What's in a Test?

Looking at your horse's blood, your veterinarian can tell if he's anemic or fighting infection or an immune-mediated disease.

Photo: Erica Larson, News Editor

Let’s take a test. 

Got your No. 2 pencil? 

Here we go. 

(Hint: The answer is always C.)

1) Your horse isn’t doing well. After completing a physical examination, your veterinarian says that she would “like to run some tests.” Is she looking for:

A. A report stating that your horse has disease X and needs treatment Y
B. The answers to the mysteries of the universe
C. Clues as to which organ systems might be affected and possible causes of the clinical signs
D. Whether your horse should be allowed to drive

As nice as it would be for the answer to be A, in good multiple choice tradition, C is correct. The words “tests,” “blood work,” “samples,” and even “urinalysis” offer little detail as to outcomes. What do some of the more common diagnostic tests actually tell us? What do positive or negative results mean? What is “normal”? What’s the difference between an antigen-detecting test and an antibody titer? What’s up with those false positive or false negative results that sometimes appear? And, the question most of us want answered: Why can’t my vet just run one test that tells her exactly what she needs to know?

Laboratory Test Interpretation 101

Kathleen Freeman, DVM, PhD, Dipl. ECVP, FRCPath, MRCVS, a clinical pathologist in Scotland, sympathizes with this feeling but points out that “there are very few lab tests that provide a perfect answer every time. Lab work still needs to be interpreted in context with the animal’s clinical condition and using multiple results.”

Diagnostic testing is less like a true or false exam and more like a combination of fill-in-the-blank and multiple-choice questions. The answer might be A) and B), none of the above, all of the above, or sometimes B) but only if C) is also present.

Much of a test’s value depends on the reasons for running it, and no diagnostic test is 100% accurate. Pathologists refer to the terms specificity and sensitivity to identify how likely a positive or negative result is to be accurate. In a perfect world, every test would be 100% sensitive and 100% specific. This means, respectively, that the test would pick up every case of the disease and that every animal it deemed positive would in fact have the disease. However, most tests are either more specific (meaning a negative is truly negative, but some positive cases might get missed) or sensitive (meaning they are more likely to catch all positive cases, but they might also pick up some unaffected horses). Freeman likens this concept to a smoke detector: “You want your smoke alarm to go off if there is a fire, but you want it to be specific for fire; you don’t want it to go off when someone lights a match.”

The likelihood of a test returning a positive result for a specific horse also impacts how likely a positive result will confirm disease. This is known as a test’s positive predictive value. Freeman explains that if a disease is extremely common in a given area—such as pigeon fever in California—a positive test result is more likely to be accurate than a positive result in an area where the disease is rare or unreported. “You have to consider the prevalence of the disease in interpreting the result,” she says.

Often, unless a horse shows signs that point toward a specific disease—giant abscess in the throatlatch area screaming “Strangles!” for instance—a veterinarian will suggest a profile or comprehensive evaluation of multiple body systems such as a complete blood count (CBC)/biochemical panel and/or urinalysis. These tests indicate which general body areas might be affected or steer the practitioner toward likely causes, but they do not give a yes/no answer. Tests tend to return results relative to a “reference interval,” or what scientists commonly call a “normal” range. But what is normal? And what does a single test result outside that interval mean?

“I tell people that a reference interval is a general guideline,” Freeman says. “It’s based on a 95% probability, so you’d expect about 5% of animals to have a normal outside the interval.”

This means when a veterinarian runs multiple tests, he or she commonly sees results outside the reference interval in healthy horses or that are not significant to the horses’ current condition. Usually such results are only a little above or below the interval, but sometimes they can be moderately different.

“Blood Work” Basics

Blood work is not a day job for vampires. Although the term could technically apply to just about any diagnostic test run on a sample of whole blood, plasma, or serum, veterinarians usually use it to refer to the combined CBC/chemistry profile.

Complete Blood Count and Chemistry Profile Explained

We've taken a visual route of describing a typical blood test and what its results might be for your horse. View the full infograpic at TheHorse.com/BloodTest.

