Most patients have never seen a corneal topography scan. Even fewer have heard of a topography quality factor, known as TQF. Yet when this small piece of metadata on a topography report carries a warning flag, it changes which measurements the surgeon should trust and which should be set aside.
What TQF Means
TQF is a quality score the topography device itself assigns to each acquired scan. It reflects whether the device managed to acquire a complete, well-focused, low-motion image of the cornea across its entire mapped area. A high TQF means the scan is reliable. A low TQF, sometimes shown as a single warning character on the report, means that part of the corneal surface was incompletely imaged.
The most common cause of a low TQF in routine clinic patients is not pathological. It is the upper eyelid sitting over the superior cornea, which causes the topography rings to be partially obscured in that sector. Tear film instability, fixation drift and patient blinking are other common contributors.
Why This Matters for IOL Calculation
High-quality optical biometers like the Lenstar LS900 use a specifically engineered dual-zone keratometric measurement for IOL power calculation¹. OCT topography platforms such as the REVO FC produce a different kind of keratometric value derived from a tomographic acquisition across the entire mapped area². When the topography is quality-passed, the two values agree closely. When the topography is TQF-flagged, they may not.
In that situation, the Lenstar value is the value that enters the IOL calculation. The REVO topography continues to be interpreted, but qualitatively, for overall corneal shape, posterior surface elevation, pachymetric distribution, and astigmatic regularity. The two measurements are answering related but not identical questions over different parts of the cornea, and direct comparison of their numeric outputs in the presence of an acquisition artefact would generate a clinically meaningless number³.
What We Do at Blue Fin Vision®
- Every topography scan is reviewed for quality before any K-value is used.
- If TQF is compromised, the topography is repeated where possible. If the artefact persists for anatomical reasons, the Lenstar K is used for the calculation.
- The REVO topography remains clinically valuable for what it does uniquely well, corneal shape, posterior surface, pachymetry, regardless of TQF status of the keratometric ring acquisition.
A TQF warning is not a failure of the device. It is the device telling the surgeon which value to trust.
Clinical Takeaway
When topography quality is compromised, Blue Fin Vision® uses Lenstar keratometry for the IOL calculation and interprets the REVO topography qualitatively for corneal shape, posterior surface, and pachymetric distribution.
References
- Domínguez-Vicent A, Venkataraman AP, Dalin A, Brautaset R, Montés-Micó R. Repeatability of a fully automated swept-source optical coherence tomography biometer and agreement with a low coherence reflectometry biometer. Eye Vis (Lond). 2023;10(1):24. doi:10.1186/s40662-023-00343-4
- Hoffer KJ, Shammas HJ, Savini G. Comparison of 2 laser instruments for measuring axial length. J Cataract Refract Surg. 2010;36(4):644-648. doi:10.1016/j.jcrs.2009.11.007
- Sikorski BL, Suchon P. OCT Biometry (B-OCT): a new method for measuring ocular axial dimensions. J Ophthalmol. 2019;2019:9192456. doi:10.1155/2019/9192456
Related Topics
- Double Biometry for Every Patient
- Why Blue Fin Vision® Performs Double Biometry for Every Cataract Patient
- How a 0.01 mm Difference Can Change Cataract Surgery Outcomes
- Why Most Cataract Clinics Only Use One Biometry Machine, and Why We Don’t
- How Double Biometry Helps Reduce Cataract Surgery Enhancements
- Lenstar vs REVO FC: Why Blue Fin Vision® Uses Both Before Lens Surgery
- What Patients Can Expect During a Blue Fin Vision® Biometry Workup
- Why Trifocal and Toric IOLs Need More Precise Biometry Than Standard Lenses
- Why Topography Quality Matters, and What a TQF Warning Tells Us
