Blood work: revelations, limitations for cattle

When thorough history and physical examination fail to yield a diagnosis in difficult cases, many practitioners turn to blood samples for a complete blood count and chemistry panel, hoping these tests will identify the problem.

They naturally are disappointed when that is not the case. The fact is, these tests do not always point to a diagnosis, but still they can be helpful. Normal blood work can rule out some diseases. And if there are abnormalities, they might aid in establishing a prognosis and/or developing a therapeutic plan, even if a specific diagnosis is lacking.

Interpretation usually is similar across species, but there are a few peculiarities in cattle. When interpreting findings, published normal values can be used; but, ideally, normal ranges should be established for each particular laboratory, clinic and machine.

Complete blood count

In most cases, a complete blood count (CBC) is not going to be helpful in determining a specific diagnosis, but it can be helpful in determining the severity of a problem and a prognosis.

When anemia is suspected after a physical examination, a packed-cell volume (PCV) is helpful in assessing the severity of the anemia. Although the normal range is 24 percent to 46 percent, in my experience the PCV is usually in the upper 20s in adult cattle and slightly higher in calves.

Care must be taken to interpret a PCV in light of the hydration status of the animal. An anemic animal that is dehydrated might have a normal PCV. A red blood cell (RBC) count does not offer any more information than the PCV, but RBC morphology should be evaluated to make sure cell size isn't changed enough to affect the PCV.

RBC indices (MCV, MCHC and MCH) might be helpful in discovering the type of anemia (regenerative versus non-regenerative) if physical examination and other laboratory tests are inconclusive. Reticylocytosis in cattle is indicative of regeneration. The reticulocyte count does not need to be corrected in cattle like it does in dogs and cats.

Total protein (TP) levels usually are interpreted with the PCV, and hydration status also must be considered. Anemia and hypoproteinemia suggest acute blood loss in the last few days. The clinician often will already know acute blood loss has occurred from the history and physical examination. In my experience, there is no magic value for PCV and TP when trying to decide whether a transfusion is necessary. Many times, the animal's condition will lead to a decision.

Acute vs. chronic blood loss

If blood loss is chronic, animals have time to adapt and might show only mild clinical signs with a PCV of less than 10.

There is no time for adaptation with acute blood loss from hemorrhage, and animals might show signs of severe weakness and respiratory distress with a PCV of 15. If the animal is not showing signs of distress, a transfusion might not be necessary for saving the animal's life, but it might speed recovery. It also might facilitate better milk production in dairy cows.

Signs of regeneration (reticulocytes, nucleated RBCs, increased MCV) should accompany a low PCV and TP of acute blood loss after 72 hours. With chronic blood loss anemia or hemolytic anemia, the TP is less likely to be low. Hemolytic anemia might be accompanied by icterus on physical examination, and if the hemolysis is intravascular (copper toxicity, Clostridium novyi. leptospirosis), hemoglobinuria usually is noted.

Chronic blood loss starts as a regenerative anemia. But in most cases, it is not diagnosed until the chronic loss of iron leads to a non-regenerative anemia. It's difficult to determine whether non-regenerative anemia is from chronic blood loss (true iron deficiency) or from chronic disease (pseudo iron deficiency, anemia of chronic disease). In both cases, serum iron levels might be low.

True iron deficiency anemia is more likely to have low RBC indices (microcytic and hypochromic), whereas anemia of chronic disease might have normal indices. However, there is too much overlap to rely on these values. Again, a physical examination might help.

For example, evidence of hardware disease on physical examination should make one suspect anemia of chronic disease. If the physical exam does not help differentiate these two types of anemia, and other blood work is normal, especially globulins and fibrinogen, a total iron-binding capacity (TIBC) and/or serum ferritin should be run.

If the TIBC is high or the TIBC is normal and the serum ferritin is low, the anemia is due to iron deficiency. If the TIBC is low, the anemia is from chronic disease. Determining the difference is important because, even if chronic blood loss is stopped, these animals might have trouble regenerating RBCs without parenteral iron supplementation.

Total protein easily can be evaluated with a refractometer, although a chemistry panel is required to determine albumin and globulin changes specifically. The difference in plasma protein and serum protein is mainly due to fibrinogen. Increases in total protein are due to dehydration or inflammation.

Physical examination and/or globulins and fibrinogen are needed to determine the exact cause. Hypoproteinemia most often is caused by lack of adequate protein in the diet or protein loss. Liver disease usually does not result in a low protein in ruminants. Nutrition problems and chronic parasitism should be ruled out first. These animals might be anemic also. If these are ruled out, loss from the gastrointestinal tract, urinary tract or into the peritoneal or pleural cavities should be suspected.

Loss from the gastrointestinal tract is difficult to prove without an intestinal biopsy, so a thorough physical examination is necessary to rule out pleuritis and peritonitis. A urinalysis also should be performed. Results that show proteinuria without evidence of infection point to amyloidosis. If these are ruled out, loss from the gastrointestinal tract can be assumed.

Johne's disease should be next on the differential list. Other inflammatory bowel diseases such as lymphocytic-plasmacytic or eosinophilic enteritis occur but are rare.

