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Dr McGregor's Equine Blood Analysis Series

This is the third in the series on Blood Interpretation in the racehorse and covers the areas that give insight into various tissue functions. This is important in localising problems and monitoring responses to changes in training or treatments.

We will look at the following enzymes: AST, Alkaline Phosphotase, CPK, GGT, and the chemicals Bilirubin, Urea and Creatinine. These readings give us a good look at liver and muscle function and a lesser look at bowel, lung, kidneys, nervous system and skin.

Other readings including proteins, electrolytes, and special tests will give us good cross-matches for some of these tissues and will, as has been previously stressed, mean you must look at the TOTAL readings before putting undue reliance on any one result.

Alkaline Phosphotase

This enzyme appears in large amounts in liver, spleen, bone, gut-lining and kidneys. In Blood Interpretations, it's main use is as an indicator of bone activity, particularly in young horses as an indicator of growth spurts.

In older horses, it is used to indicate excessive bone turnover often associated with arthritis, pedal bruising, or spinal bone changes. In older horses normal levels are 70-100 while in two-year-olds the normal level is 130-170, depending on size. Three-year-olds read out between 120-150 and, again, size and sex need to be taken into account.

Alk. Phos. will also rise in tissue damage to the spleen, gut, lungs, liver and particularly the biliary system (gall bladder and bile duct from from liver to gut), so levels need to be compared to other enzymes before full bone activity can be assessed.

Horses with gut problems often have elevated Alk. Phos. with low chloride and low globulin readings, while horses with chronic airway and throat diseases often have elevated Alk. Phos. with raised Monocytes. Both these conditions will have stressed White Cells.

Thus, Alk. Phos. is a reading to be used with others and/or to assess bone functions.

Relating to bone function, Alk. Phos. readings over 200 in two-year-olds are often associated with shin fractures, severe pedal bruising and excessive shin soreness, and frequently indicate a warning to "back off".

In pacers, raised Alk. Phos. readings can indicate sessamoiditis and hock problems.

I find Alk. Phos. to be an excellent measure to follow treatment of a leg bone injury or change in training. It is one enzyme that seems to be universally measured in the same way and there is very little, if any, laboratory variation.

AST - Aspartate Transaminase

This enzyme is found in several tissues but is most significant in liver and muscle in the horse.

It has a long half-life which means the number you get reflects muscle changes over several days and sometimes weeks. AST levels rise in early preparation work and when young horses start working and generally level out at about twelve to fourteen weeks.

Normal levels are between 300-360iu but are usually up after racing. Levels up to 500 normally indicate increased muscle turnover from building up or minor muscle damage from lameness ... examples of sore gluteal muscles with a sore knee or fetlock are likely to cause some moderate rises.

Levels up to 50 000 are not uncommon in full-blown "tying-up" episodes, often evident in two-year-old colts or three-year-old fillies.

Remember though, if a "tying-up" episode is acute, the AST from that episode may be normal when tested close to the event and rise over several days often when the horse is returning to normal. So, in effect AST is an overall long-term assessment of muscle functions.

An interesting area I have found over the past twenty years has been in the LOW readings of AST. These are readings below 300 and are associated with either underwork or under effort on the part of the horse, relative to their current stage of fitness.

This can sometimes explain a disappointing performance and indicate a change in work pattern to improve fitness.

CPK - Creatine Phosphokinase

This is a very accurate measure of current muscle activity and so is a good indicator, for example, of what happened in the morning workout.

Levels are usually around 80-120 and can double without a problem after work. Higher levels, for example, 500-700 in non- or light-working horses can indicate cardiac muscle damage. CPK is mostly used when checking on "tying-up" episodes, where levels up to 100 000 are not uncommon and, as CPK rises and falls very quickly, it can be used to closely monitor such epidsodes and assess progress. There are about 50 causes of "typing-up" and both the above enzymes can prove valuable in assessing and treating various ones.

CPK levels rise and fall through larger ranges earlier in a preparation and tighten into a narrower range when the horse is attaining good fitness levels for the racing required.

GGT (Gamma Glutamyl Transferase)

This is a very accurate enzyme with which to measure liver tissue, and has a normal reading between 20-60iu.

The enzyme is sensitive to mild liver damage and levels increase from 60-150 often after intake of drugs or some nutrient changes. A bleeding reduction supplement and summer weeds will both push GGT over 100 and if acute liver disease is present, GGT readings will rise from 1000 to 10 000 but these causes (which include biliary obstruction, cancer, liver fluke, bacterial/viral hepatitis) are those reflected in severe liver disease and are reasonably rare in racing-age horses.

Again, as with AST, a GGT reading below 20 will indicate an underactive liver. There seems to be a condition I call Flat Liver Syndrome where the liver activity is below the level required to allow maximum improvement in fitness. This is commonly seen in horses after long spells or after grazing for periods on poor-quality, high-weed pastures. It appears the liver does not "tone up" to the workload of training in the way the muscles, bones and respiratory system do. Horses with this problem respond well to high levels of B-Group Vitamins.


This pigment which is mostly from old red blood cells and nitrogen metabolism is a by-product eliminated through the biliary system.

In itself, it does not do any damage but the elimination route is the same as many of the toxic body products, so when the Bilirubin is high so too are a large number of other toxins.

A normal Bilirubin in adult animals is below 25 units and is higher in the very young. It rises and falls with the severity of exercise and the speed with which it falls is a very good indicator of the recovery rate of a horse from work or racing.

Often following a hard race, a horse will have a Bilirubin of 80 but if this falls quickly to say 50 and 30 within the next two days, the horse is recovering well and usually ready for fast work.

If the rate of fall is 80-70-60-50 the horse is unlikely to eat well until the fourth day and will still require two more days to be ready for fast work again.

Horses that try really hard often have very high Bilirubin readings and are affected for some time. A top example was the great competitor Heron Bridge (wins included 1984 Newmarket in 1:08) who was often 100 after a race and took from ten to fourteen days to fully recover before he could be worked hard again. This meant racing "underdone" which produced more toxins on raceday and therefore created an ongoing circle.

As a rule of thumb, a Bilirubin over 50 will stop most fillies and mares from eating well and over 60 will slow or stop colts and geldings.

High Bilirubin levels can occur from two areas:

  • ONE - the destruction of red cells and other tissues from hard work usually greater than the horse is conditioned for.

  • TWO - reduced drainage and elimination of toxins through the gallbladder/bile duct from liver to gut.

The other obvious problem could, of course, be a combination of both.


This product is a breakdown of protein and nitrogen metabolism usually raised in horses with urinary problems.

This is mostly restricted to colts and stallions with crystals and/or stones. Normal levels are 3.5 - 5.0


This chemical is really only to measure major kidney damage and the reading is unlikely to be high unless the horse is suffering from kidney failure at which point he is near death.

Any reading above 200 indicates a very sick horse.

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