Health & Medical

Most of the numbers on your pet's blood results are not diagnoses. They are clues, several are red herrings, and a fair few of the values printed in bold red mean nothing whatsoever in the animal sitting in front of you. That is not a criticism of the test. It is how laboratory medicine works, and once you understand it, the sheet the receptionist hands over stops being intimidating.

Start with the reference intervals down the right-hand side, because they cause more anxiety than anything else on the page. Those ranges come from testing healthy animals and taking the middle chunk of results, which by design leaves a small percentage of perfectly well individuals outside the range. A greyhound normally runs a higher packed cell volume and a lower white cell count than a labrador. A growing puppy is not a small adult. A slightly flagged value in an animal who feels fine is often just that animal being themselves.

The liver enzymes are where most confusion starts. ALT lives inside liver cells and appears in the blood when those cells leak or die. A raised ALT tells you liver cells have been damaged and nothing at all about whether the liver is still doing its job. A dog can post a spectacular ALT after one toxin exposure and recover completely, while a cat with an end-stage liver may show an ALT that looks almost reasonable because there is little tissue left to leak. Height does not equal severity.

ALKP is misread even more often. It rises with cholestasis, meaning bile is not flowing as it should, but in dogs there is also a separate steroid-induced form, so a dog on prescribed steroids or making too much of its own cortisol can show a high ALKP with no primary liver disease. Bone produces it too, which is why a growing puppy routinely has an ALKP that would worry you in a ten-year-old. Cats are different again: feline ALKP has a much shorter half-life and no steroid-induced form, so a genuine rise in a cat carries more weight. If your vet adds a GGT or bile acid test, that is separating these possibilities rather than fishing.

Kidney values carry a limitation that deserves to be better known. Urea and creatinine only start climbing once a substantial majority of functioning kidney tissue has already gone, so a normal creatinine does not certify healthy kidneys. It certifies that enough remain to keep the number down. Creatinine is also swayed by muscle mass, exactly the problem in the patients you most want to catch: a wasted old cat can post a creatinine within range while genuinely in trouble, while a heavily muscled working dog sits naturally higher with no disease at all. SDMA was introduced to address this. It rises earlier and is far less affected by muscle mass, though it is not infallible.

Whichever markers are used, blood is only half the test. Urine is the other half. If a pet's kidneys can still concentrate urine, that tells you something no blood value can, and reading kidney numbers without a urine specific gravity beside them is like reading one page of a two-page letter. If your vet asks for a sample, it matters.

Glucose is the classic trap, particularly in cats. Adrenaline drives blood sugar up, and a cat who has endured a carrier, a car journey, a waiting room full of dogs and a needle has plenty on board. Stress hyperglycaemia in cats can reach levels that look convincingly diabetic and mean nothing. That is why a single high glucose in a cat is not a diagnosis and why fructosamine, which reflects the average over the preceding couple of weeks, settles the question. Dogs show the effect far less, so a high glucose in a dog carries more weight, but even then your vet will want urine and repeat testing before acting.

On the haematology side, the packed cell volume is the headline. Low means anaemia, and the next question is whether the bone marrow is responding, judged by counting young red cells called reticulocytes. A regenerative anaemia suggests blood is being lost or destroyed; a non-regenerative one points at production failure, chronic disease or marrow trouble. Dehydration concentrates the blood and can hide an anaemia, so an animal who arrives dry may look fine on paper and show the true picture only after fluids.

White cells are read as a pattern rather than a total. A neutrophilia with immature cells appearing suggests active inflammation or infection. A neutropenia is more worrying. And there is the stress leukogram, driven by cortisol: neutrophils rise while lymphocytes and eosinophils fall, reflecting a stressed or unwell body in general rather than any one disease. Excited young cats can produce the opposite, with lymphocytes up purely from the adrenaline of the visit. Platelets deserve a footnote, because feline platelets clump in the sample notoriously easily and give a falsely low count. Very low platelets in a cat with no bruising or bleeding is usually an artefact, confirmed by eye on the smear.

A few practical things quietly wreck results. A pet fed shortly before sampling produces fatty, cloudy serum that interferes with several tests. Slightly damaged red cells release their contents and skew potassium. Some breeds have their own quirks, such as the Akita and Shiba, whose red cells hold unusually high potassium. Good laboratories flag these, but it is another reason results are read by a person, not an analyser.

Which leads to the most useful habit an owner can adopt. Ask for a copy of every panel and keep them together. A creatinine that has quietly doubled over two years while never once leaving the normal range says far more than one mildly abnormal number in isolation, and only a file of old results shows you that. A baseline taken while your pet is well, ideally in middle age, is one of the more valuable things in the record. Single snapshots start conversations. Trends answer questions.