Aneurysm Growth Rate Calculator
Aneurysm Growth Rate Calculator
Millimetres per year from two diameters and an interval — with the measurement error printed next to it, because inter-observer spread over a short interval produces a growth rate that is pure noise.
Aneurysm growth rate
mm per year from two scans41 mm, then 44 mm, 12 months apart
Formula
- later and earlier diameter
- the maximum aortic diameter in millimetres on each scan, measured the same way on both. The ESVS 2024 guideline’s recommendation 10 asks for “consistent calliper placement in an orthogonal plane perpendicular to the aorta”
- x 12 / months
- the annualisation, and it is linear. That is an assumption: RESCAN found growth accelerating with diameter, so a rate measured across a long interval is an average of a changing quantity. A zero or negative interval is refused rather than divided by
- the published anchors
- RESCAN’s pooled analysis of individual participant data found mean growth in men of 1.3 mm per year at 3.0 cm and 3.6 mm per year at 5.0 cm, with about 0.5 mm per year added per 0.5 cm, and rupture rates rising in parallel from 0.05 to 0.64 per 100 person-years
- the error bar, which is the point
- Matthews and colleagues report inter-observer reproducibility coefficients in the anteroposterior plane of plus or minus 4.6 mm leading-edge-to-leading-edge, 4.5 mm inner-to-inner and 4.8 mm outer-to-outer, with a range across 16 earlier studies of plus or minus 1.7 to 12.6 mm. Over a 6-month interval, 4.5 mm of spread alone annualises to 9 mm per year
- inner-to-inner versus outer-to-outer, and ultrasound versus CT
- a few millimetres each, with no conversion for either. ESVS 2024 states that “it is still not possible to suggest one calliper placement over another”, and the published difference is a spread of up to about 5 mm rather than a fixed offset. CT reads larger than ultrasound: Kontopodis and colleagues’ review tabulates 2.6, 4.1 and 9.6 mm in three different studies. A growth rate computed across a change of modality is measuring the scanner
Worked example
41 mm, then 44 mm, 12 months apart
(44 - 41) x 12 / 12 = 3 x 1 = 3.0 mm per year
Three intervals, same two diameters. Over 6 months, (44 - 41) x 12 / 6 = 6.0 mm per year; over 36 months, 1.0 mm per year. The same 3 mm is a rate above RESCAN's 5.0 cm mean, at it, or below its 3.0 cm mean, depending only on how long you waited
Which is why the short interval is the trap. Matthews and colleagues report an inter-observer reproducibility coefficient of about plus or minus 4.5 mm; over 6 months that spread alone annualises to 9 mm per year, larger than the entire measured change here
Refusal. Enter an interval of 0 and the calculator returns no answer rather than infinity. Enter a negative interval and it does the same
A 3 mm change is inside the measurement spread. Matthews and colleagues record calliper-placement differences of up to about 5 mm, so 41 to 44 mm is compatible with an aorta that has not grown at all — the single most useful thing this page can tell a reader
If the first scan was ultrasound and the second CT, some of the 3 mm is the scanner: Kontopodis and colleagues' review tabulates CT axial reading 2.6 to 9.6 mm larger than ultrasound across different studies
What the published growth rates actually are
| Diameter | Mean growth (men) | Rupture rate | Source |
|---|---|---|---|
| 3.0 cm | 1.3 mm/year (95% CI 1.0 to 1.5) | 0.05 per 100 person-years (0.03 to 0.07) | RESCAN pooled analysis |
| 5.0 cm | 3.6 mm/year (95% CI 3.3 to 3.9) | 0.64 per 100 person-years (0.43 to 0.95) | RESCAN pooled analysis |
| Each additional 0.5 cm | About +0.5 mm/year | Roughly doubles | RESCAN pooled analysis |
The measurement error, which is usually larger than the signal
| Source of variation | Reported size | Source |
|---|---|---|
| Calliper placement, between methods | Up to about 5 mm | Matthews 2021, citing two earlier studies |
| Inter-observer, anteroposterior plane | About plus or minus 3.4 to 5.3 mm across four studies | Matthews 2021 |
| Inter-observer, across 16 earlier studies | Plus or minus 1.7 to 12.6 mm | Matthews 2021 |
| Intra-observer, across 16 earlier studies | Plus or minus 0.9 to 4.0 mm | Matthews 2021 |
| CT axial versus ultrasound | CT larger by 2.6 to 9.6 mm, depending on the study | Kontopodis 2016 |
A rate with no error bar is not a measurement
The arithmetic is a subtraction and a division: the change in maximum diameter, annualised by the interval between the two scans. What makes it worth a page is that the answer is routinely smaller than the error on the inputs, and nothing in the calculation says so.
