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Table 2 Summary of variation between patients and ratio of variability attributable to instrument and observers

From: Identifying more reliable parameters for the detection of change during the follow-up of mild to moderate keratoconus patients

  SP S(r&R) r&R TV RU
Anterior
 K1 (D) 4.14 0.10 0.28 4.14 1.00
 K2 (D) 3.11 0.19 0.53 3.11 0.99
 Km (D) 3.66 0.19 0.53 3.66 0.99
 Astigmatism (D) 1.02 0.16 0.46 1.04 0.91
 Q-value, 30 deg 0.83 0.08 0.24 0.84 0.98
 Rper (mm) 0.07 0.04 0.10 0.08 0.76
 Rmin (mm) 0.52 0.11 0.32 0.53 0.90
Posterior
 K1 (D) 1.06 0.09 0.24 1.07 0.99
 K2 (D) 0.78 0.14 0.39 0.79 0.94
 Km (D) 0.96 0.06 0.18 0.96 0.99
 Astigmatism (D) 0.24 0.09 0.25 0.26 0.68
 Q-value, 30 deg 0.82 0.19 0.54 0.84 0.85
 Rper (mm) 0.42 0.06 0.18 0.42 0.97
 Rmin (mm) 0.48 0.09 0.26 0.49 0.92
Summary data
 Kmax front (D) 4.56 1.09 3.03 4.69 0.88
 Corneal volume (mm3) 4.39 1.34 3.72 4.59 0.79
 KPD (D) 0.47 0.15 0.41 0.49 0.83
 AC volume (mm3) 48.84 14.65 40.60 50.99 0.92
 AC angle (deg) 15.47 4.91 13.60 16.23 0.80
 AC depth (mm) 0.72 0.04 0.12 0.72 1.00
 Pupil diameter (mm) 0.46 0.18 0.51 0.49 0.84
Pachymetry
 Pupil centre (μm) 28.20 11.00 30.49 30.27 0.72
 Apex (μm) 13.76 11.54 31.98 17.96 0.42
 Thinnest (μm) 10.28 9.84 27.28 14.24 0.36
Corneal volume (mm3)
 3 mm diameter 0.05 0.06 0.17 0.08 0.34
 5 mm diameter 0.14 0.14 0.40 0.20 0.35
 7 mm diameter 0.85 0.44 1.23 0.96 0.59
Indices
 ISV 42.03 3.12 8.64 42.15 1.00
 IVA 0.68 0.05 0.14 0.68 0.98
 KI 0.13 0.03 0.09 0.14 0.85
 CKI 0.05 0.01 0.03 0.05 0.96
 IHA 17.95 18.40 51.01 25.71 0.39
 IHD 0.04 0.00 0.00 0.04 0.96
Elevation map
 Front at TCT (μm) 18.33 4.82 13.36 18.95 0.85
 Back at TCT (μm) 55.01 4.64 12.85 55.21 0.99
Ectasia map
 Df 6.41 2.30 6.38 6.81 0.75
 Db 5.57 0.77 2.14 5.62 0.99
 Dp 2.94 1.45 4.01 3.28 0.60
 Dt 0.12 0.13 0.37 0.18 0.39
 Da 0.10 0.18 0.51 0.21 0.21
 D 4.13 0.46 1.28 4.16 0.99
  1. K1, K2 = Keratometry readings 1,2; Km = Mean keratometry reading; Rper = Mean radius of curvature in the 7-9 mm area of the cornea; Rmin = Minimum radius of curvature; KPD = Keratometric power deviation; AC = Anterior chamber; ISV = Index of surface variance; IVA = Index of vertical asymmetry; KI = Keratoconus index; CKI = Centre keratoconus index; IHA = Index of height asymmetry; IHD = Index of height decentration; SR = Between observer standard deviation; R = The limits of reproducibility; TR = The log of the ratio between the limits of reproducibility of keratoconus patients and normal subjects; SP = Between patient standard deviation; S(r&R) \( =\sqrt{Sr^2+{SR}^2} \); TV = Total variation \( =\sqrt{SP^2+{Sr}^2+{SR}^2} \); Df = Deviation of the front elevation map; Db = Deviation of the back elevation map; Dp = Deviation of average pachymetric progression; Dt = Deviation of the minimum thickness; Da = Deviation of the apex thickness; D = Belin/Ambrosio ectasia total deviation value
  2. Example of reading the table: Kmax front has an SP value of 4.56 D, which represents the average between patient variability in this parameter; S(r&R) represents the combined limits of repeatability and reproducibility: 1.03 D, TV of 4.78 D represents the total in Kmax variability due to repeatability, reproducibility and between-patient variability. The associated RU value of 0.88 indicates that this parameter has considerable noise with respect to the range of the dynamic range