Cognitive
The one system with no blood test — screened by a validated questionnaire.
Cognition is the exception in this library: there is no blood marker for it. An ageing pet's mental change cannot be read from a chemistry panel, so it is assessed the way it is in people — with a structured, validated questionnaire that turns everyday observations into a trackable score.
Cognitive screening (the CCDR) is the tool for dogs — a short owner questionnaire that flags whether canine cognitive dysfunction ("dog dementia") is likely, using the DISHAA pattern of signs (disorientation, altered interactions, sleep–wake disturbance, house-soiling, activity changes, anxiety). Cats develop an analogous feline cognitive dysfunction, screened through careful behavioural assessment.
Two principles carry over from the rest of the library. First, a screen raises the question — the overlapping causes (pain, sensory loss, thyroid and blood-pressure problems) must be excluded by a veterinary work-up before cognitive dysfunction is settled on. Second, the trajectory matters more than the snapshot: a baseline taken while a pet is still bright, repeated over time, is what catches a subtle decline early enough to manage it well.