EIPH is most commonly diagnosed by endoscopic examination of the trachea and larger bronchi, with the optimal timing for endoscopy being 60–90 minutes after hard exercise.[15] This post-exercise delay allows time for blood within the lungs to travel to the trachea.[17] Blood can usually be detected in the trachea or bronchi for 1–3 days after an episode of EIPH, but may be present for up to a week.[18] The amount of blood visible in the trachea at the time of examination is most commonly graded on a scale of 0 (no blood) to 4 (airways awash with blood).[19] In some mildly-affected horses, endoscopy must be performed on multiple occasions to confirm a diagnosis of EIPH.[5]