Interpretation of the CBC

Clients look to us not just to read numbers but to tell them what they mean and to help them to make decisions based on interpreting those numbers in light of other available information. They want to know when something is an emergency, when following trends is appropriate, and when to relax. Most importantly, they want to know why we are asking for more testing or even asking them to take their pet to the ER. Being as confident and straightforward as possible when interpreting and discussing lab results helps clients to understand our reasoning and what they will gain from following our recommendations and improves compliance.

Here are some basics when interpreting a CBC and explaining our concerns, what we are looking for with further testing, and even why we aren’t worried about certain abnormalities!

Pearls

• All lab work results should be interpreted in light of the clinical appearance of the patient.
• If the lab work doesn’t match the clinical appearance of the patient, question the validity of the results, and consider repeating vs. additional testing.
• Be careful not to over interpret mild abnormalities.
• Small deviations can be normal for a particular patient. Most labs attune to 95% of the population.
• Be prepared to justify a recommendation for repeating testing and/or performing further testing.
• Know when to worry, when to monitor, and when to say that, to the best of your knowledge, everything is okay!

General Principles

• Always check any previous lab work to see if results today are similar to results in the past. Don’t panic about values that have been unchanged for a long time, especially if there has been previous work-up.
• If you are unsure about the significance or accuracy of a CBC finding, and in-house blood smear is a great tool!
• In cases of significant abnormalities in sick patients, a pathology review is always a great recommendation!

Interpretation of Results

• Red blood cell indices:

  • HCT and total number of RBCs are the most important values to look at first.
  • If anemia is found, look at the other indices to see if the anemia appears regenerative (reticulocytosis present, anisocytosis or increased MCV present) or non-regenerative (low reticulocytes, normal-sized or smaller RBCs).
    • Regenerative anemias are commonly caused by blood loss or hemolysis.
    • Non-regenerative anemias can be secondary to kidney disease, endocrinopathies, other chronic disease, toxin exposure, certain infectious agents, nutritional deficiencies, or neoplasia that is affecting the bone marrow.
  • If there is erythrocytosis, think about the following:
    • Breed – sight hounds normally have a high normal to elevated HCT and RBC count
    • Dehydration – look at the patient’s clinical status, total protein, and electrolytes to help to determine if the patient is hemoconcentrated.
    • Splenic contraction can happen in times of stress of illness or emotional stress of an appointment
    • Think of polycythemia vera after you have thought about the rest of the above, and in light of symptoms. You may find it, but IT IS RARE.

• White blood cells indices: 

  • Neutropenia

    • Interpret in light of clinical appearance and history.
    • Think about toxins.
    • Think about medications that can have toxic effects (classic example is chemo, but when in doubt, look it up on Plumb, VIN, or in another drug handbook you feel comfortable with.)
    • Think about severe infection/sepsis or a localized infection like an abscess that can act like a neutrophil sink.
    • Some breeds (like greyhounds) can have lower neutrophils than others and be perfectly normal.
    • Think about immune mediated disease.
    • Think about retroviruses in cats.
    • If there are no symptoms and there is no readily apparent cause, repeating the CBC in 3-7 days is reasonable to check validity of results and determine if the abnormality is persistent or transient.
    • If there is a potential cause that can be identified, correct the potential underlying cause and then repeat the CBC.
    • Other diagnostics should be performed with consideration for clinical appearance of patient and the rest of the lab work results.
    • If other signs of autoimmune disease are apparent, an autoimmune panel should be considered. Neutropenia can be seen with lupus.
    • The best way to evaluate persistent neutropenia where a primary disease/cause is not readily apparent is a bone marrow biopsy. Internal medicine consult is helpful in these cases.

  • Mature neutrophilia

    • Interpret in light of clinical appearance and history.
    • Consider stress leukogram, more commonly in dogs.
    • Is the patient on steroids?
    • Could the patient be Cushingoid?
    • Look for sources of infection or inflammation, including cancer, if clinically indicated.
    • Chronic myelogenous leukemia is not common and is a diagnosis of exclusion.

