There are several possible explanations for unexpected readings. Do not rule out the possibility that your results may have physiological explanations. Retinal oximetry is a new field of science and our understanding retinal oxygenation is still being improved. Artificial or confounding factors to your study may include:
- The image quality might be poor. Cataract or interference from eye lashes or poorly dilated pupils can make a large difference. The images must have good contrast and the illumination of the retina must be even.
- The alignment of the image may make a difference, see the paper by Palsson et al in IOVS.
- Different protocol used for selecting vessel segments. Not everyone use the same protocol and thus it can give different values.
- The degree of pigmentation can influence the results.
- Low or nonstandard illumination, make sure you choose sufficient flash settings e.g. 50Ws.
- The age might affect the outcome. Studies show slightly less oxygen saturation in the venules in older people (but the difference in arterioles is very small, if any). See Geirsdottir et al.