The white-cell army, the red-cell picture, and the markers of active inflammation.
This panel reads the blood's cellular defences and the inflammatory signals that ride alongside them. The white cell count sums a mixed immune army, and the differential — led by the neutrophils — shows which part is reacting and to what. The acute-phase proteins add a fast, sensitive gauge of how much inflammation is present and, above all, whether it is settling: CRP in dogs, serum amyloid A in cats.
The red-cell side of the count travels with it: haematocrit and haemoglobin measure oxygen-carrying capacity and diagnose anaemia, while platelets report on the blood's ability to clot — with the important caveat that feline platelets clump and can read falsely low.
Two themes run through it all. First, the total is only a headline — the differential and the smear carry the meaning. Second, these markers are at their most powerful tracked over time: a falling CRP or SAA, a recovering haematocrit, a rising platelet count are the objective signals that an illness — and its treatment — is heading the right way.
CRP is a protein that rises quickly with inflammation anywhere in the body and falls as it settles, which makes it an excellent way to track whether an inflammatory process is improving. CRP is the dog's main inflammatory marker — cats use serum amyloid A instead.
Serum amyloid A is the cat's main inflammatory marker — the feline counterpart to CRP in dogs. It rises sharply with inflammation and falls as it resolves, so its trajectory is an excellent way to tell whether an inflammatory process is settling.
The white cell count reflects the immune system's activity. Both high and low values matter, but the total is only the headline — the pattern across the differential (which types of white cell have changed) tells you far more than the number alone.
Neutrophils are the immune system's front-line responders to infection and inflammation, and they are usually the largest part of the white cell count. Shifts here — up or down — help characterise what the total count is reacting to, and a supporting detail called a 'left shift' adds important context.
Haematocrit is the proportion of blood made up of red cells — the body's oxygen-carriers. A low value means anaemia; a high one usually means dehydration or, less often, overproduction. Both ends carry meaning, and the trend and the reticulocyte count tell you what kind of process is at play.
Haemoglobin is the oxygen-carrying pigment packed inside red blood cells, and it tracks closely with the haematocrit. Together they describe how well the blood is carrying oxygen, so a low haemoglobin is essentially another way of measuring anaemia.
Platelets are the tiny cell fragments that form clots to stop bleeding. A very low count raises a genuine bleeding risk and is taken seriously — but in cats especially, platelets often clump in the sample and make the reported number falsely low, so a low result is confirmed before it is believed.
Lymphocytes are the arm of the immune system that handles longer-term and viral responses and makes antibodies. Stress and steroids lower them, so context matters — and a persistent, marked rise is one of the flags that warrants a closer look for a lymphoid disorder.
Monocytes are the clean-up and repair cells of the immune system — they clear debris and sustain longer-running inflammation. A raised count often accompanies a chronic, healing, or tissue-damaging process, so it's read as part of the pattern rather than a diagnosis on its own.
Granulocytes are the front-line infection-fighting white cells — mostly neutrophils, plus eosinophils and basophils. Some analysers report them as a single group; the absolute count is the meaningful figure, and it's interpreted much like the neutrophil count it largely represents.
Eosinophils are the white cells that respond to parasites and allergic inflammation, so a raised count classically points to worms, fleas, or allergic disease. A low count is usually just the body's stress response and rarely means anything on its own.
Basophils are the rarest white cells, involved in allergic and parasitic responses. They are so scarce that a low count is meaningless, and even a raised count is uncommon — when it does occur it usually travels with eosinophils and points towards parasites, allergy, or (classically) heartworm.
Reticulocytes are young red blood cells, and counting them answers the single most important question about any anaemia: is the bone marrow responding? A high count means a regenerative anaemia (blood loss or destruction); a low one means the marrow isn't keeping up.
Globulins are largely antibodies, so a persistently raised value reflects chronic immune stimulation or inflammation, while a low value can signal an antibody or protein-loss problem. Read with albumin, globulin helps tell an immune process from a simple protein leak.
The albumin-to-globulin ratio is a derived value that captures the balance between the liver's main protein and the immune system's antibodies. A falling ratio flags chronic inflammation or an antibody-producing process, and it is followed as a pattern rather than optimised to a target number.
The NLR divides neutrophils by lymphocytes to capture the balance between the immune system's front line and its longer-term arm. It's a cheap, emerging marker of systemic inflammation and stress that has shown prognostic promise — but it is highly non-specific, and a routine stress response raises it just as readily as disease.
Iron studies exist to answer one question: is an anaemia caused by true iron deficiency, or by chronic inflammation? Both lower serum iron, but they behave differently — iron deficiency raises the carrying capacity (TIBC) and empties the stores (ferritin), while inflammation lowers capacity and raises stores.
Allergen-specific IgE blood panels do not diagnose allergy — atopic dermatitis is a clinical diagnosis made by excluding other causes of itch. Their real purpose is to select which allergens to include in immunotherapy once that diagnosis is already made, which is a genuinely useful but much narrower job.
The red cell count is the number of oxygen-carrying cells in the blood, sitting alongside haematocrit and haemoglobin as a measure of oxygen-carrying capacity. A low value indicates anaemia; a high one usually means dehydration or overproduction — and it's read together with the red-cell indices.
MCV is the average size of the red blood cells. Small cells point towards iron issues or chronic disease; large cells often accompany a regenerative response to anaemia. It's one of the red-cell indices used to characterise what type of anaemia is present.
MCH is the average amount of oxygen-carrying pigment in each red blood cell. It's read together with MCV and MCHC to characterise the type of any anaemia, and it tends to track closely with cell size.
MCHC is how tightly packed the oxygen-carrying pigment is inside red cells. A low value suggests a regenerative or iron-poor picture; a high value is almost always a sample artefact, since cells cannot truly be over-filled with pigment.
RDW measures how variable the red cells are in size. A rising value is an early clue that the marrow is releasing a new population of differently-sized cells — as in a regenerative response — so it can flag change before the average indices move.
MPV is the average size of the platelets. Larger, younger platelets can indicate the marrow is actively replacing them — a useful clue when the platelet count is low. In cats this figure is often unreliable because platelets clump so readily.
Serum 25-hydroxyvitamin D is the storage form of vitamin D and, unlike in people, in dogs and cats it comes almost entirely from the diet rather than sunlight. Lower status tracks inflammation, several cancers, and cardiac and kidney disease — but laboratory methods disagree markedly, so there is no settled universal reference band.