Barrett True K (Post-LASIK) Calculator
Use this tool to generate an educational estimate of true corneal power and IOL power after refractive surgery.
No-history K (Shammas-style):
K = 1.14 × Kpost − 6.8History K:
K = Kpre + (SEpre(corneal) − SEcurrent(corneal))IOL estimate (simplified SRK-style):
P = A − 2.5(AL) − 0.9(K) − 1.5(Target)
Why post-LASIK IOL calculation is difficult
Standard cataract formulas assume a normal relationship between the front and back surfaces of the cornea. After LASIK or PRK, that relationship changes. In myopic LASIK, the anterior cornea is flattened, and many conventional keratometry methods overestimate corneal power. That can lead to selecting an IOL that is too low, leaving the patient unexpectedly hyperopic after surgery.
The Barrett True-K approach was designed to improve IOL planning in exactly this scenario. It can incorporate historical data when available, but it also has no-history pathways when older refractive records are missing.
What this calculator does
This page gives a practical, quick estimate for educational use. It does three things:
- Generates a post-refractive true corneal power estimate.
- Shows a standard-K vs adjusted-K IOL power comparison.
- Provides a rounded IOL suggestion in common power steps.
If you choose History Available, the tool blends a history-derived K value with a no-history estimate. If you choose No-History, it relies only on current keratometry.
How to use it step-by-step
1) Select your mode
Use History Available if you have old charts (pre-LASIK K and pre-LASIK refraction). Use No-History if you only have current post-LASIK measurements.
2) Enter corneal and biometric data
At minimum, enter post-LASIK K, axial length, A-constant, and target refraction. If using history mode, also enter pre-LASIK K and spherical equivalents.
3) Run the estimate and review the delta
Pay attention to the difference between standard-K IOL power and adjusted-K IOL power. A larger difference usually means post-refractive correction is materially changing your plan.
Input guidance and practical tips
K readings
- Use reliable mean K from topography/tomography when possible.
- If measurements vary between devices, reconcile before entering values.
Axial length
- Use optical biometry if available.
- Re-check unexpectedly short or long values for acquisition error.
A-constant
- Use lens-specific optimized values from your biometer/IOL database.
- A generic A-constant can shift estimated lens power by meaningful amounts.
Target refraction
- For distance-dominant goals: often near plano to -0.50 D.
- Adjust based on patient preference, fellow eye status, and monovision plans.
Important limitations
This implementation is intentionally simplified and cannot replicate the full proprietary Barrett True-K engine, which uses richer modeling and integration with biometry systems. Treat this calculator as a teaching or pre-screening aid.
- Not a substitute for validated clinical software.
- Not suitable as a stand-alone surgical planning tool.
- Should be cross-checked with multiple formulas and clinical judgment.
Suggested real-world workflow
- Collect stable keratometry, axial length, and historical refractive records if available.
- Run at least two post-refractive formulas (e.g., Barrett True-K, Haigis-L, ASCRS approach).
- Compare outliers and confirm lens constant optimization.
- Counsel patient about refractive uncertainty and enhancement possibilities.
Quick FAQ
Is this the official Barrett True-K calculator?
No. It is an educational approximation for understanding trends and sensitivity.
Can I use this for post-RK eyes?
Not recommended. Post-RK eyes often need a separate strategy because corneal behavior and stability differ significantly.
What if I have no pre-LASIK records?
Use No-History mode here, but in clinical practice you should still compare several validated no-history formulas before final lens selection.