Blood testing

is very limited from a nutritional aspect. A blood test allows a vet to learn information about a horse’s health which can only be found from collecting a sample of blood and having it analysed. 

This includes a CBC (complete blood count) and blood chemistry that analyses the chemical components in the blood. 

Blood tests are a tool for diagnosing non nutritional wellness states.

When a vet takes a vet sample to be analysed in a lab, the blood is separated into the red and white blood cells and the plasma. Minerals and electrolytes are measured in the plasma portion (or serum after the blood has clotted), not the whole blood sample. Blood levels do not tell us anything about what horses are absorbing because all blood from the intestines passes through the liver before going to general circulation. The kidney is also involved in storing (and excreting) minerals.

A study on wild Muskoxen is one example of the disconnect between serum and tissue (liver) levels “Comparison of liver and serum concentrations of copper, selenium and vitamin E showed that the concentration in one tissue was a relatively poor indicator of the concentration in the other.”

The study Serum ferritin as a measure of stored iron in horses shows how serum iron does not reflect liver iron in horses. “Several measures have been used to monitor iron adequacy. Packed cell volume, hemoglobin, serum iron, serum total iron-binding capacity and bone marrow hemo-siderin are used, but none are adequate.” Notable quote “Occasionally, horses are given large amounts of iron to improve performance. Although iron deficiency could limit erythrocyte production and other functions related to nonhematologicaltissues, it probably only occurs in blood loss.”

Another example is in the chapter on Cystic Fibrosis Nutrition in people, link is at the bottom of this page. Amanda Leonard comments “Plasma zinc concentrations are not an adequate measure of zinc deficiency, and a deficiency can be present with normal serum zinc concentrations.”

It is well established that minerals share pathways for absorption and can influence the absorption of other minerals. Blood concentrations of  minerals and electrolytes are not a reliable indicator of whole body status. Or to put it another way, a horse can have a normal blood level of a mineral/electrolyte but be severely deficient or be excessively over-supplemented. Blood levels of minerals and electrolytes are tightly controlled in a range, exceeding or going under these levels can lead to death. To prevent this, nutrients can be drawn into the blood to maintain correct concentrations. For example, sodium from the intracellular spaces between cells. When sodium is drawn into the blood, the skin can present with the ‘tent’ look when pinched as an indication of dehydration. Calcium will be drawn from the skeleton if needed. Horses eat high levels of potassium which is not an issue, the blood level will barely change. Horses will excrete the excess of potassium and other electrolytes including sodium and chloride via the kidneys in the urine, and can be as quick as within an hour.

Also worth noting that blood concentrations can change due to exercise and health, hydration status, time of day, time of feeding. A blood test is a tool for diagnosing an unwellness state, for example, an infection.

One exception is phosphorus where the blood phosphorus level can be an indicator of diet, as shown in the study called Inorganic phosphorus of horse serum, the effect of age and nutrition, published in 1934 but still cited today in research. Results from blood serum testing for inorganic phosphorus content was used as a base for a 1952 study on foals by IP Earle.

Since the question of hair testing vs blood testing vs pasture testing often comes up…….
A 2015 study compared blood testing vs hair testing which found (not surprisingly) that hair testing was superior to blood testing. Since we know that blood testing is extremely limited it’s not saying much about the validity of hair testing.

When published it did cause a stir stating “Hair mineral analysis is a suitable tool for evaluating mineral status in horse. Unfortunately the study design didn’t compare with liver nutrient status so wasn’t able to verify hair levels with body levels of nutrients. The statement that hair testing is a suitable tool is a stretch.

Selenium can be detected in hair. Harold Hintz, Ph.D., a professor of animal nutrition at Cornell University, said, “Hair analysis may be useful in identifying horses being fed high-selenium diets, those in the toxic range.” Although Hintz has been investigating the possibility of hair analysis as a means of evaluating nutritional adequacy of trace minerals on and off for over 20 years, he questions the merit of the testing for the majority of minerals. “As routine procedure, it’s not generally accurate.” Veterinarian and nutritionist Sarah Ralston shares the sentiments of Hintz. Ralston, a faculty member at Rutgers University, agrees that hair analysis may be “vaguely useful” for selenium status, but believes values from blood samples are more representative of true levels [needs to be whole blood]. In terms of other minerals, Ralston cited one study in which horses were fed escalating amounts of calcium. The calcium content of hair samples, however, did not correspond with increasing blood levels, once again suggesting the questionable merit of the procedure.
More on hair testing research.

Further reading

Links may change over time. If a link doesn’t work, search the title in your search engine.

Blakely BR, Kutz SJ, Tedesco SC and Flood PF (2000) Trace mineral and vitamin concentrations in the liver and serum of wild Muskoxen from Victoria Island J. Wildl/. Dis.Apr;36(2):301-307
https://www.ncbi.nlm.nih.gov/pubmed/10813612

Earle IP (1952) Inorganic phosphorus content and phosphatase activity of the blood serum of foals J. Anim. Sci. Feb;11(1):191-195
https://academic.oup.com/jas/article-abstract/11/1/191/4761266

Ghorbani A, Mohit A and Kuhi HD (2015) Effects of dietary mineral intake on hair and serum mineral contents of horses J. Equine Vet. Sci. 35(4):295-300
https://www.sciencedirect.com/science/article/abs/pii/S0737080615000441

Leonard A (2015) Cystic Fibrosis Nutrition
https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/zinc-deficiency

Pearson PB (1934) Inorganic phosphorus of horse serum, the effect of age and nutrition J.Bio. Chem. 106(1)
http://www.jbc.org/content/106/1/1.full.pdf

Ralston SL (2004) Diagnosis of Nutritional Problems in Horses
https://esc.rutgers.edu/fact_sheet/diagnosis-of-nutritional-problems-in-horses/

Smith JE, Moore K, Cipriano JE and Morris PG (1984) Serum ferritin as a measure of stored iron in horses J. Nutr. 114:677-681
https://www.researchgate.net/profile/Kateri_Moore/publication/16480512_Serum_Ferritin_as_a_Measure_of_Stored_Iron_in_Horses/links/004635342c4460fe31000000.pdf

Thumbnail from https://www.vet.upenn.edu/about/press-room/publications/new-bolton-post/winter-2015/blood-transfusion-story