When we see a high PCV, it’s tempting to think, “Lots of red blood cells!”

Sometimes that’s exactly what’s happening.

But sometimes the patient simply doesn’t have enough plasma.

And that’s why a high PCV in dogs shouldn’t automatically be interpreted as a primary red blood cell problem.

Trying to learn more about PCV’s in animals? Take a look at 7 Essential Facts About Manual PCV Every Veterinary Nurse Should Know

What does a high PCV mean?

A high PCV means that the proportion of blood occupied by red blood cells is above the relevant reference interval.

There are two broad possibilities:

The patient has too many red blood cells, or

The patient has reduced plasma volume, making the red cell proportion appear higher.

The second situation is particularly important in everyday practice.

1. Dehydration

One of the most common reasons for a high PCV in dogs is dehydration.

When a patient loses fluid without losing the same proportion of red blood cells, the blood becomes more concentrated.

The result is an increased PCV.

This is why PCV can be a useful piece of information when assessing hydration.

However, it should never be used alone.

Look at the patient.

Check mucous membranes, skin tent, pulse quality, heart rate, blood pressure where appropriate and other laboratory findings.

2. Gastrointestinal fluid losses

Vomiting and diarrhoea can result in significant fluid loss.

If enough fluid is lost, haemoconcentration can develop and the PCV may rise.

A dog arriving after several days of severe diarrhoea may therefore have a surprisingly high PCV.

Treating the underlying fluid deficit can cause the PCV to move back towards the patient’s normal range.

3. Reduced water intake

Patients that aren’t drinking can become dehydrated for many reasons.

Pain, nausea, weakness, access issues or systemic disease can all contribute.

Again, the PCV may increase as plasma volume falls.

4. Excitement and splenic contraction

This is particularly relevant in cats.

Stress and excitement can cause splenic contraction, releasing stored red blood cells into circulation.

The result can be a temporary increase in PCV.

So if you’ve just wrestled a furious cat through a consultation room door, don’t be surprised if the blood results reflect the experience.

5. Relative erythrocytosis

Relative erythrocytosis occurs when the red cell mass itself isn’t necessarily increased, but plasma volume is reduced.

Dehydration is a classic example.

The important distinction is that the problem is really one of concentration rather than excessive red blood cell production.

6. Absolute erythrocytosis

Less commonly, a high PCV can occur because the patient genuinely has an increased red cell mass.

This is known as absolute erythrocytosis.

There are primary and secondary causes.

Primary erythrocytosis involves inappropriate red blood cell production, while secondary erythrocytosis can occur when increased erythropoietin stimulates red cell production.

The latter can be associated with chronic hypoxia or inappropriate erythropoietin production.

These cases require further investigation by the veterinary team.

7. Sample and interpretation issues

Before getting too excited about an unexpected result, consider whether the result makes sense.

Could there have been a sampling issue?

Was the sample handled appropriately?

Was the PCV performed correctly?

Does the result fit with the rest of the patient’s blood work?

A surprising result deserves a second look.

What does total solids tell us?

Total solids can be particularly useful alongside a high PCV.

For example, an increased PCV with increased total solids may support haemoconcentration in the appropriate clinical context.

However, neither result should be interpreted in isolation.

The patient could have other reasons for increased total solids, so the entire clinical picture matters.

What should veterinary nurses monitor?

If a patient has a high PCV and dehydration is suspected, nursing observations are incredibly important.

Monitor according to the patient’s condition and practice protocols, including:

• Hydration status
• Mucous membranes
• Heart rate
• Respiratory rate
• Pulse quality
• Blood pressure where appropriate
• Urine output
• Bodyweight
• Fluid intake
• Fluid losses

Bodyweight can be particularly useful when monitoring fluid balance because relatively small changes can represent meaningful fluid shifts in small patients.

Why trends matter

Just like a low PCV, a high PCV becomes much more meaningful when you know where the patient started.

For example, a dehydrated patient may have:

Admission PCV: 62%
After fluid therapy: 54%
Later: 47%

The falling PCV may reflect correction of haemoconcentration.

That is much more informative than simply seeing a single value of 62%.

Final thoughts

A high PCV in dogs doesn’t automatically mean that the patient is producing too many red blood cells.

In everyday clinical practice, dehydration and haemoconcentration are important considerations.

The trick is to put the PCV into context.

Look at hydration, total solids, previous PCVs, other blood results and — most importantly — the patient.