The statistical normal reference range for IOP in humans is 10-21 mmHg, and the magnitude of monocular fluctuations and the symmetrical difference between the two eyes are ≤4 mmHg. This means that there is a difference in IOP between the two eyes, but ≤4 mmHg is normal. Therefore, sometimes the IOP is high in the left eye and normal in the right eye, and sometimes it is the other way around, but as long as it does not exceed 4 mmHg, it is normal. If there is a big difference in IOP between the two eyes, it depends on whether there are any factors affecting the IOP measurement at that time. In addition, the diagnosis of glaucoma cannot be made only by looking at the IOP but also the changes of fundus and visual field.