Pet Anaesthesia: What Bloodwork Checks and Why Fasting Times Are Set

When a vet schedules your pet for a procedure under general anaesthesia, the consent form arrives with two instructions that can feel like formalities: bring your pet in for pre-operative bloodwork, and nothing to eat after a certain hour the night before. Neither is a formality. Anaesthetic drugs are processed almost entirely by the liver and kidneys. A pet with compromised organ function metabolises those drugs differently, sometimes dangerously slowly, and the only way to know before the needle goes in is to look at the blood. The fasting rule, meanwhile, exists because of a reflex that anaesthesia suppresses: the gag reflex. Together, these two steps do more to prevent anaesthetic deaths in companion animals than any single drug choice or monitoring device.

A pre-anaesthetic blood panel in dogs and cats typically covers two broad areas: a complete blood count and a biochemistry profile. The complete blood count looks at red blood cells, white blood cells, and platelets. Anaesthesia lowers blood pressure and reduces oxygen delivery; a pet that is already anaemic going in, with a low red blood cell count, has less reserve to handle that drop. Platelet levels matter because some procedures carry a bleeding risk, and a pet with thrombocytopenia, an abnormally low platelet count, may not clot reliably.

The biochemistry profile is where organ function becomes legible. Alanine aminotransferase and alkaline phosphatase are the primary liver markers. Elevated levels suggest hepatocellular stress or bile duct involvement, which changes how the anaesthetist selects and doses drugs, because most anaesthetic agents are hepatically metabolised. Blood urea nitrogen and creatinine reflect kidney function; the kidneys clear many drugs and their metabolites, and a pet in early renal insufficiency, which often shows no symptoms at all, may retain drugs far longer than expected, prolonging recovery and raising the risk of respiratory depression. Blood glucose matters particularly in cats and in diabetic dogs, where stress and fasting interact in ways that can tip an already-borderline animal into hypoglycaemia or hyperglycaemia under anaesthesia. Electrolytes, sodium, potassium, chloride, affect cardiac rhythm. Hypokalaemia, low potassium, is a documented cause of arrhythmia under general anaesthesia in cats, and it is entirely correctable before the procedure if caught in time.

A healthy six-month-old Labrador being neutered carries a different risk profile from a twelve-year-old Persian cat going in for a dental. Most veterinary practices use age-tiered panels: a basic screen for young, apparently healthy animals, and a more comprehensive panel, often adding a thyroid function test in cats over seven, for middle-aged and senior patients. Hyperthyroidism is the most common endocrine disorder in older cats, and it accelerates the heart rate and raises blood pressure in ways that interact badly with certain anaesthetic induction agents. Testing for it before a dental procedure costs far less than managing a hypertensive crisis on the table.

Breed-specific risks also shape what gets checked. Brachycephalic breeds, Bulldogs, Pugs, French Bulldogs, Persian cats, have anatomical airway narrowing that makes intubation harder and recovery from anaesthesia more precarious. While bloodwork does not show airway anatomy, the pre-operative appointment that accompanies it gives the vet a chance to assess airway grade and plan accordingly. Greyhounds and other sighthounds have a documented sensitivity to barbiturate-based induction agents because of their low body fat ratio, which affects drug distribution; knowing the breed in advance changes the drug selection before a single test result comes back.

General anaesthesia suppresses the laryngeal reflex, the involuntary closure of the airway that prevents stomach contents from entering the lungs. In a conscious animal, vomiting triggers an immediate protective response. Under anaesthesia, that response is gone. If a pet vomits or regurgitates during induction or recovery, stomach contents can be inhaled into the lungs, causing aspiration pneumonia, which carries a mortality rate that makes it one of the most serious anaesthetic complications in veterinary medicine.

The fasting window is calibrated to gastric emptying time, which differs meaningfully between species. In dogs, solid food typically clears the stomach in six to eight hours under normal circumstances; the standard fast for dogs is eight to twelve hours for solids, with water often permitted until two hours before the procedure. Cats empty their stomachs more slowly and are also prone to stress-induced hypoglycaemia and hepatic lipidosis if fasted too long, a condition in which fat mobilises to the liver faster than it can be processed, causing acute liver dysfunction. For this reason, many feline anaesthesia protocols now recommend a shorter fast of four to six hours for food, rather than the overnight window once applied uniformly. Kittens, toy-breed puppies, and diabetic animals may be given specific individual fasting instructions because their glucose regulation cannot tolerate even a standard fast without monitoring.

Pre-operative bloodwork does not always clear an animal for anaesthesia as planned. It sometimes delays a procedure, adjusts the drug protocol, or prompts additional diagnostics. A dog with a creatinine reading above the reference range may need intravenous fluid support during anaesthesia to protect kidney perfusion, and the anaesthetist will select agents with lower renal clearance. A cat with elevated liver enzymes may have its induction agent switched from one that relies heavily on hepatic metabolism to one that is more rapidly cleared by other routes. An animal found to be anaemic may have its elective procedure postponed until the underlying cause is identified and the red cell count stabilised.

These adjustments are the point. The bloodwork is not a formality that gives the clinic paperwork to file, it is the information that allows the anaesthetic plan to be written for this specific animal, not for the average animal of this species and weight. A twelve-year-old cat with normal bloodwork and a twelve-year-old cat with early chronic kidney disease require meaningfully different protocols, and the difference between them is invisible on physical examination alone.

Pre-anaesthetic screening and species-appropriate fasting are the two interventions with the clearest evidence base for reducing anaesthetic mortality in companion animals, according to the American College of Veterinary Anesthesia and Analgesia. The blood panel does not guarantee a safe outcome, no test does, but it converts an unknown into a known, and known risks are manageable ones. The fast does the same for the airway: it removes a variable that anaesthesia cannot safely handle. Both steps exist because the margin for error under general anaesthesia is narrow, and the preparation happens entirely before the animal is sedated.