Blood contains red blood cells (RBCs), leukocytes or white blood cells (WBCs), platelets, serum (the clear yellowish liquid part), and fibrinogen. A veterinarian uses a CBC to look at all the cell types (RBCs, WBCs, and platelets) and their size, shape, and composition. Veterinarians might also add a fibrinogen (an indicator of inflammation) measurement to this test.

2) A CBC can tell you if it’s possible that:

A. The horse is anemic
B. The horse is losing blood
C. A), B), and D)
D. The horse has an immune-mediated disease or an infection

Yes, the answer is C. However, please note the phrasing in the question: “if it is possible.” A CBC is a snapshot. It shows what is happening in the horse’s bloodstream at the moment the sample is drawn. It doesn’t show what is going on in the bone marrow or the spleen. It doesn’t necessarily tell you if an infection is developing or in the recovery phase, or if a true infection is present at all.

This is why Freeman says it is important to run follow-up blood work to look for trends. Is the RBC count rising or is an existing anemia getting worse? Do neutrophil (a type of white blood cell capable of engulfing and destroying bacteria and other disease agents, immune complexes, and cell debris) numbers continue to increase or are they returning to normal?

And what happens with the serum?

Serum chemistry panels are not one test; rather, they’re a collection of tests run on one sample. While the panel offered might differ slightly among laboratories, clinicians generally analyze the following:

  • Electrolytes Changes in sodium, potassium, chlorine, bicarbonate, and calcium can indicate excessive electrolyte storage or loss and offer clues as to metabolic abnormalities as well as how to treat the horse with fluid or electrolyte replacement therapy.
  • Glucose This sugar is one of the body’s main energy sources. Glucose levels can become elevated under physiologic stress such as infection or insulin resistance. Low blood glucose (hypoglycemia) can indicate nutritional or metabolic loss and can be a particularly critical condition for young foals.
  • Proteins The two primary serum proteins are albumin and globulin. Levels of albumin, which is produced in the liver, might be low due to kidney failure or liver damage. Both albumin and globulin levels might drop in cases of severe diarrhea, parasitism, or blood loss. Globulin, which is associated with antibody production, can become elevated in infectious disease cases.
  • Bilirubin Veterinarians most commonly check levels of this breakdown product of heme, a component of red blood cells, to assess for liver damage. However, both red blood cell destruction (hemolysis) and diminished eating can cause bilirubin levels to rise.
  • Enzymes When cells are damaged and cell membranes rupture, enzymes are released into the blood. While enzymes such as alkaline phosphatase might be released by more than one tissue (e.g., skeletal muscle, heart muscle, liver, and bone), others are more specific to particular tissues. Muscles release creatinine kinase (CK), for instance, as a result of acute damage. These CK levels might become very high within minutes to hours following an insult to the muscle. 
  • BUN Blood urea nitrogen (blood ammonia, also sometimes called urea) is a breakdown product of protein metabolism by the liver that the kidneys filter from the body. An elevated BUN might indicate kidney disease, while a low BUN could indicate liver damage.
  • Creatinine Kidneys excrete this muscle breakdown product. Veterinarians can use creatinine levels, particularly when paired with BUN, to assess kidney function. However, creatinine levels also appear elevated in cases of dehydration.

Here, Trigger, Aim for this Cup

3) Urinalysis results might indicate:

A. Kidney function and hydration status
B. Infection, blood loss, and urinary tract stones
C. All of the above

A urine sample contains more clues than you might think. Color, clarity, and even smell can reveal a lot. For instance, if a horse that has “tied up” recently expels dark brown or reddish urine, he might have significant muscle damage and could be excreting kidney-damaging muscle proteins. Other clues from urinalysis include:

  • Urine specific gravity (USG) This measurement indicates the urine’s concentration. A normal horse should be able to produce more concentrated urine when less hydrated and produce a dilute urine when healthy or overhydrated.
  • pH levels As herbivores, most horses have alkaline (pH greater than 7) urine. A lower (more acidic) urine pH can indicate metabolic disturbances, muscle damage, or infection.
  • Protein Protein loss in the urine might indicate kidney damage or loss of muscle protein (myoglobin) or blood protein (hemoglobin).
  • Urine sediment Clinicians can centrifuge urine samples and examine the sediment at the bottom, usually comprised of cells and crystals. High white blood cell counts or the presence of bacteria can indicate a bacterial infection. Normally, veterinarians expect to find large numbers of crystals in horses’ urine, as these help regulate their calcium levels. But if a horse shows signs of colic or urinary blockage and has elevated red blood cell counts in his urine, your veterinarian should check for a urinary tract stone. 

