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Jared Nielsen, Diabetic Retinopathy Clinical Research Network; Equations to Convert Retinal Thickness Measurements from Spectral Domain OCT Machines to “Equivalent” Time Domain OCT Measurements. Invest. Ophthalmol. Vis. Sci. 2013;54(15):2405.
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To formulate equations to convert central subfield (CSF) thickness from spectral domain (SD) OCT to a predicted equivalent value on time domain (TD) OCT.
Prospective observational study of persons with diabetes enrolling: a) both eyes with at least one eye with center-involved diabetic macular edema (TD CSF thickness >250μm on a Stratus [Carl Zeiss Meditec] machine), and b) eyes without retinal thickening and no or very mild diabetic retinopathy (TD/SD on a Spectralis [Heidelberg] machine cohort only). Each study eye underwent two replicate TD scans followed by two replicate SD scans (either on a Spectralis or Cirrus [Carl Zeiss Meditec] machine. Within each SD cohort, the TD/SD scan pair from a random half-sample of eyes was used to create a conversion equation to transform CSF thickness from the respective SD-OCT value to a predicted value on the TD machine. The remaining half-sample was used to evaluate the equation.
The study enrolled 540 eyes with TD/Cirrus paired scans and 758 eyes with TD/Spectralis paired scans, with median (25th, 75th percentile) TD CSF thickness of 290μm (247, 371) and 239μm (199, 308), respectively. For each cohort, initial TD measurement was within 10% of the replicate TD measurement in 92%. Conversion equations predicted a TD thickness within 10% of the observed thickness 86% and 89% of the time for TD/Cirrus and TD/Spectralis cohorts, respectively. For the TD/Cirrus and TD/Spectralis cohorts, respectively, the mean converted TD thickness was 322μm and 266μm whereas the mean observed TD thickness was 322μm and 267μm. For any single observed Cirrus or Spectralis value the half-width of the 95% prediction interval for the converted TD value was +38μm and +41μm, respectively. For example, an individual with a Cirrus CSF thickness of 300μm had predicted TD value of 259μm (95% prediction interval: 220, 297); and a Spectralis CSF thickness of 300μm predicted TD value to be 235μm (95% prediction interval: 194, 276).
Conversion equations may be used to transform CSF values obtained on a SD-OCT to a TD scale for group comparisons. However, the CSF conversion equations do not appear to predict TD values for an individual accurately enough to warrant use of these conversion equations confidently in clinical decision-making at the patient level.
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