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Chylack LT, Wolfe JK, Singer DM, Leske MC, Bullimore MA, Bailey IL, Friend J, McCarthy D, Wu SY. The Lens Opacities Classification System III. The Longitudinal Study of Cataract Study Group. Archives of ophthalmology. 1993;111:831-6. PMID 8512486

One-paragraph summary

LOCS III was built to fix four named defects of LOCS II: unequal intervals between reference standards, only one standard for colour grading, integer-only grading, and wide tolerance limits. Standards were selected from the Longitudinal Study of Cataract slide library at the Center for Clinical Cataract Research in Boston. The system provides six slit-lamp images for grading nuclear colour (NC) and nuclear opalescence (NO), five retroillumination images for cortical cataract (C) and five for posterior subcapsular cataract (P), all on a decimal scale with regularly spaced intervals. The 95% tolerance limits fell from 2.0 units for each class under LOCS II to 0.7 (NO), 0.7 (NC), 0.5 (C) and 1.0 (P) under LOCS III, with interobserver agreement described as excellent.

Key findings

  • Four structural changes from LOCS II: decimalised scale, regularly spaced intervals, separate NC and NO standards, expanded standard set.
  • 95% tolerance limits: 2.0 (all classes, LOCS II) → 0.7 NO / 0.7 NC / 0.5 C / 1.0 P (LOCS III).
  • Grading remains a subjective slit-lamp comparison against photographic standards, by design.
  • Companion work from the same group reported LOCS II kappa 0.85–1.00 and LOCS III intraclass correlations 0.67–0.94, against objective systems with much higher internal reliability (fast spectral scanning colorimetry r₁ = 0.96–0.98; nuclear mean density r₁ = 0.97; percent area opacity r₁ = 0.92–0.96) (Chylack 1993, PMID 8302524).

Limitations

  • Subjective by construction; reproducibility depends on the grader and on coexisting opacities — LOCS II work showed that more advanced coexisting opacities degraded agreement for nuclear (but not cortical or PSC) diagnosis in 3,646 eyes (Maraini 1991, PMID 2071351).
  • Calibrated against other graders, never against visual function or disability.
  • Rarely used in routine clinical practice, which limits the pool of experienced graders (Mackenbrock 2024, PMID 38242135).
  • No published conversion function linking LOCS III to the WHO simplified system (Thylefors 2002, PMID 11821974), the decimalised Oxford system (Sparrow 2000, PMID 10652171), or device metrics.

Why it matters

LOCS III became the field's common currency: it is the scale against which every objective and machine-learning grading system is validated, and the scale used to define entry criteria and endpoints in cataract trials, including the AREDS antioxidant trial's photographic grading approach (AREDS Research Group 2001, PMID 11594943). Its dominance is also the field's measurement ceiling — because objective systems have higher internal reliability than the standard they are validated against, "agreement with LOCS III" caps how good a new method can appear (PMID 8302524; Mirzaie 2022, PMID 36160089).

Cited by wiki pages

  • classification-and-grading