Biometry & IOL power calculation
Calculation of IOL power
1. Refractive:
IOL power = 18+ 1.25 X Preoperative refraction (SE)
2. Theoretical formulae.
3. Second generation formulae.
4. Third generation formulae: Holladay II, SRK-T & Hoffer Q.
5. AI hybrid.
Holladay’s Data Screening Criteria to identify unusual measurement and require further validation. Repeat measurement if:
1. Axial length < 22.0 mm or > 25.0 mm.
2. Average corneal power < 40.0 Diopters or > 47.0 Diopters.
3. Calculated emmetropic IOL power > 3.0 Diopters of average power for the specific lens type.
4. Between eyes, the difference in:
a. Average corneal power > 1.0 Diopter.
b. Axial length > 0.3 mm.
c. Emmetropic IOL power > 1.0 Diopter.
Optiwave refractive analysis: Independent of AL & K readings.
The Impact of Optical Biometry
In ultrasound biometry, axial length measurement error alone accounted for 54% to 68% of the total prediction error. In optical biometry, the source of error from axial length measurement decreased substantially from 0.65 D to 0.43 D or 30 to 40% of the total prediction error.
Despite the improvement in AL measurement, this precision is not reflected in reducing prediction error. This less than encouraging improvement was probably overshadowed and supplanted by the ACD prediction error, a function of IOL power calculation formulas.
RCOphth guidelines:
In axial length < 22.00 mm, use Haigis or Hoffer Q.
In axial length between 22.00 and 26.00 mm, use SRK/T, or Barrett Universal II if it is installed on the biometry device and does not need the results to be transcribed by hand.
In axial length > 26.00 mm, use Haigis or SRK/T
Modern Evidence: Haigis vs Hoffer Q
Multiple studies show Haigis often matches or exceeds Hoffer Q in short eyes:
Moschos et al (2014): Haigis had lowest median absolute error (MedAE) in 55 eyes (AL <22 mm), outperforming Hoffer Q, Holladay 1, SRK/T.
Eom et al (2014): Equivalent MedAE (0.40 D) overall, but Haigis superior in shallow ACD (<2.5 mm), where differences widen as ACD decreases (R²=0.644). Shallow ACD exacerbates Hoffer Q's tendency to predict hyperopic shifts.
2025 meta-analysis (1178 eyes): Haigis ranked highly (with Kane, Olsen), though Kane edged out overall.
Recent study (2025): Kane best overall for AL <22 mm, but Haigis competitive.
Pathophysiology note: Short eyes risk postoperative hyperopia due to posterior IOL displacement from crowded bags; Haigis uses three IOL constants (a0,a1,a2) optimising effective lens position (ELP) via preoperative ACD.
When to repeat biometry?
· Previous measurements are > 4 years old.
· IOL exchange is required.
· Patient has had previous corneal surgery or a progressive corneal disease.
· Had a major eye operation since last biometry
· Axial length may have changed due to eye disease
· Considerable change in measurement system/devices.
· Recalculate as necessary if there is a change in type or supplier of the IOL used routinely.
Patient safety terminology
National Reporting & Learning System/ Learn From Patient Safety Events
1. Adverse incident:
2. Near Misses:
3. Never event:
4. Unexpected outcome:
