[From Tufts August 2012 Issue]
Like detectives piecing together clues, veterinarians conduct blood tests when they suspect a health problem. Equally important, the tests can uncover diseases even when signs aren’t evident. A complete blood count and basic blood chemistry panel are especially important for middle-age and older dogs — beginning at about 5 to 8 years old.
“The older a dog gets, the more likely it is to have disease processes,” says says Linda Ross, DVM, a specialist in small animal medicine at Cummings School of Veterinary Medicine at Tufts University. “The CBC is valuable to check for anemia, the presence of infection or certain problems with blood clotting. Blood chemistry tests look for problems in various organs.”
Those tests, also called serum chemistry tests, measure important constituents of the serum, the liquid portion of the blood that doesn’t contain cells. They provide different information than a CBC, which examines the size, type and number of blood cells.
“Some diseases may cause few, if any, clinical signs early,” Dr. Ross says. After reviewing the results of basic blood tests, “more specific tests for certain conditions could be recommended, but the routine tests can tell if the dog is anemic, has diabetes, kidney disease or liver disease.”
Tests for Geriatric Pets
Preventive health guidelines from the American Veterinary Medical Association and American Animal Hospital Association recommend annual blood tests for dogs from middle- to senior age and every six months for geriatric pets to establish a baseline for future monitoring.
Baseline testing of a healthy pet is valuable when the dog becomes ill suddenly, Dr. Ross says. For example, if blood work is normal, but six weeks later, new tests show kidney dysfunction, treatment may be different than for chronic kidney disease. “From a medical standpoint, when kidney disease comes on acutely, that makes us think of different causes,” she says.
Chronic, low-grade liver or kidney disease can go unnoticed for years, but when an early diagnosis is established with blood and urine tests, “there are certainly things we can do to help the animal feel better and prolong life,” Dr. Ross says.
Veterinarians routinely order blood tests before surgery for dogs whose breed, weight or history put them at risk for diabetes or other diseases. They also advise annual blood tests for heartworm and other common parasitic diseases, especially if the dogs live in endemic regions. A common, four-panel blood test (the SNAP 4Dx from IDEXX Laboratories) enables veterinarians to test in-house for mosquito-borne heartworm disease and three tick-borne diseases: Lyme, ehrlichiosis and anaplasmosis.
Minimum testing for ill dogs or those undergoing routine geriatric and wellness exams also includes urinalysis, Dr. Ross says. “That gives us a lot of basic information about what’s going on in a dog’s system.”
Complete blood counts can provide information about:
White blood cells, or leukocytes
An elevation in the white blood cell count may suggest infection or inflammation because white cells mobilize against foreign organisms and material. Changes in the types of cells can sometimes indicate bone marrow cancer or leukemia.
In addition to the total number of white blood cells, a CBC also checks the number of five specific types. Veterinarians consider the total number of cells of each type and analyze subtotals as a percentage of the total white blood count called the differential. Changes in the differential can give clues about the type of problem, its severity and causes. An example is Addison’s disease, in which the reduced production of adrenal gland hormones sometimes causes changes in the proportion of specific types of white cells.
Red blood cells, or erythrocytes
Red cells far outnumber white cells, giving blood its color. A CBC analyzes the number of red blood cells and the amount of hemoglobin they’re carrying. Hemoglobin, an iron-containing protein that binds to oxygen, enables the bloodstream to carry oxygen from the lungs to the tissues. The hematocrit, or packed cell volume, measures the volume percentage of red cells.
“Anemia is an abnormal (low) red blood cell count. It can be caused by many diseases, including chronic kidney disease or intestinal bleeding,” Dr. Ross says.
The size, shape and appearance of red blood cells also aid in diagnosis. For example, tiny blood cells can signify blood vessel abnormalities in the liver. Large red cells in an anemic dog mean bone marrow is responding by producing more cells in response to bleeding or red cell destruction. The most common cause of red destruction is immune-mediated hemolytic anemia.
The number and size of blood platelets provide an assessment of clotting ability and bone marrow function. “If platelets are low, blood is liable not to clot,” Dr. Ross says. “That could be because of infections like Ehrlichia or immune-mediated disease where the body itself is destroying the platelets or disorders affecting platelet production in the bone marrow.”
Low numbers also could mean a bone marrow problem or other disorder. High numbers or sizes could signify bone marrow disease or cancer.
A basic serum chemistry panel measures the functioning of major organs and systemss. Some of the key measurements provided are:
Glucose, or blood sugar levels
High glucose (hyperglycemia) may indicate diabetes mellitus. Low blood sugar (hypoglycemia) can be a complication of insulin therapy or poor nutrition in puppies. Low glucose also can be seen with a systemic bacterial infection, a pancreatic tumor, Addison’s disease or liver disease.
Electrolytes, which are dissolved minerals such as sodium, potassium, phosphorus, chloride and calcium, play an important role in heart and muscle contraction, nerve conduction and hydration. Abnormal electrolyte levels can sometimes be life threatening. High phosphorus levels can indicate malfunctioning kidneys, while a low ratio of sodium to potassium can suggest Addison’s disease. Excessive potassium can produce cardiac arrhythmias. Inadequate potassium causes weakness, and low sodium and chloride may result in dehydration and shock. High calcium levels can be seen with certain cancers, disorders of the parathyroid glands or ingestion of products containing vitamin D.
BUN and creatinine levels
Blood urea nitrogen and creatinine, two common byproducts of protein metabolism, are filtered by the kidneys and normally found in small quantities in the blood. High BUN and creatinine levels suggest kidney problems. However, these levels do not rise until at least three-fourths of kidney function is lost. BUN readings can also rise due to dehydration and can fall because of severe liver disease.
Liver enzymes are vital in liver function. Three liver enzymes — alanine aminotransferase, alkaline phosphatase and gamma glutamyltransferase — are commonly measured. Elevated liver enzymes occur with diseases of the liver and/or gall bladder.
Proteins in the blood
Typical chemistry panels measure the total serum protein in the blood and the amount of albumin and globulin — two proteins associated with liver and immune functions. Sufficient protein levels are essential for healthy organs, bones, muscles, tissues and hair.
Low serum protein indicates blood loss or an illness preventing production of protein or absorption of nutrients, such as liver or pancreatic disease. Low protein can also mean its loss from intestinal or kidney disorders.
“Low globulin in conjunction with a low albumin level gives us a hint as to why this is occurring,” Dr. Ross says. “One cause is a protein-losing enteropathy, an intestinal disease which causes loss of protein into the feces.”
An elevated total serum protein can indicate dehydration, shock or infection. “In some diseases, the globulins become elevated due to chronic immune stimulation,” Dr. Ross says, for example, due to an infection or multiple myeloma, a cancer of certain blood cells.
In the end, while expensive, high-tech diagnostics like magnetic resonance imaging and computed tomography can sometimes provide very valuable diagnostic information, “You can never substitute an MRI for a CBC or the other way around,” Dr. Ross says. Routine blood testing is expensive and remains the first step in diagnosis, proving that a dog is healthy or raising a red flag that a problem exists.
Fran Pennock Shaw is a writer in Lancaster, Pennsylvania.