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SRK II: historical educational model

Explores the SRK II 1988 formula and its documented adjustments. This teaching implementation neither selects a lens for surgery nor replaces modern biometry and constants.

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Check the population, units and version. The result shows the formula or classification; it does not make an automatic treatment decision.

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Method · Limits of application

SRK II 1988; historical implementation for teaching

Historical model with limited precision, especially in short and long eyes. Does not integrate modern biometry or optimized constants and does not select a lens for surgery.

Conditions of use

Check the conditions of application before calculating. The result presents the documented formula or criteria; it does not establish a diagnosis or prescription.

Scope of this implementation

Historical model with limited precision, especially in short and long eyes. Does not integrate modern biometry or optimized constants and does not select a lens for surgery.

Parameters in this edition

Lens A constant
Axial length · mm
Mean keratometry (K) · D
Target postoperative refraction · D
Historical/educational use of the SRK II formula, without selecting an implant for surgery?

Sources and documentation

Method version

SRK II 1988; historical implementation for teaching

Formula and criteria

P = adjusted A − 2.5 L − 0.9 K. A adjustment: +3 if L < 20; +2 if L < 21; +1 if L < 22; 0 if L < 24.5; −0.5 otherwise. Target refraction: subtract R × 1.25 if P > 14, or R if P ≤ 14.

Population and limitations

Historical model with limited precision, especially in short and long eyes. Does not integrate modern biometry or optimized constants and does not select a lens for surgery.

Synthetic cases checked (9)

Synthetic cases for checking the implementation, with no patient data.

Case 1

reference-001

Inputs

Lens A constant: 118.4
Axial length: 23.5 mm
Mean keratometry (K): 44 D
Target postoperative refraction: -1 D
Historical/educational use of the SRK II formula, without selecting an implant for surgery?: Yes

{
  "a": 118.4,
  "al": 23.5,
  "k": 44,
  "alvo": -1,
  "contexto": "1"
}

Expected result

{
  "pe": 20.05,
  "p": 21.3,
  "adj": 0,
  "factor": 1.25
}

Actual result

{
  "pe": 20.050000000000004,
  "p": 21.300000000000004,
  "adj": 0,
  "factor": 1.25
}

Case 2

reference-002

Inputs

Lens A constant: 118.4
Axial length: 19.99 mm
Mean keratometry (K): 44 D
Target postoperative refraction: 0 D
Historical/educational use of the SRK II formula, without selecting an implant for surgery?: Yes

{
  "a": 118.4,
  "al": 19.99,
  "k": 44,
  "alvo": 0,
  "contexto": "1"
}

Expected result

{
  "adj": 3,
  "pe": 31.825,
  "p": 31.825,
  "factor": 1.25
}

Actual result

{
  "pe": 31.82500000000001,
  "p": 31.82500000000001,
  "adj": 3,
  "factor": 1.25
}

Case 3

reference-003

Inputs

Lens A constant: 118.4
Axial length: 20 mm
Mean keratometry (K): 44 D
Target postoperative refraction: 0 D
Historical/educational use of the SRK II formula, without selecting an implant for surgery?: Yes

{
  "a": 118.4,
  "al": 20,
  "k": 44,
  "alvo": 0,
  "contexto": "1"
}

Expected result

{
  "adj": 2,
  "pe": 30.8,
  "p": 30.8,
  "factor": 1.25
}

Actual result

{
  "pe": 30.800000000000004,
  "p": 30.800000000000004,
  "adj": 2,
  "factor": 1.25
}

Case 4

reference-004

Inputs

Lens A constant: 118.4
Axial length: 21 mm
Mean keratometry (K): 44 D
Target postoperative refraction: 0 D
Historical/educational use of the SRK II formula, without selecting an implant for surgery?: Yes

{
  "a": 118.4,
  "al": 21,
  "k": 44,
  "alvo": 0,
  "contexto": "1"
}

Expected result

{
  "adj": 1,
  "pe": 27.3,
  "p": 27.3,
  "factor": 1.25
}

Actual result

{
  "pe": 27.300000000000004,
  "p": 27.300000000000004,
  "adj": 1,
  "factor": 1.25
}