Lab work still needs to be interpreted in context with the animal's clinical condition and using multiple results.

Dr. Kathleen Freeman

The Whole Barn is Sick. Now What?

Sometimes either the CBC/fibrinogen or the clinical signs themselves clearly indicate an infectious pathogen (virus, bacterium, or parasite) has invaded the horse. Diagnostic testing options for infectious diseases are a little less broad than blood chemistry, CBC, or urinalysis, but black-and-white answers are still rare.

4) Veterinarians can detect infectious diseases using:

A. Bacterial or fungal culture
B. Antibody detection tests
C. A, B, and D
D. Antigen detecting tests

Nicola Pusterla, PhD, Dipl. ACVIM, a professor in the department of medicine and epidemiology in the University of California, Davis, School of Veterinary Medicine, says that while veterinarians still use conventional diagnostic tests such as microbiological culture (spreading a biologic sample on a nutrient medium and checking for bacterial growth), quicker biochemical assays involving techniques such as mass spectrometry and genetic analysis known as polymerase chain reaction (PCR) testing allow for much faster organism identification than in the past.

Besides timing limitations, drawbacks to direct analyses such as culturing include bacteria that are tricky to grow, or other factors (e.g., administration of antibiotics) that might compromise proliferation.

Antigen detection tests such as polymerase chain reaction find even small numbers of an organism. Pusterla says these tests are more sensitive than culture. “The beauty of these tests is the ability to screen for a variety of pathogens,” he adds. An example of such screening would be the “respiratory panels” performed on nasal swabs to check for a variety of respiratory pathogens such as equine influenza virus and equine herpesviruses-1 and -4 and Streptococcus equi subspecies equi (the agent that causes strangles).

However, in some cases the horse might have already entered the recovery phase of the disease and is no longer shedding antigen, or the pathogen might infect an area of the body that is challenging to sample. An example Pusterla cites is equine protozoal myeloencephalitis (EPM): The organisms that cause EPM infect the brain or spinal cord and are, thus, impossible to isolate in the live horse because a sample of the central nervous system would likely kill the horse. In this case, says Pusterla, indirect testing is the only way. “Indirect assessment of exposure relates to antibody detection,” he explains. 

The horse’s immune system creates antibodies in response to exposure to or infection by a pathogen. However, antibody levels persist in blood even after an infection has cleared. Therefore, sometimes it can be tricky with a positive/negative test to know whether a horse is still sick or has simply been exposed to the disease agent. For this reason, Pusterla says many tests are moving toward a more quantitative evaluation that produces a number that veterinarians can interpret along with clinical signs to indicate disease probability as well as disease course: Is the horse getting worse, reaching a plateau, or recovering?

Take-Home Message

Diagnostic testing is rarely a one-size-fits-all proposition. Both Freeman and Pusterla stress the importance of evaluating laboratory results in the context of the animal’s clinical signs and the likelihood of a given disease appearing in that animal.

Veterinarians frequently conduct follow-up tests to get a complete picture of the horse’s condition and recovery progress. Repeat testing or, in the case of an infectious disease outbreak, testing multiple horses, gives veterinarians a clearer picture of the disease situation and can help them plan treatment protocols and duration

About the Author

Christy Corp-Minamiji, DVM

Christy Corp-Minamiji, DVM, practices large animal medicine in Northern California, with particular interests in equine wound management and geriatric equine care. She and her husband have three children, and she writes fiction and creative nonfiction in her spare time.

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