Haematocrit in dogs, normal, low and high readings explained

What the haematocrit measures

The haematocrit is the proportion of blood volume taken up by red blood cells. On a report it appears as Hct, HCT or PCV, short for packed cell volume, given as a percentage or in litres per litre, so 45 % is the same as 0.45 L/L: in a spun tube of blood the red cells then form a little under half of the column and the rest is plasma. Two methods are in use and they do not give quite the same figure, one measured by centrifuging a microcapillary tube, the other calculated by the analyser from the red cell count and the mean cell volume. The value is always read alongside the haemoglobin and the red cell count, which move in the same direction. The American spelling, hematocrit, means the same test.

What counts as a normal value in dogs, and why the laboratory's range is the one that applies

In adult dogs the usual values sit at roughly 41 to 58 %, according to the veterinary diagnostic laboratory at Cornell University using an Advia 2120 analyser. That figure is an indicative order of magnitude and nothing more: every laboratory sets its own limits according to its analyser and method, and only the range printed beside the result on your own report tells you whether your dog's value is inside it or outside it.

The gap between dogs and cats is wide, and it is the most common mix-up. Cats have a physiologically lower haematocrit, around 31 to 48 % at Cornell, and other laboratories drop their lower limit as far as 28 %. A haematocrit of 33 % is anaemia in an adult dog and an ordinary value in a cat: a figure that is perfectly normal in a dog would read as low in a cat, and crossing the two ranges leads either to needless worry or to missing something. Age works the same way, since puppies between six and twelve weeks old have a physiologically lower haematocrit than adults.

What vets look into when the value falls outside the range

Below the lower limit the dog is anaemic, and the first question is not yet the cause but whether the bone marrow is responding. That is settled by the reticulocyte count, reticulocytes being young red cells newly released from the marrow, read as an absolute number rather than a percentage: a percentage is measured against an already reduced red cell count, so it can look respectable while production remains inadequate. Plenty of reticulocytes means a regenerative anaemia and points towards blood loss or destruction of red cells, so towards bleeding, a parasite, an immune mechanism or a toxin. Low reticulocytes mean a non-regenerative anaemia and send the question back to production itself, towards chronic kidney disease, established inflammation, marrow disease, a deficiency or a hormonal disorder. Reticulocytes take a few days to appear, which is why a very recent anaemia can look non-regenerative at first and a repeat sample is requested.

Above the upper limit, two groups need separating. The blood may simply be concentrated, with no real rise in the number of red cells, which happens with dehydration or loss of plasma: the vet then weighs up total protein, albumin, hydration status and kidney values. Or the red cell mass really is increased, which is polycythaemia, either secondary to chronic lack of oxygen or to excess erythropoietin production or primary and arising in the marrow, and attention turns to the heart, the lungs, the kidneys and the circumstances of sampling.

What this value does not tell you on its own

A high haematocrit is not necessarily abnormal. Greyhounds, whippets, salukis and other racing sighthounds have a physiologically higher haematocrit than the average dog, and a value flagged as above range may be that dog's own normal. An excited, stressed or firmly restrained dog contracts its spleen and releases red cells into the circulation, which lifts the figure temporarily. A drip running does the opposite, diluting the blood and lowering it, and in the first hours of a haemorrhage the figure can stay normal because the blood lost takes plasma and cells away in the same proportion. Dehydration, conversely, masks an anaemia by concentrating the red cells that remain.

The plateletcrit, often printed a few lines below, concerns platelets and their volume, not red cells. And the decision to give a blood transfusion is never taken on a single figure: it depends on how quickly the anaemia developed, on how well the dog is coping and on the wider picture, and it belongs to a veterinary surgeon registered with the RCVS who has examined the dog.

Questions to ask your vet

1. What is your laboratory's reference range, and how far outside it is the result?
2. Was the reticulocyte count reported as an absolute number, not a percentage?
3. Could hydration, stress or the blood sampling itself have affected this figure?
4. Does my dog's breed justify a haematocrit higher than the printed range?
5. What repeat test do you suggest, and which further investigations?

What is a normal haematocrit for a dog?

In adult dogs the usual values sit at roughly 41 to 58 %, according to the veterinary diagnostic laboratory at Cornell University using an Advia 2120 analyser. That is an indicative order of magnitude and it does not replace the range printed on your report, which is the only one that applies because it depends on the laboratory's analyser and method. Two laboratories can therefore judge the same result differently. Cats sit considerably lower, around 31 to 48 %, and some laboratories take their lower limit down to 28 %.

What does a low haematocrit mean in a dog?

A value below the laboratory's lower limit means anaemia, without saying what is causing it. The vet starts with an absolute reticulocyte count to see whether the bone marrow is responding. Plenty of reticulocytes points towards blood loss or destruction of red cells, while low reticulocytes point towards a production problem such as chronic kidney disease, established inflammation or marrow disease. Because reticulocytes take a few days to appear, a very recent anaemia can look non-regenerative on the first sample.

My dog's haematocrit is high, should I be worried?

A figure above the range does not automatically mean a blood disorder. Dehydration or loss of plasma concentrates the blood and lifts the value without the number of red cells having genuinely risen, and an excited or firmly restrained dog contracts its spleen, which raises the result temporarily. Racing sighthounds also run a physiologically higher haematocrit than other breeds. The vet separates these from true polycythaemia by weighing up protein, hydration, the heart, the lungs and the kidneys.

At what haematocrit does a dog need a blood transfusion?

There is no cut-off figure that triggers a transfusion, and no number should be read that way. The decision is a clinical one and takes in how quickly the anaemia developed, how well the dog is coping, the suspected cause and what is planned over the following hours. A dog whose anaemia came on slowly will often tolerate a value that would be poorly tolerated after a sudden drop. That decision rests with the veterinary surgeon examining the dog.