The canal of Schlemm fills promptly and completely with blood on gonioscopy with a special contact lens if conditions for aqueous outflow are normal and the intraocular pressure is also normal. With normal applanation readings and blood appearing promptly within the whole circumference of the angle, the outflow conditions can be presumed normal in persons over the age of 45.
Schirmer KE. Gonioscopic Assessment of Blood in Schlemm's CanalCorrelation With Glaucoma Tests. Arch Ophthalmol. 1971;85(3):263-267. doi:10.1001/archopht.1971.00990050265001