurysm screening observed 18 deaths related to aneurysm in the group of men invited for screening and 25 in the control group.1 The corresponding reduction in mortality was 39% (relative risk 0.61, 95% confidence interval 0.33 to 1.11), which the authors summarise as showing that screening did not reduce overall death rates. The authors have fallen into the common trap of interpreting a non-significant difference as evidence of no difference.
The stated conclusion is all the more surprising given the available evidence from other randomised trials (table). In each trial, the number of aneurysm related deaths in the men invited for screening is lower than in the control group, and so the relative risks are all below 1. The widths of the confidence intervals vary according to the size and power of the trial. The largest trial, the multicentre aneurysm screening study (MASS), shows a significant benefit.2 So does the Danish trial, based on the published aneurysm related mortality in hospital.3 The Chichester and Australian trials, were too small to show the difference convincingly.4
Table 1.
Most recent published results from the randomised trials of abdominal aortic aneurysm screening in men
|
Age range at recruitment (duration of follow up) (years)
|
No of participants
|
No of aneurysm related deaths
|
Relative risk (95% CI)
|
|||
|---|---|---|---|---|---|---|
| Trial | Invited | Control | Invited | Control | ||
| Australian1 | 65-83 (5) | 19 352 | 19 352 | 18 | 25 | 0.61 (0.33 to 1.11) |
| MASS, UK2 | 65-74 (4) | 33 839 | 33 961 | 65 | 113 | 0.58 (0.42 to 0.78) |
| Denmark3 | 65-73 (5) | 6 339 | 6 319 | 6 | 19 | 0.32 (0.11 to 0.59) |
| Chichester, UK4 | 65-80 (10) | 3 000 | 3 058 | 24 | 31 | 0.79 (0.53 to 1.40) |
But it does not take a formal metaanalysis to deduce the high level of evidence, across the four trials, that screening reduces mortality related to aneurysm by the order of 40% (corresponding to a relative risk of 0.60). Speculation about possible reasons for the differences between the results of the trials is unhelpful, when what is more notable is their consistency.
Competing interests: None declared.
References
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