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Dr McGregor's Equine Blood Analysis Series

Blood analysis has been part of horse racing now for over forty years with Dr Percy Sykes in Sydney being the major contributor to linking detectable variations in the blood picture with performance traits in the horse. Extensive work has been carried out both on horses and humans to assess response to training, signs of early over-training and correction of problems which allow horses to work closer to that knife- edge that will have them finishing off their races to the best of their ability.

In this article, we will look at the components that make up the red series. That is, the measurements to do with the red blood cells. In later articles, we willexamine the white series and the chemistry of the blood which will give an overall picture of the horse.

Firstly, it should be stated that the taking of blood samples needs to be standardised as much as possible. To this end, two separate blood sample times are preferable to assess the bloods. I believe that the best time is first thing in the morning before feeding and working and as soon as the lights are switched on. This gives a constant reference as it is the lowest metabolic point for the day for the horse. The other acceptable time is two hours after work but variations in work days, temperature on the day, sweating patterns and stable routine will all cause variations in the red series. Best to sample first thing and then get it off to the lab as quickly as possible.

With a red series, there are six standard readings which I will discuss separately.

  • Haemoglobin: This is the protein within the red blood cell which carries the oxygen. The more haemoglobin present, the more oxygen can be carried. However, when the amount of red cells gets too large then the blood flow is restricted by the density (thickness) of the blood and flow is then affected.

    Most laboratories measure haemoglobin the same way and the range for optimum oxygen capacity is 14.5 - 15.5 gm/100 ml of blood. Levels below 12.5 and above 16.0 will affect performance. The lower reading indicates an anaemic state and the higher reading can indicate dehydration. Excitement at collection time can also cause high readings. These can be differentiated from dehydration by the examination of the blood proteins.

    The only thing that causes haemoglobin to rise is work. All that additives do is provide the raw materials to make haemoglobin, no work means no rise in haemoglobin. Horses in the paddock do perfectly well on 10.5 - 12.5 and when they return to the stables, these are normal readings. The aim of training is to stress the system so the bone marrow makes more haemoglobin for the workload, thus, a gradual increasing workload is best. When we get up to fast work, a strong, hard workout for that stage of the preparation will destroy up to a gram of haemoglobin/100 ml of blood, that is 13 drops to12. So when we are fast working, we need time to rebuild after a fast day and putting too many fast days close together, can drop the haemoglobin. Haemoglobin levels do not indicate fitness but they can indicate how well the horse has built-up and whether the build-up prep was uneventful.

  • Packed Cell Volume: This is mostly the percentage of cells within the blood. The red cells have the larger percentage as there are usually 6-8 million/ml as against white cells of 6-8 thousand/ml. PCV is used to assess dehydration and anaemia. Optimum levels are close to 40% with levels below 35% and above 45% likely to indicate problems. This reading is also affected by excitement at collection. PCV also rises as fitness increases, often though, it keeps rising when problems occur because dehydration is a common sequel of work-related problems.

  • Mean Corpuscular Haemoglobin Concentration: MCHC is a measure of the amount of haemoglobin in each red blood cell. Usually this is in a tight range of 35- 39%. It cannot be above 40% so any reading with a figure greater than 40% is wrong. Variations in MCHC usually reflect problems in the other readings as most labs calculate MCHC by dividing the haemoglobin by PCV and multiplying by 100.

  • Mean Corpuscular Volume: MCV is the size of the red blood cell and is important in two fields. One relates to anaemia cause diagnosis. If the figure is high the horse has anaemia, the cause is usually blood loss (ulcers, bleeder), Vitamin B6, B12, Folic Acid or niacin deficiencies or gut upsets causing reduced production of these vitamins. If the figure is low with anaemia, this usually indicates iron deficiency but also copper or pyridoxine.

    The other relates to the efficiency of the bone marrow. A fresh horse has a high MCV and is making good quality, new (large) red blood cells. A horse that is stale is not making new cells and his MCV is lowish. This varies between labs but a common numerical assessment would be that if the lab range was 43-50, then 50 is very fresh, not fit. 48 is good activity in a horse tightening up and ready to race. 46 is a horse nearing its peak or needing tapering. 45 is a horse that is losing it and starting to train off and 43 is in the paddock. This assessment must be done looking at a total blood but it is a good indicator of freshness. Thus sprinters may be raced pre-peak if freshness and excitement is their main virtue. Meanwhile, stayers are best pushed to good activity pre-race and usually will be in taper mode when stringing staying races together.

  • Total Red Cell Count: This is rarely used except as a crosscheck on calculations of PCV, MCHC and MCV.

  • Erythrocyte Sedimentation Rate: This is the rate that red blood cells settle in a solution. That sounds like "no big deal" but it is important because red blood cells have an electric charge and as the horse gains fitness, this charge can increase while stress, disease states and pregnancy can cause a decrease in the electric charge and so cells clump together and fall faster. The density of the blood influences ESR and so each PCV reading has an ESR range. In horses without other stresses in their blood, the ESR can give an indication of wellness and I believe cardiovascular fitness. The lower the ESR, the fitter the horse.                                                                   

    Ranges are:
    PCV = 35 ESR = 43 -13
    PCV = 37 ESR = 28 - 8
    PCV = 39 ESR = 19 -3
    PCV = 40 ESR = 8 - 0

PCV = 45 ESR = 3 - 0

Horses with figures closer to the lower end of the range are fitter and healthier than horses that are at the higher end of the range or above.

Finally, one interesting factor with red blood cells is the recent USA work which indicates that horses have two types of red blood cells. The first is a rigid cell similar in consistency to a basketball. The other is more like a water-filled balloon which is able to change shape in confined areas. Horses with more of the less rigid cells will find it easier to move blood into the muscles and the lungs and so may be able to maintain greater period of stamina work than their more rigid cell competitors. On average though, horses have 40% rigid and 60% fluid cells. A horse with 5% rigid cells would have a tremendous advantage, all else being equal, and this may be an area that explains why some horses are superior to their fellows. It is very interesting and its only problem is that you need an electron microscope to tell the difference. However, possibilities in assessment of future performance ability will mean this area will continue to be researched.

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