How Likely Is AAA Screening to Find an Aneurysm?
If 1,000 men aged 65 go for NHS AAA screening in England today, about 993–994 will not have an abdominal aortic aneurysm (AAA) detected.
About 6–7 will.
That is based on the latest NHS screening figures.
But there is an important distinction. Finding an AAA is not the same as finding an aneurysm that is likely to rupture soon.
What does AAA screening actually find?
The NHS defines an AAA as an aorta measuring 3.0 cm or more.
The results are divided into three groups:
small: 3.0–4.4 cm
medium: 4.5–5.4 cm
large: 5.5 cm or more
Across the UK screening programmes between 2013 and 2023, about 80% of AAAs found at the initial scan were small, around 10% were medium and roughly 5–10% were large.
So, if we apply that historical size distribution simply to illustrate today's detection rate, 1,000 men would look roughly like this:
993–994: no AAA detected
about 5: small AAA
fewer than 1: medium AAA
fewer than 1: large AAA
Those last three figures are an illustration, not observed 2025/26 size-specific figures.
If a small AAA is found, how likely is it to rupture?
It depends considerably on its size.
In an NHS study of 18,652 men under surveillance, the annual risk of rupture was about 0.03% for small AAAs and 0.28% for medium AAAs. For AAAs measuring 5.0–5.4 cm, it was about 0.40%.
These figures apply to men being monitored by the NHS screening programme, not to aneurysms left without surveillance.
A separate analysis of more than 15,000 people with AAAs below 5.5 cm found that, in men, the rupture rate approximately doubled for every additional 0.5 cm in diameter.
For a man with a 3.0 cm AAA, the analysis estimated that the risk of rupture would remain below 1% over about 8½ years.
This was a modelled estimate from men in surveillance studies, not the result of leaving men with 3.0 cm aneurysms unmonitored for eight years.
The studies used in this analysis recruited patients between 1983 and 2008, so these are historical population estimates rather than precise predictions for men screened today.
So the actual measurement matters. A 3.0 cm AAA and a 4.4 cm AAA are both described by the NHS as “small”, but they do not carry the same risk.
How quickly can an AAA grow?
On average, AAAs grow, but how quickly varies considerably. Starting size matters.
The same analysis estimated:
| Starting size | Estimated time to a 10% chance of reaching 5.5 cm |
|---|---|
| 3.0 cm | 7.4 years |
| 4.0 cm | 3.2 years |
| 5.0 cm | 0.7 years |
These are modelled population estimates, not predictions for an individual man. Growth rates varied considerably between studies.
Reaching 5.5 cm does not mean an aneurysm is about to rupture. It is the usual size at which men are referred to a vascular service to consider treatment. Men can also be referred earlier if an aneurysm grows rapidly.
Smaller AAAs are normally monitored rather than repaired immediately. Randomised trials found no survival advantage from early repair of AAAs below 5.5 cm, and repair itself carries risks.