Start with the published growth rates, so there is something to compare against. The RESCAN pooled analysis found mean growth in men of 1.3 mm per year at a diameter of 3.0 cm, rising to 3.6 mm per year at 5.0 cm, with rupture rates roughly doubling over the same step. Those are the magnitudes in play: single millimetres per year, accelerating with size.
Now the error. Matthews and colleagues measured the reproducibility of maximum aneurysm diameter from ultrasound images and report inter-observer coefficients in the anteroposterior plane of about plus or minus 4.5 to 4.8 mm for inner-to-inner and outer-to-outer calliper placement, a range across sixteen earlier studies of plus or minus 1.7 to 12.6 mm, and differences of up to about 5 mm between calliper methods. The transverse plane is worse. CT also reads larger than ultrasound, by 0.8 to 9.6 mm across the studies Kontopodis and colleagues tabulate. So a single diameter carries an uncertainty of several millimetres, and a difference of two carries more.
Divide that by a short interval and the result is noise with a decimal point. Over six months, plus or minus 4.5 mm of spread alone annualises to plus or minus 9 mm per year — larger than the fastest mean growth rate RESCAN published, and larger than the 10 mm per year the NHS screening programme uses as a referral trigger. That is why published surveillance intervals run from months to years rather than weeks, and why the surveillance interval schedules key on diameter rather than on growth. Inner-to-inner against outer-to-outer is a few millimetres and there is no conversion for it: ESVS 2024 states that “it is still not possible to suggest one calliper placement over another”. The only remedy is to measure both scans the same way and to treat a small difference over a short interval as the absence of information it is. This category owns the limb and the named vessel. The cost of an operation on this aorta belongs to the Revised Cardiac Risk Index, and the aortic size index would consume body surface area by link rather than rebuild it.
Frequently asked questions
How fast does an abdominal aortic aneurysm normally grow?
The RESCAN pooled analysis of individual participant data found mean growth in men of 1.3 mm per year (95 per cent CI 1.0 to 1.5) at a diameter of 3.0 cm and 3.6 mm per year (3.3 to 3.9) at 5.0 cm, with about 0.5 mm per year added for each additional 0.5 cm of diameter. Those are cohort means; growth in an individual aneurysm is episodic rather than steady.
Why does a short interval give a meaningless growth rate?
Because the measurement error stays the same size while the denominator shrinks. Matthews and colleagues report inter-observer reproducibility of about plus or minus 4.5 mm; over three months that spread alone annualises to plus or minus 18 mm per year, five times the fastest published mean growth rate.
Does it matter whether the diameter is measured inner-to-inner or outer-to-outer?
Yes, by a few millimetres, and there is no conversion between them. The ESVS 2024 guideline says that “it is still not possible to suggest one calliper placement over another”, and Matthews and colleagues record that different methods have been reported to differ by up to about 5 mm — enough to cross a surveillance-interval band or a repair threshold. What matters is that both scans used the same convention.
Can I compare an ultrasound diameter with a CT diameter?
Not without allowing for the modality. Kontopodis and colleagues’ review tabulates CT axial reading larger than ultrasound by 2.6, 4.1 and 9.6 mm in three different studies. A growth rate computed across a change of modality is partly measuring the scanner.
What growth rate triggers referral?
The NHS Abdominal Aortic Aneurysm Screening Programme’s care pathway carries growth of more than 1 cm in 12 months as a referral trigger alongside a diameter of 5.5 cm or more. That is a screening programme’s operational rule, not a statement about this aorta, and other bodies publish different schedules.
Related calculators
References
- RESCAN Collaborators. Surveillance intervals for small abdominal aortic aneurysms: a meta-analysis. NIHR Health Technology Assessment programme report, Southampton: NIHR Journals Library.
- Matthews EO, Pinchbeck J, Elmore K, et al. The reproducibility of measuring maximum abdominal aortic aneurysm diameter from ultrasound images. Ultrasound J. 2021;13:13.
- Kontopodis N, Lioudaki S, Pantidis D, Papadopoulos G, Georgakarakos E, Ioannou CV. Advances in determining abdominal aortic aneurysm size and growth. World J Radiol. 2016;8(2):148–58.
- Wanhainen A, Van Herzeele I, Bastos Goncalves F, et al. Editor’s Choice — European Society for Vascular Surgery (ESVS) 2024 Clinical Practice Guidelines on the Management of Abdominal Aorto-Iliac Artery Aneurysms. Eur J Vasc Endovasc Surg. 2024;67(2):192–331.
- Public Health England / NHS England. NHS Abdominal Aortic Aneurysm (AAA) Screening Programme care pathway. London: GOV.UK, updated 2024.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/