Can Red Blood Cell Count Predict a Horse’s Racing Ability?

By Dr Duncan Houston

Red blood cells are essential for athletic performance because they carry haemoglobin, which transports oxygen from the lungs to working muscles. That makes red blood cell count, packed cell volume and haemoglobin tempting numbers to use when assessing a racehorse.

The problem is that a blood sample measures what is circulating at that particular moment. In horses, those values can change dramatically with excitement, pain, exercise, dehydration and contraction of the spleen.

Bloodwork can reveal reasons a horse may be unable to perform normally. It cannot reliably tell you which horse will be fastest.

Quick Answer

No. A resting red blood cell count, packed cell volume or haemoglobin concentration cannot reliably predict a horse’s racing ability.

These values are affected by splenic contraction, hydration, excitement, breed, medication and the timing of blood collection. Standardised exercise testing can contribute useful information, but racing performance depends on cardiovascular function, respiratory health, soundness, muscle metabolism, training and many other factors. In one Thoroughbred study, post-exercise packed cell volume was not significantly correlated with Timeform performance ratings. (PubMed)

What Do Red Blood Cell Count, Haemoglobin and Haematocrit Measure?

These terms are related, but they are not interchangeable.

Blood value	What it measures	What it can tell you
Red blood cell count	The number of circulating red blood cells within a volume of blood	Helps identify anaemia or erythrocytosis
Haemoglobin	The concentration of oxygen-carrying haemoglobin in the blood	More directly reflects the blood’s oxygen-carrying component
Packed cell volume	The percentage of blood occupied by red cells after centrifugation	Commonly used to assess red cell concentration and hydration
Haematocrit	Usually an analyser-calculated estimate based on red cell count and cell size	Often used clinically in a similar way to packed cell volume
Total red cell volume	The total quantity of red cells within the horse’s entire circulation	More relevant to total oxygen transport, but not measured by a routine CBC
Total haemoglobin mass	The total amount of haemoglobin in the body	Physiologically important, but different from haemoglobin concentration

A routine complete blood count measures concentrations within the sample, not the horse’s total red cell volume or total haemoglobin mass. A concentrated sample may reflect dehydration rather than a genuinely larger red cell mass. (eClinpath)

Why Are Red Blood Cells Important for Racing?

Haemoglobin binds oxygen in the lungs and carries it to working tissues. During intense exercise, a racehorse’s muscles require an enormous and rapidly increasing oxygen supply.

The amount of oxygen reaching the muscles depends on more than red blood cells. It also depends on:

• Pulmonary ventilation and gas exchange

• Cardiac output

• Blood volume

• Circulation through the muscles

• Haemoglobin concentration

• The muscles’ ability to extract and use oxygen

• Temperature regulation

• The horse’s metabolic efficiency

Reducing red cell volume experimentally can reduce oxygen delivery and shorten the time a horse can exercise before fatigue. This confirms that adequate red cell mass matters. However, excessively concentrated blood increases viscosity and may make circulation less efficient. The ideal situation is not simply the highest possible haematocrit. (PubMed)

Why Are Horses Different From Most Other Athletes?

Horses have a highly muscular spleen that acts as a reserve tank for red blood cells.

Up to approximately 30 percent of a horse’s red blood cells may be stored in the spleen. When adrenaline rises during excitement, fear, pain or strenuous exercise, the spleen contracts and releases many of those cells into circulation. (eClinpath)

This can rapidly increase:

• Red blood cell count

• Packed cell volume

• Haematocrit

• Haemoglobin concentration

• Circulating blood volume

• Oxygen-carrying capacity

Splenic contraction can raise haematocrit by approximately 40 percent above the individual horse’s baseline. At maximal exercise, systemic haematocrit may rise from the mid-30s at rest to approximately 60 to 70 percent. (Merck Veterinary Manual)

The effect is temporary. When excitement and adrenaline settle, values caused by splenic contraction should generally move back towards baseline within approximately 40 to 60 minutes. (Merck Veterinary Manual)

This is why a horse sampled quietly in the stable can have very different results from the same horse sampled:

• Immediately after transport

• At the racetrack

• After a gallop

• During pain or colic

• While frightened by needles

• Following sedation or tranquillisation

A resting result therefore does not reveal the horse’s maximum exercise-related oxygen-carrying capacity.

Why Can’t a Resting Blood Test Predict Racing Ability?

The spleen changes the result

Two horses with similar total red cell reserves may have different resting counts simply because one horse has released more cells from the spleen at the time of collection.

Hydration changes the concentration

Dehydration reduces plasma volume and makes red cell count, haematocrit and haemoglobin concentration appear higher. This is relative erythrocytosis, not proof that the horse has produced more red cells. (Merck Veterinary Manual)

Excitement can mimic a high result

A nervous Thoroughbred may develop substantial splenic contraction while being restrained for blood collection. The result may reflect adrenaline rather than exceptional fitness.

Sedation can lower the circulating value

Tranquillisers and anaesthetic medications can relax the spleen, allowing red cells to move out of active circulation. This may lower red cell count, haematocrit and haemoglobin without representing true anaemia. (eClinpath)

Breed affects reference ranges

Thoroughbreds and other hot-blooded breeds tend to have different erythrogram values from many Warmblood and heavier breeds. A recent comparison found significantly higher red blood cell counts in Thoroughbreds than in the other populations studied. (Wiley Online Library)

A concentration is not the same as total capacity

A horse can have a high haematocrit because plasma volume is low. Another may have a larger total blood volume and red cell mass but a less impressive concentration because the plasma compartment is also larger.

Racing performance is multifactorial

Poor performance can involve airway obstruction, lower airway inflammation, gastric disease, exercise-induced muscle disease, cardiac arrhythmias, lameness and several simultaneous conditions. A normal CBC cannot rule these problems out. (PMC)

What Does the Racing Research Show?

In one study, researchers collected blood from Thoroughbreds following an 800-metre gallop and during a separate standardised treadmill test. They assessed packed cell volume, haemoglobin, heart rate, lactate and other measurements against Timeform performance ratings.

Packed cell volume was not significantly correlated with racing quality. Blood lactate following a strenuous standardised treadmill test did show a relationship with performance, illustrating that a carefully controlled dynamic test may provide more information than a red cell concentration alone. (PubMed)

Other training research has similarly concluded that no single routine haematological measurement reliably predicts racing performance. (Animal Bioscience)

This does not mean red blood cells are unimportant. It means that one blood value cannot summarise the entire athletic system.

Does a Higher Haematocrit Ever Help Performance?

Adequate total red cell volume supports oxygen delivery, and reducing red cell volume substantially can reduce exercise capacity.

However, there is an important physiological trade-off.

As haematocrit rises:

• Oxygen-carrying capacity may increase

• Blood also becomes more viscous

• The heart must move thicker blood

• Vascular resistance may rise

• Pulmonary pressures may increase

• Tissue perfusion may become less efficient

In Standardbred trotters with red cell hypervolaemia, removing approximately 22 percent of blood volume reduced oxygen delivery and shortened exercise time to fatigue, showing that those horses relied on their enlarged red cell volume. The same intervention also reduced blood viscosity, vascular resistance, pulmonary pressures and the severity of exercise-induced pulmonary haemorrhage. (PubMed)

At the other extreme, an endurance horse receiving multiple haematologic supplements developed erythrocytosis, poor performance and exercise-associated colic. After the supplements were stopped and the haematocrit normalised, the horse successfully returned to longer-distance competition. (ScienceDirect)

The practical conclusion is simple:

Too few red cells can limit oxygen delivery. Too many can impair circulation. The highest result is not automatically the best result.

What Is Bloodwork Actually Good For in a Racehorse?

Bloodwork is valuable for identifying health problems that could limit performance.

It can help detect:

• Anaemia

• Dehydration

• Persistent erythrocytosis

• Inflammation or infection

• Muscle injury

• Electrolyte abnormalities

• Liver or kidney disease

• Protein loss

• Blood loss

• Red blood cell destruction

• Some toxicities

• Trends during training or recovery

Bloodwork is excellent at finding reasons a horse should not be performing normally. It is much less useful for proving that a healthy horse will perform exceptionally.

A normal blood panel also does not exclude:

• Subtle lameness

• Dynamic upper airway obstruction

• Moderate equine asthma

• Exercise-induced pulmonary haemorrhage

• Cardiac arrhythmias that occur only during exercise

• Gastric ulceration

• Poor conditioning

• Rider or equipment factors

• Inappropriate training load

Many performance-limiting abnormalities only become apparent during exercise testing. (PMC)

When Does a Low Red Blood Cell Count Matter?

A low red blood cell count, haematocrit or haemoglobin may represent genuine anaemia, but the result must be interpreted with the horse’s clinical condition and sampling circumstances.

Signs associated with clinically important anaemia can include:

• Reduced energy

• Poor recovery after exercise

• Weakness

• Reduced appetite

• Pale gums

• Increased resting heart rate

• Increased respiratory rate

• A heart murmur

• Reduced tolerance of normal work

The speed at which the anaemia developed matters. A horse with slowly developing anaemia may compensate surprisingly well, while acute severe blood loss can cause shock and death. (Merck Veterinary Manual)

Blood loss

Possible causes include:

• External wounds

• Internal haemorrhage

• Gastrointestinal ulceration

• Blood-sucking parasites

• Urinary or reproductive tract bleeding

• Coagulation disorders

• Bleeding tumours

• Recent surgery or trauma

An important clinical trap is that packed cell volume may initially remain normal after acute haemorrhage. The horse loses plasma and red cells together, and splenic contraction may temporarily maintain the measured concentration. The PCV and protein concentration may not fall until fluid shifts occur over the following hours. (eClinpath)

Red blood cell destruction

Haemolytic anaemia may result from:

• Equine infectious anaemia

• Equine piroplasmosis in relevant regions

• Immune-mediated disease

• Certain toxins or plants

• Drug reactions

• Neonatal isoerythrolysis in foals

• Other infectious diseases

A horse with haemolysis may have yellow gums or eyes, dark urine, fever, weakness or rapid deterioration. (Merck Veterinary Manual)

Reduced red blood cell production

Possible causes include:

• Chronic inflammatory disease

• Chronic infection

• Kidney disease

• Bone marrow disease

• Neoplasia

• Severe malnutrition

• Rare mineral deficiencies

• Certain medications or toxins

Anaemia of chronic inflammatory disease is commonly mild to moderate and improves by addressing the underlying condition, not by simply supplying iron. (Merck Veterinary Manual)

Horses are unusual when assessing regeneration

Unlike dogs and many other species, horses generally do not release meaningful numbers of reticulocytes into peripheral blood after developing anaemia. A routine reticulocyte count is therefore not a reliable way to assess whether the equine bone marrow is responding. Trends in cell size, bone marrow examination and the clinical history may be required. (eClinpath)

Is Iron Deficiency Common in Horses?

No. True iron deficiency is uncommon in adult horses.

Forage, soil and most commercial feeds provide substantial amounts of iron. When iron deficiency does occur, it is more likely to result from prolonged external blood loss than from a simple dietary shortage. (Merck Veterinary Manual)

Potential sources include:

• Chronic gastrointestinal bleeding

• Significant parasite burdens

• Repeated urinary bleeding

• Long-term external blood loss

• Rare severe dietary imbalance

Iron deficiency generally produces small, poorly haemoglobinised red cells, described as microcytic and hypochromic. Low serum iron by itself does not confirm deficiency because inflammation can also lower circulating iron availability. (eClinpath)

Should an anaemic horse receive iron?

Not automatically.

Iron supplementation has not been shown to improve oxygen-carrying capacity or red cell production in horses that are not iron deficient. Excess iron can interfere with copper metabolism, contribute to liver disease and create toxicity. Injectable iron can be dangerous in horses and should not be used casually. (Merck Veterinary Manual)

The mistake I see most often is treating the laboratory number while ignoring why it became low.

A horse with anaemia from inflammation, internal bleeding, kidney disease or bone marrow disease will not be cured by adding a red-coloured “blood builder” to the feed bucket.

What About Blood Builders, Vitamin B12 and Haematologic Supplements?

Many products marketed to performance horses contain combinations of:

• Iron

• Copper

• Cobalt

• Vitamin B12

• Folic acid

• Amino acids

• Herbal ingredients

• Other claimed haematologic stimulants

These products should not be used as a substitute for diagnosis.

Supplementation may be appropriate when a specific deficiency has been documented or a complete ration has been professionally assessed as inadequate. It is unlikely to improve racing performance in a horse already receiving a balanced diet.

Multiple unnecessary haematologic products may also create excessive mineral intake or interfere with the interpretation of later blood testing. A published case of supplement-associated erythrocytosis demonstrates that attempting to push red cell values higher can be counterproductive. (ScienceDirect)

When Does a High Red Blood Cell Count Matter?

The most common reasons for an increased result are:

• Splenic contraction from excitement, pain or exercise

• Dehydration

• Recent strenuous work

These are relative increases. They do not necessarily mean the horse has produced an abnormal total quantity of red blood cells. (Merck Veterinary Manual)

A persistent increase may occasionally result from:

• Chronic inadequate oxygenation

• Severe respiratory or congenital cardiac disease

• Inappropriate erythropoietin production

• Certain tumours

• Exogenous erythropoietin administration

• Rare primary erythrocytosis

Clinical signs of significant erythrocytosis can include:

• Lethargy

• Poor performance

• Weight loss

• Dark or unusually thick-looking blood

• Deep red or purple mucous membranes

• Increased heart rate

• Increased respiratory rate

• Hypertension

• Delayed capillary refill

• Thrombosis or abnormal bleeding

Persistent absolute erythrocytosis is uncommon, but it should not be celebrated as superior athletic physiology. (Merck Veterinary Manual)

What About Erythropoietin and Blood Doping?

Erythropoietin, commonly called EPO, is a hormone that stimulates red blood cell production.

Controlled research in Standardbred horses has demonstrated that recombinant human EPO can increase aerobic capacity and exercise performance. That does not make its use safe, ethical or permitted. (PubMed)

Horses given recombinant human EPO can produce antibodies against the foreign protein. Those antibodies may also attack the horse’s own erythropoietin, causing severe suppression of red cell production and potentially life-threatening anaemia. This was documented in horses that developed erythroid hypoplasia following treatment. (PubMed)

Recombinant EPO and its synthetic analogues are banned substances under major equine anti-doping systems, including the 2026 FEI Equine Prohibited Substances List. HISA also classifies EPO and EPO mimetics as banned substances in US Thoroughbred racing. (inside.fei.org)

There is no legitimate performance shortcut here. Manipulating red cell production can end a racing career, threaten the horse’s life and result in serious regulatory consequences.

How Worried Should You Be About an Abnormal Result?

There is no universal PCV number that determines urgency in every horse. Breed, laboratory reference intervals, hydration, sampling conditions and the horse’s clinical signs all matter.

Risk level	What it may look like	What to do
Low risk	One mildly abnormal value, horse bright and performing normally, sample taken after transport, excitement, sedation or exercise	Review the sampling conditions and arrange a standardised repeat within one to two weeks
Moderate risk	Persistent mild anaemia or erythrocytosis, reduced performance, weight loss, slow recovery or an unexplained trend across several samples	Arrange a veterinary examination and diagnostic review within several days
High risk	Marked abnormality with pale or deep-red gums, weakness, fever, reduced appetite, dark urine, tachycardia, muscle pain or significant performance loss	Contact your veterinarian the same day and stop strenuous exercise
Critical	Collapse, severe weakness, active haemorrhage, breathing difficulty, white or yellow gums, inability to stand or rapidly worsening clinical signs	Treat this as an emergency and seek immediate veterinary care

A mildly abnormal number in a comfortable horse is very different from a similar result in a horse that is weak, tachycardic or actively bleeding.

When Is This an Emergency?

Seek immediate veterinary help if your horse has:

• Collapse or inability to stand

• Severe weakness

• Active external bleeding

• Suspected internal haemorrhage

• Very pale, white, yellow or purple gums

• Severe breathing difficulty

• A markedly increased resting heart rate

• Dark red or brown urine with weakness or jaundice

• Fever with rapid deterioration

• Severe colic

• Neurological abnormalities

• Uncontrolled bleeding

• A significant recent injury

• A profoundly abnormal result accompanied by clinical illness

Do not wait for a repeat blood test when the horse is unstable.

Remember that acute blood loss may not immediately produce a low PCV, so a normal early result should not overrule signs of shock or haemorrhage. (eClinpath)

What Should You Do Next?

1. Do not treat one number in isolation

Review the whole CBC, not just the red blood cell count.

Important accompanying values include:

• Haemoglobin

• PCV or haematocrit

• Mean cell volume

• Mean cell haemoglobin concentration

• White blood cell count

• Platelets

• Total protein

• Albumin

• Blood smear findings

The combination often distinguishes dehydration, inflammation, blood loss and true red cell disease more effectively than one value alone.

2. Record how the sample was collected

Tell your veterinarian whether the horse had:

• Recently exercised

• Travelled

• Been frightened or excited

• Experienced pain

• Received sedation

• Eaten or drunk normally

• Sweated heavily

• Received intravenous fluids

• Started any new supplement or medication

Without this context, a technically correct laboratory result can be clinically misleading.

3. Repeat the test under standardised conditions

For a resting baseline, blood should ideally be collected:

• When the horse is calm

• Before strenuous exercise

• In a normally hydrated state

• At a similar time of day

• Using the same laboratory where possible

• With medication and sedation recorded

When splenic contraction is suspected, a repeat after the horse has remained calm for at least 40 to 60 minutes, or on another quiet day, may provide a more representative resting value. (Merck Veterinary Manual)

4. Investigate persistent abnormalities

Depending on the findings, your veterinarian may recommend:

• Repeat CBC and blood smear

• Serum biochemistry

• Total protein and albumin

• Kidney and liver assessment

• Muscle enzymes

• Urinalysis

• Faecal egg count

• Faecal occult blood testing in selected cases

• Infectious disease testing

• Abdominal or thoracic ultrasound

• Rectal examination

• Gastroscopy

• Coagulation testing

• Bone marrow sampling in unexplained cases

Iron testing should be targeted to horses with supporting evidence such as microcytosis, hypochromasia and suspected chronic external blood loss.

5. Investigate the horse, not only the blood

When the main complaint is poor performance, assessment may also need to include:

• Full physical examination

• Lameness examination

• Resting and exercising cardiac evaluation

• Dynamic upper airway endoscopy

• Post-exercise respiratory assessment

• Bronchoalveolar lavage

• Gastroscopy

• Exercise testing with heart rate and lactate monitoring

• Evaluation for exertional muscle disease

• Review of training load, recovery and nutrition

In a study of 259 non-lame Standardbred racehorses referred for poor performance, multiple subclinical disorders were common, including moderate equine asthma, dynamic upper airway obstruction, muscle disease and gastric ulceration. (PMC)

6. Stop unnecessary supplements before adding more

Provide your veterinarian with photographs or labels of every supplement being used. Several products may contain the same minerals under different ingredient names.

Do not add iron, cobalt, injectable vitamins or haematologic products solely because a red cell value appears lower than expected.

Common Mistakes Trainers and Owners Make

Using resting PCV to select the most talented horse

A high resting PCV may reflect excitement or dehydration rather than superior total red cell mass.

Assuming higher is always better

Excessive red cell concentration increases blood viscosity and may impair performance rather than enhance it.

Giving iron without diagnosing iron deficiency

True iron deficiency is rare in adult horses, and blind supplementation may cause nutritional imbalance or toxicity.

Comparing results collected under different conditions

A calm stable sample cannot be fairly compared with one taken after transport, exercise or a stressful veterinary procedure.

Comparing different laboratories without checking reference intervals

Laboratories use different analysers, methods and reference populations. Small variations may reflect methodology rather than a real physiological change.

