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.