Case 5

reference-005

Inputs

Lens A constant: 118.4
Axial length: 22 mm
Mean keratometry (K): 44 D
Target postoperative refraction: 0 D
Historical/educational use of the SRK II formula, without selecting an implant for surgery?: Yes

{
  "a": 118.4,
  "al": 22,
  "k": 44,
  "alvo": 0,
  "contexto": "1"
}

Expected result

{
  "adj": 0,
  "pe": 23.8,
  "p": 23.8,
  "factor": 1.25
}

Actual result

{
  "pe": 23.800000000000004,
  "p": 23.800000000000004,
  "adj": 0,
  "factor": 1.25
}

Case 6

reference-006

Inputs

Lens A constant: 118.4
Axial length: 24.5 mm
Mean keratometry (K): 44 D
Target postoperative refraction: 0 D
Historical/educational use of the SRK II formula, without selecting an implant for surgery?: Yes

{
  "a": 118.4,
  "al": 24.5,
  "k": 44,
  "alvo": 0,
  "contexto": "1"
}

Expected result

{
  "adj": -0.5,
  "pe": 17.05,
  "p": 17.05,
  "factor": 1.25
}

Actual result

{
  "pe": 17.050000000000004,
  "p": 17.050000000000004,
  "adj": -0.5,
  "factor": 1.25
}

Case 7

reference-007

Inputs

Lens A constant: 116.8
Axial length: 29 mm
Mean keratometry (K): 44 D
Target postoperative refraction: 0 D
Historical/educational use of the SRK II formula, without selecting an implant for surgery?: Yes

{
  "a": 116.8,
  "al": 29,
  "k": 44,
  "alvo": 0,
  "contexto": "1"
}

Expected result

{
  "adj": -0.5,
  "pe": 4.2,
  "p": 4.2,
  "factor": 1
}

Actual result

{
  "pe": 4.199999999999996,
  "p": 4.199999999999996,
  "adj": -0.5,
  "factor": 1
}

Case 8

reference-008

Inputs

Lens A constant: 114
Axial length: 24 mm
Mean keratometry (K): 40 D
Target postoperative refraction: -2 D
Historical/educational use of the SRK II formula, without selecting an implant for surgery?: Yes

{
  "a": 114,
  "al": 24,
  "k": 40,
  "alvo": -2,
  "contexto": "1"
}

Expected result

{
  "adj": 0,
  "pe": 18,
  "p": 20.5,
  "factor": 1.25
}

Actual result

{
  "pe": 18,
  "p": 20.5,
  "adj": 0,
  "factor": 1.25
}

Case 9

reference-009

Inputs

Lens A constant: 110
Axial length: 24 mm
Mean keratometry (K): 40 D
Target postoperative refraction: -2 D
Historical/educational use of the SRK II formula, without selecting an implant for surgery?: Yes

{
  "a": 110,
  "al": 24,
  "k": 40,
  "alvo": -2,
  "contexto": "1"
}

Expected result

{
  "pe": 14,
  "p": 16,
  "factor": 1
}

Actual result

{
  "pe": 14,
  "p": 16,
  "adj": 0,
  "factor": 1
}

Source for the active edition

Echo-Son · PIROP PAB33 manual rev.9 · 2020 · section 10.2

Additional reading and history

Sanders DR, Retzlaff J, Kraff MC. Comparison of the SRK II formula and other second generation formulas. J Cataract Refract Surg, 1988.

Identifier: lente-intraocular-srk-ii

Original clinical content: Brazilian Portuguese.

Code and reproducibility

This tool’s code on GitHub

Download code and tests

Sources, version and reproduction instructions

The code tests are technical checks. Clinical approval and professional translation review are recorded separately.

Relationship with cancer research

General context — relationship to be defined

A resource for general clinical contexts. This edition does not assign a specific oncological relationship; selecting it for a study requires a justified question, population and purpose.