  • Bands and toxic neutrophils

    • Look at a smear if the CBC was done in-house to confirm.
    • Bands indicate inflammation and/or infection. The higher the numbers and the more toxic changes, the more severe the inflammation.
    • High numbers of bands with toxic changes are indicative of a potentially poor prognosis. If toxic changes are present, more aggressive diagnostics and therapeutics are warranted.

  • Lymphopenia

    • This is usually a result of stress in an asymptomatic cat or dog.
    • In a sick patient, think about viral infection, lymphangiectasia, chylous effusion, lymphoma, and zinc deficiency.

  • Lymphocytosis

    • Mild is likely to be reactive/indicative of an inflammatory response.
    • Think about Addison’s in a dog depending on the rest of the lab work and clinical picture.
    • More severe might be part of an inflammatory response, but leukemia and lymphoma should be considered and discussed.

  • Eosinopenia

    • Likely not a significant finding.
    • Depending on the rest of the clinical picture, consider stress, Cushing’s, or exogenous steroids.

  • Eosinophilia

    • Think parasites, allergy, Addison’s, paraneoplastic, fungal disease depending on history.
    • Consider hypereosinophilic syndrome in Rottweilers and cats depending on the rest of the clinical picture, work up, and previous treatments.
    • Eosinophilic leukemia is possible, but is not common.

  • Monocytopenia

    • Don’t overinterpret – monocytopenia alone is unlikely to be clinically significant.

  • Monocytosis

    • Most commonly part of a stress leukogram (think endogenous and exogenous steroids, especially when combined with a mature neutrophilia).
    • In a sick pet, think about chronic inflammation or infection, and cancer.

• Platelets

  • Thrombocytopenia

    • If found on an asymptomatic pet utilizing in-house machine, it should always be confirmed by blood smear, sending to the reference lab, or repeating with a new sample.
    • Platelet clumping is common, especially in cats, and it sometimes can lead to a marked decrease in a machine platelet count.
    • ITP is the first cause we think about and should be considered immediately in cases of true thrombocytopenia
    • Also consider decreased production associated with a drug, toxin, primary bone marrow issue, etc.
    • Depending on the clinical picture, consider sequestration in diseases that cause splenomegaly, consumption (DIC, local thrombosis, Vitamin K antagonism), and loss secondary to severe hemorrhage.

  • Thrombocytosis

    • Transient thrombocytosis can happen secondary to epinephrine release or recent blood loss.
    • Reactive thrombocytosis is common and can happen secondary to a multitude of diseases and disorders. If a patient is sick, look for the primary process. Even hypo- and hyperthyroidism can cause thrombocytosis.
    • Primary thrombocytosis is a rare myeloproliferative disorder with platelet counts of >1,000,000 cells/uL. This is a zebra!!

Remember that this is basic information to help you in your thought process when looking at a routine CBC.

If there are abnormalities in the RBC or platelet counts, or there is a clinical reason to be suspicious of an early process associated with those cell lines, look carefully at the reticulocyte count, reticulocyte hemoglobin level, MCV, MCH, MPV, etc., for nuances that will help you to fine-tune your interpretation. Anisocytosis and polychromasia are often indicative of a regenerative response and can be early indicators of impending anemia in a pet that still has a normal HCT. Similarly, large platelets are also a sign of increased production that may be associated with a compensatory response to consumption.

The more normal blood smears you have the opportunity to do, the more confident you will be when you are faced with an abnormal CBC on an in-house machine in a sick pet! Remember that admitting that you aren’t sure about what you are seeing when you have clinical concerns is a powerful tool, not a weakness. Ask a colleague to review a smear with you, or send for pathology review and compare their interpretation to what you saw. Clients tend to appreciate it when we admit that we are not omniscient and we need a specialist’s help!