Why Blood Work Can't Tell You What to Feed Your Pet

Written by:

Ken Tudor, DVM
Published: November 07, 2013

Why Blood Work Can’t Evaluate Your Pet's Nutritional Status: Case Study

An English Bulldog was admitted to the teaching hospital at the Ohio State University Veterinary Medical Center for coughing and respiratory difficulty. Initial tests indicated that the dog was in congestive heart failure. An echocardiograph (a chest ultrasound) confirmed that the dog had an enlarged heart (dilated cardiomyopathy) sometimes associated with taurine (an amino acid) deficiency.

Further questioning of the owners revealed that they were feeding a homemade lentil, rice, and potato diet. Suspicious of the diet, the doctors performed a special test for blood taurine levels. The levels for this dog were 2nmol/ml. Normal levels are between 60-120nmol/ml. The dog had a complete recovery with taurine supplementation and switch to a balanced diet.

The preliminary blood screen, the same routine blood work your vet performs on your pets, was normal. Veterinary evaluation and blood work prior to the clinical symptoms would have suggested that this pet was healthy and its diet adequate. This case demonstrates that routine blood work will not reveal the nutritional adequacy of a diet.

Why Is This Important?

An ever growing number of pet owners are feeding homemade and raw diets. A recent study indicated that 95% of homemade recipes are nutritionally inadequate. Owners are relying on blood tests performed by their veterinarians to evaluate their dogs' diets.

Unfortunately, as the case above points out, the routine blood screening that veterinarians use to evaluate their patients says little about the diet. With the exception of very distinct changes in red blood cell size with iron or vitamin B-12 deficiency, your veterinarian cannot evaluate your pet’s diet based on routine blood work.

What Does Your Pet's Routine Blood Work Evaluate?

Complete Blood Count (CBC): Routine blood work measures the number, size and hemoglobin content (molecule responsible for transporting oxygen and carbon dioxide) of red blood cells. The number and type of infection fighting white blood cells are identified. The platelet (cells important for blood clotting) numbers are also indicated.

Serum Biochemistry: Chemistries evaluate liver function, kidney function, and pancreas function by measuring the levels of specific enzymes or chemicals. Cholesterol, triglycerides, total and specific proteins, and glucose levels are also measured. Calcium, magnesium, potassium, sodium, and chloride are the only minerals measured. Many labs will also include an enzyme that evaluates muscle damage and thyroid hormone levels.

 Why Is Blood Work for Pets Inadequate?

The mammalian body has a tremendous capacity to hormonally, chemically, and mechanically adjust to nutrient inadequacies. Having evolved with a constant inadequate diet, these adaptions were vital for species survival.

Let’s work through some nutritional deficiencies.

Calcium, Phosphorus, or Magnesium Deficiency: If blood levels start to decrease, hormones are released that act to free these minerals from the bone. Until bone mass is near depletion, blood levels of these minerals will remain normal. These deficiencies will be missed unless your vet evaluates your pet’s bone density.

Chloride, Potassium, and Sodium Deficiency: In light of falling blood levels, hormonal changes will signal the kidneys to retain these minerals rather than eliminate them in the urine. This mechanism maintains necessary blood levels of these critical minerals despite possible dietary insufficiency.  

Protein Deficiency: As long as there is muscle tissue, it can be utilized to maintain protein blood levels. The protein that is measured in routine blood work does not evaluate individual amino acids that may be missing from the diet (like our Bulldog friend above). Until symptoms of specific amino acid deficiencies or muscle loss are evident, your vet will be unable to verify the protein adequacy in your pet's diet.

Vitamin and Mineral Deficiency: Vitamins and minerals are necessary to carry out various chemical reactions of the body. Routine blood testing does not measure vitamin or mineral levels other than those mentioned above. Deficiencies will not be apparent until the decrease or absence of chemical reactions causes clinical symptoms. Special blood tests, not routine screens, are needed to verify the adequacy of vitamins and minerals.

The Bottom Line on Blood Testing for Nutritional Deficiencies

It is generally near end-stage dysfunction (again, like our Bulldog above) that nutritional deficiencies are evident. Only diet analysis can determine the nutritional status of a diet. Simply feeding a variety of meats, carbohydrates, oils, fruits, and vegetables, adding a vitamin/mineral/calcium supplement, and then relying on routine veterinary blood testing will not guarantee that your pet’s diet and health are adequate.

Dr. Ken Tudor

Image: Tyler Olson / Shutterstock