Topography and Keratoconus Screening Card: Cut-offs and Staging
Last updated: October 2026
What to check, in order, when a topography or tomography report lands on your desk: is this keratoconus (or a suspect), how severe is it, is it progressing, and is the cornea safe for refractive surgery?
Step 1: quality and pattern
- Check the quality score, blinks, tear film and centration first. Dry eye and contact lens warpage (stop soft lenses about 1–2 weeks and rigid lenses about 3–4 weeks before) mimic ectasia.
- Look at both eyes side by side. Asymmetry between eyes is often the earliest sign.
- Red-flag axial patterns: inferior steepening, asymmetric bowtie with skewed radial axes (SRAX), and "crab-claw" (PMD-like) patterns.
Step 2: front-surface (Placido) indices
| Index | Suspect | Keratoconus |
|---|---|---|
| Central K | Over 47.2 D | Over 48.7 D |
| I–S value (inferior minus superior power, 3 mm) | Over 1.4 D | Over 1.9 D |
| Inter-eye central K difference | Over 1.0 D | |
| SRAX (skew of the radial axes) | Over 21° | |
| KISA% (combines K, I–S, astigmatism and SRAX) | 60–100% | Over 100% |
Rabinowitz-McDonnell criteria. Placido indices miss early posterior-surface disease, so always look at tomography when available.
Step 3: tomography (Pentacam-type) checks
| Parameter | Normal | Suspicious | Abnormal |
|---|---|---|---|
| BAD-D (Belin-Ambrósio final D) | Under 1.6 | 1.6–2.6 | 2.6 or more |
| Anterior elevation at the thinnest point (8 mm best-fit sphere) | Under +12 µm | +12 to +15 µm | Over +15 µm |
| Posterior elevation at the thinnest point (8 mm best-fit sphere) | Under +17 µm | +17 to +20 µm | Over +20 µm |
| Thinnest pachymetry | Over 500 µm | 470–500 µm | Under 470 µm (interpret with the pattern) |
| Inter-eye thinnest-point difference | Under 30 µm | 30 µm or more | |
| Thinnest point location | Displaced inferotemporally away from the apex, with a rapid "fall-off" of the pachymetry progression curve, is suspicious | ||
Device note: these cut-offs are for Pentacam-style Scheimpflug systems. Galilei, Sirius, MS-39 and swept-source OCT tomographers use their own indices and normative databases, so do not transfer numbers between machines. The BAD-D grey zone also depends on age: a borderline value is more worrying at 20 than at 55.
Step 4: stage it (Belin ABCD)
| Stage | A: anterior radius (3 mm zone) | B: posterior radius (3 mm zone) | C: thinnest pachymetry | D: best distance VA |
|---|---|---|---|---|
| 0 | Over 7.25 mm (under 46.5 D) | Over 5.90 mm | Over 490 µm | 20/20 or better |
| I | Over 7.05 mm (under 48.0 D) | Over 5.70 mm | Over 450 µm | Worse than 20/20 |
| II | Over 6.35 mm (under 53.0 D) | Over 5.15 mm | Over 400 µm | Worse than 20/40 |
| III | Over 6.15 mm (under 55.0 D) | Over 4.95 mm | Over 300 µm | Worse than 20/100 |
| IV | Under 6.15 mm (over 55.0 D) | Under 4.95 mm | 300 µm or less | Worse than 20/400 |
Each parameter is staged separately (for example A2 B3 C1 D1), with a suffix for scarring.
Step 5: is it progressing? (cross-linking decision)
- The global consensus (2015) defines progression as a consistent change in at least two of: steepening of the anterior surface, steepening of the posterior surface, and thinning (or faster change in thickness from periphery to the thinnest point).
- A commonly used practical trigger is an increase in Kmax of 1 D or more over 12 months on repeated, good-quality scans from the same device.
- Children and teenagers progress faster: many surgeons cross-link at diagnosis in paediatric keratoconus.
- Standard epithelium-off (Dresden) cross-linking needs at least about 400 µm of stroma after epithelial removal. Thinner corneas need modified (hypo-osmolar or customised) protocols.
Step 6: refractive surgery safety screen
| Check | Safer | Caution or avoid |
|---|---|---|
| Residual stromal bed (LASIK) | 300 µm or more | Under 250–300 µm |
| Percent tissue altered: (flap + ablation) ÷ preoperative CCT | Under 40% | 40% or more (high ectasia risk) |
| Tomography | Normal in both eyes | Any suspicious BAD-D, elevation or pachymetry pattern: avoid LASIK; consider surface ablation only with great care, or phakic IOL |
| Epithelial thickness map (if available) | Even epithelium | Epithelial thinning over a steep area with a surrounding "doughnut" of thicker epithelium suggests early ectasia |
Red flags on any report: inferior steepening with a skewed bowtie, posterior elevation over +20 µm, a thinnest point under 470 µm or far from the apex, inter-eye asymmetry, or a young patient with a rising cylinder. Repeat the scan, then decide.
Related: how to read topography and tomography (detailed tips), keratoconus notes and refractive surgery for residents.
Key sources
- Rabinowitz YS, McDonnell PJ. Computer-assisted corneal topography in keratoconus. Refract Corneal Surg 1989;5:400–8; and the KISA% index (Rabinowitz & Rasheed, J Cataract Refract Surg 1999)
- Belin ABCD keratoconus staging (Oculus supplement, EyeWorld Asia-Pacific 2018)
- Gomes JA et al. Global consensus on keratoconus and ectatic diseases. Cornea 2015;34:359–69
- Santhiago MR et al. Association between the percent tissue altered and post-LASIK ectasia. Am J Ophthalmol 2014;158:87–95
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