Blood chemistry results can be extremely helpful in zeroing in on medical problems, but they often aren't clear cut. One of the most common findings is elevation in enzymes that could have come from the liver, but this doesn't always mean the horse has liver disease.
To help you understand the results from the blood tests your veterinarian ordered, we've included a list of liver-related tests commonly found on routine blood chemistry screens and situations where they might be abnormal.
GAMMA GLUTAMYL TRANSFERASE (GGT)
This enzyme is the most specific for liver and also the most sensitive. However, GGT may show mild-to-moderate elevations in horses receiving one or more drugs that are processed by the liver without indicating any liver damage per se.
Plants Toxic to the Liver
Senecio jacobaea (tansy ragwort) is found in many U.S. Regions.
A variety of commonly found plants can be toxic to the liver, and liver disease in grazing horses occurs with some regularity. However, many of these plants retain their toxicity when dried and baled into hays. This is one important reason why it's wise never to feed hays containing unidentified plants/weeds. Some examples are:
Senecio jacobaea (tansy ragwort) - Pacific, Pacific northwest, Northeast, Great Lakes region
Crotalaria (Rattlebox) - East of the Rockies, New Mexico, Arizona, Utah Trifolium hybridum (Alsike clover) - Nationwide except Texas. Panicum coloratum (Kleingrass) - Texas, New Mexico Lantana - Southern states from coast to coast, plus Utah
Amsinckia (Fiddleneck) - Several varieties nationwide, particularly in the Western half of the country
Heliotropium (various types) - Nationwide except Wisconsin, Michigan and Minnesota
This list is not meant to be comprehensive. Plant-related liver toxicity is possible anywhere. Information on distribution is from the USDA Plants Database (http://plants.usda.gov/index.html).
GGT is also commonly mildly to moderately elevated in horses in hard work, especially early in the conditioning period and/or if the horse does not have adequate antioxidant reserves. Low-level toxin exposure (e.g., aflatoxins in grains, a mold toxin) may also rev up GGT activity leading to higher blood levels but without actual damage to the liver.
Horses with poorly controlled Cushing's disease or insulin resistance may also show mild elevations, probably secondary to oxidative stress and/or impaired energy generation in the liver cells.
To distinguish between those scenarios and liver damage/disease, you need to consider both how high the GGT is, and if other indicators of liver disease are also present. Upper limits of normal for most labs are usually in the range of 40 to 60 IU. Elevations of up to 300 to 400 IU can be seen without changes in any other liver function tests with exercise, drugs, or toxin exposures and don't necessarily indicate liver damage, although they should prompt a search for the cause. Higher elevations accompanied by abnormal liver-related tests may indicate liver damage.
ALKALINE PHOSPHATASE (ALP)
Often called "alk phos," this is another enzyme in high concentration in liver, but also in spleen, intestinal tract, bone and lungs. If the elevation is from liver disease, GGT will also be elevated to a similar or greater extent.
ALP is normally elevated in young, growing horses and young horses at some stages of training because it is involved in the formation and remodeling of bone.
Horses on chronic corticosteroids (such as for allergies or lung disease), and horses with high production of cortisol caused by uncontrolled Cushing's syndrome may also show elevations in ALP since corticosteroids can cause bone resorption. Fractures and inflammatory conditions of bone (e.g., sesamoiditis) may cause mild elevations. A typical upper lab limit for this enzyme is 250 IU.
ASPARTATE TRANSAMINASE (AST)
This is another enzyme widely distributed throughout the body, but in highest levels in liver and muscle. To distinguish between the two, it's necessary to look for elevations of other enzymes pointing to either muscle (CPK) or liver (GGT and other liver function tests). If only AST and GGT are elevated, this doesn't distinguish between exercise or liver disease as a cause. Repeat testing, including CPK, should be done close to a bout of exercise to look for abnormal muscle enzymes. Upper limit is around 400 IU.