Ignoring total protein and hydration

A high PCV with increased protein and clinical dehydration means something different from a high PCV with normal protein after excitement.

Assuming normal bloodwork rules out disease

Many important causes of poor performance are not visible on a routine CBC.

Chasing numbers instead of trends

An individual horse’s consistent baseline is often more informative than whether one result sits near the high or low end of a broad population range.

How Should Bloodwork Be Used in a Racing Programme?

Blood monitoring is most useful when it is consistent.

A sensible approach includes:

• Establishing a healthy resting baseline

• Using the same laboratory where practical

• Sampling under similar conditions

• Recording exercise and transport before collection

• Tracking hydration and body weight

• Recording all medications and supplements

• Comparing results with training response and recovery

• Investigating meaningful trends rather than reacting to every small fluctuation

• Using standardised exercise testing when evaluating fitness

Pre-exercise and post-exercise samples can be useful when collected as part of a defined veterinary protocol. Randomly timed post-work samples are much harder to interpret because spleen contraction, sweating and plasma volume shifts change rapidly.

The objective is not to produce the highest red cell value. It is to maintain a healthy horse whose cardiovascular, respiratory and musculoskeletal systems work effectively together.

Will a Horse With an Abnormal Red Blood Cell Count Be Okay?

Often, yes, particularly when the change is mild, transient and caused by excitement, sedation or hydration.

The outlook depends entirely on the cause.

• Splenic contraction usually resolves as the horse settles.

• Dehydration generally improves when the fluid deficit and its cause are corrected.

• Mild anaemia from chronic inflammation may resolve after the underlying disease is treated.

• Iron deficiency can improve when chronic blood loss is controlled and true deficiency is correctly supplemented.

• Acute haemorrhage or severe haemolysis can be life-threatening.

• Bone marrow disease may require prolonged investigation and treatment.

• Persistent erythrocytosis requires assessment for hypoxaemia, inappropriate erythropoietin production or rare primary disease.

The number alone cannot provide a prognosis. The trend, cause and horse’s clinical condition matter most. (Merck Veterinary Manual)

Frequently Asked Questions

Does a high haematocrit mean a horse will run faster?

No. Adequate red cell mass supports oxygen delivery, but a high concentration may result from excitement or dehydration. Excessive haematocrit also increases blood viscosity and may impair circulation.

What is a normal PCV for a racehorse?

Reference intervals vary between laboratories, breeds and analysers. Thoroughbreds may have higher resting erythrogram values than some other breeds. The best comparison is the laboratory’s equine interval combined with the individual horse’s previous results collected under similar conditions.

Can iron supplements make a horse race faster?

Not in a horse that is already receiving enough iron. Iron supplementation has not been shown to improve oxygen-carrying capacity in horses without confirmed deficiency, and excessive supplementation can be harmful.

Should blood be collected before or after exercise?

That depends on the question being asked. Resting health screening should use a calm, standardised pre-exercise sample. Exercise testing may include samples collected at precisely defined times before, during and after work.

Can a normal CBC rule out a medical cause of poor performance?

No. A horse can have normal routine bloodwork and still have lameness, airway obstruction, equine asthma, gastric disease, exercise-induced arrhythmia, muscle disease or inadequate conditioning.

The Real Takeaway

Red blood cells are vital to racing performance, but their importance does not make a routine blood count a talent test.

A resting RBC count, PCV or haemoglobin result is influenced by the spleen, hydration, excitement, medication, breed and sampling conditions. It tells you what was circulating at one moment, not the horse’s full oxygen-delivery capacity and certainly not how fast that horse will race.

Use bloodwork to identify disease, establish individual trends and investigate poor recovery or reduced performance. Do not use it to rank horses by potential or justify unnecessary iron and blood-building supplements.

The most useful performance assessment combines bloodwork with the horse’s history, soundness, respiratory function, cardiovascular response, exercise testing, training and recovery.

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Concerned about an abnormal blood result or an unexplained drop in performance? ASK A VET™ can help you organise laboratory trends, symptoms, training history and recovery data so you can understand what needs monitoring and when an in-person equine veterinary assessment is required.