Importance of standardizing the number of cells measured for coefficient of variation (COV) estimates of corneal endothelial cell area values as relevant to contact lens wear

Abstract

PurposeTo assess the impact of using different numbers of cells in calculations of the coefficient of variation (COV) value for normal and polymegethous endothelia MethodsFour sets of 20 non-contact specular microscope images obtained from Caucasian individuals were assessed, and categorized according to the extent of polymegethism, i.e. grade 0 (none), grade 1 (mild), grade 2 (moderate) and grade 3 (substantial). Cell areas were measured manually and then values for between 2 and 100 cells were progressively added and averaged to generate COV estimates. These were then assessed in terms of their relative values (as percentages +/- SD) in relation to the value obtained on over 100 cells. ResultsFor the 4 sets of endothelia with group-mean COV values of 25.8, 33.1, 45.1 and 56.8%, the reliability of the COV estimates realized asymptotic values of close to ±1.0, ±2.7, ±3.6 and ±5.0% with 90 cells, but with greater uncertainty with few numbers of cells, e.g. only to within ±3.4, ±5.3, ±6.1 and ±7.9% with 75 cells. ConclusionsCOV estimates for the corneal endothelium are dependent on the number of cells used in the calculations. It is recommended that every effort should be made to not only assess 75–100 cells per endothelial image, but that this number should be the same or very similar for all endothelial images in a particular data set so that the uncertainly (or estimated proportional error) in the estimates is balanced.