Evaluating fibrinogen, globulin

Although there are other acute phase proteins that increase during inflammation, fibrinogen is the one that can be readily assessed in practice. A quick way to estimate fibrinogen is to subtract the serum protein (from clotted blood) from the plasma protein (blood in anticoagulant) that's measured by a refractometer.

Multiply the total by 1,000. An increase in fibrinogen occurs approximately 24 hours after the start of an inflammatory process.

As long as the inflammation is active, the fibrinogen usually will stay elevated.

The exception is when the inflammatory process leads to severe debilitation, and the liver no longer can produce fibrinogen. Globulin levels also will increase with inflammation and remain elevated even after fibrinogen levels returns to normal.

The reason fibrinogen and globulin levels are important to evaluate is because the white blood-cell count usually does not accurately reflect inflammation in cattle. Cattle have a small, marginated pool of neutrophils, so they don't have a ready reserve to start circulating when they are needed, and neutrophilia is not always seen. Since the marginated pool is low, the bone marrow responds by putting immature neutrophils in the blood. A left shift can occur early and dramatically in cattle. A severe left shift indicates severe inflammation, but it is not the death sentence it is for small animals.

Cattle normally have many different sizes of circulating lymphocytes, so large, reactive-looking lymphocytes should not be interpreted as neoplastic cells. Also, a common mistake is to confuse persistent lymphocytosis with lymphosarcoma.

Persistent lymphocytosis (an increase in absolute lymphocyte count at least 3 standard deviations above the normal mean for three months) is an indication of BLV infection only, not the presence of tumors.

Chemistry panel

One of the most useful parts of a serum chemistry profile is the electrolyte values (sodium, potassium, chloride, total CO2). They not only help diagnose certain problems, but they can be useful in making fluid therapy decisions. Most electrolyte abnormalities in adult cattle are a high TCO2 (metabolic alkalosis), hypochloremia and hypokalemia, particularly in cattle with gastrointestinal disease.

Hypokalemia is common in anorexic animals but should be interpreted with the acid-base status. Acidosis might cause hyperkalemia, but total body potassium is rarely elevated and is usually low. Potassium can be supplemented in the face of hyperkalemia if the acidosis is corrected at the same time. Make sure that serum has not been sitting on the clot too long and that hemolysis does not occur; both of these conditions will falsely increase potassium.

Hyponatremia and hypochloremia can occur with diarrhea.

If diarrhea is absent, renal disease and a urinary obstruction or ruptured bladder should be suspected. Hyperkalemia is not a consistent finding in ruminants with a ruptured bladder.

Chemical element measurements

Calcium, phosphorus and magnesium measurements also are useful. Mild hypocalcemia is common in sick cattle, and an addition of extra calcium to fluids usually is indicated. Clinical hypocalcemia is easily recognized during physical examination, and blood work usually is unnecessary. However, in cases that relapse or with unusually high numbers of cases in a herd, blood work might be helpful to make sure phosphorus and magnesium problems are not contributing to the signs.

Beef cattle with hypocalcemia along with hypomagnesemia usually will have signs of hypomagnesemic tetany. These animals might not respond as well to magnesium administration as pure hypomagnesemia, or they might relapse. Remember that acid-base status and albumin levels will influence ionized levels of calcium, but this will not be reflected in a total calcium measurement.

Hypercalcemia is rare and usually caused by excessive administration of intravenous calcium.

Hypophosphatemia usually is due to prolonged anorexia or dietary problems. The condition is uncommon and often related to hemolysis. However, renal disease should be investigated if hemolysis is ruled out.

Blood urea nitrogen (BUN) and creatinine are indicators of dehydration and/or renal disease. Since urea is recycled in saliva and utilized in the rumen, increases in BUN do not always occur, so increases in creatinine are more sensitive.

The most common cause of azotemia is dehydration, which can be detected clinically. But renal disease and dehydration can occur concurrently so urine-specific gravity is indicated for azotemic animals.

In most species, if the urine-specific gravity is less than 1.025 in an azotemic animal, renal disease should be suspected. However, since cattle with other disease processes can have azotemia and a low urine-specific gravity with no evidence of renal disease, this needs to be a repeatable finding.

The problem comes in that these animals usually need fluid therapy, and once fluid therapy is instituted, a urine-specific gravity is not reliable. So if fluid therapy is instituted, and the BUN and creatinine return to normal in about 24 hours, the azotemia is probably prerenal. If the BUN and creatinine stay elevated, renal disease should be suspected.

Interpreting enzyme tests

Enzyme tests must be interpreted with caution because many are not specific for one disease system. The most useful enzyme is gamma-glutamyltransferase (GGT).

Elevations in GGT can occur with cholestasis and hepatocellular membrane damage in cattle. Chronic liver disease is more common and usually involves some degree of cholestasis. Also, GGT stays elevated longer than other enzymes. So elevations in GGT can indicate active or recent hepatocellular damage or chronic liver disease.

Other enzyme elevations might indicate one type of disease or another, but most indicate that the liver should be further investigated. The caveat to this is that in the face of normal enzymes, liver disease cannot be ruled out. Increases in bilirubin occur more frequently with hemolytic diseases than liver disease.

Many times, the bilirubin is normal with liver disease. High bilirubin in association with liver disease usually is a poor prognosis.

Dr. Navarre works as an extension veterinarian with Louisiana State University's Department of Veterinary Science.