The short answer for a rat
A rat health check centres on breathing, skin, and movement: silent breathing with no sneezing fits, bright eyes free of heavy reddish crusting, a clean coat over unbroken skin, no new lumps, even front teeth, and confident climbing with a strong grip. Rats are social and expressive, so behaviour is a real measure here; a rat who leaves the group, stops bruxing and boggling when relaxed, or loses interest in food he normally begs for is telling you something. Fold the check into normal free-roam handling and it takes only a couple of minutes.
Breathing and the reddish discharge question
Rats are prone to respiratory trouble, so listen before you handle: breathing should be silent, with no wheeze, click, or rattling as he moves about. Occasional sneezes in a new environment can be nothing, but repeated sneezing fits, noisy breathing at rest, or hunched posture with fluffed fur point toward a vet visit. Look at the eyes and nostrils for porphyrin, the reddish fluid rats produce around the eyes and nose; a faint trace on waking is normal, while heavy staining that he stops grooming away often travels with stress or illness and is worth recording with a date and a photo.
Coat, skin, and the lump check
Let him walk across your hands and run your fingers gently against the coat to see the skin: look for scabs around the neck and shoulders from scratching, patchy thinning, or tiny specks that could suggest parasites, especially if cage mates are scratching too. Then feel slowly over the whole body, including the belly, chest, and under the jaw, for lumps. Rats, particularly older females, develop growths readily, and the useful information is when a lump appeared and how fast it changes, so measure anything you find with your fingertips and log the size. A lump that grows quickly, ulcerates, or interferes with walking needs prompt veterinary assessment.
Teeth, tail, feet, and ageing back ends
Rat front teeth are naturally a deep yellow-orange and should meet evenly without overgrowing or breaking; a rat who drops food or chews oddly deserves a vet's look. Run your eye along the tail for sores or swelling, and check the feet for redness, swelling, or scabs on the soles, which can follow rough flooring or cage bars. In older rats, watch the hind end closely: dragging back feet, difficulty climbing, or muscle loss along the spine and hips can develop gradually, and noting when grip or climbing first changed helps the vet judge pace. Weigh weekly, since weight loss often precedes visible illness in rats.
When a rat needs the vet without delay
Book promptly for noisy or laboured breathing, a rat who stops eating, heavy porphyrin he no longer cleans, or a lump that grows, opens, or changes shape. A head tilt, loss of balance, bleeding, or sudden hind-leg weakness that leaves him unable to climb also needs timely care rather than home observation. Rats decline quickly once breathing is compromised, so treat respiratory signs as the most urgent item on this list. Keeping your dated notes on breathing, lumps, and weight in KinsLe in the Pet Passport gives the vet a timeline that a single examination simply cannot provide.