Top Takeaways from a lecture by Dr Hannah Darcy

Introduction

Anaemia in cats presents a diagnostic challenge. The approach to diagnosis – and subsequent management requires a systematic framework: determining severity, assessing regenerative response, identifying evidence of haemolysis, and prioritising key diagnostic steps. 

This case-based lecture follows a patient called “Oscar” through multiple scenarios, demonstrating how the same presenting complaint can lead to vastly different diagnoses based on subtle clinical clues. Building a solid toolkit for these cases means understanding not just what tests to run, but why each test matters and how to interpret results in context.

Key Lessons

1. Start with the Fundamentals: PCV and Total Protein Done Correctly

Packed cell volume (PCV) is a direct measurement—the percentage of a centrifuged blood tube that is solid red blood cells. Haematocrit, by contrast, is a calculated value from haematology machines and can be influenced by factors like sample evaporation or cell swelling. 

Critical practical point: Microhaematocrit tubes must be centrifuged at 10,000–12,000 RPM for three minutes. Using slower settings (e.g. those used for urine) can change how tightly red blood cells compact, producing results that vary by 5–10% without clinical reason.

Always record plasma colour when spinning PCV tubes. Clear serum, icterus, haemolysis, or lipaemia all provide immediate clues. Total protein should be measured alongside PCV – low protein with anaemia suggests blood loss; normal protein with anaemia points toward haemolysis or decreased production.

2. Regenerative versus Non-Regenerative: The Essential Distinction

Anaemia splits into two broad categories:

• Regenerative: Due to blood loss or haemolysis. The bone marrow responds by releasing immature red blood cells.
• Non-regenerative: Due to decreased production, either truly non-regenerative or pre-regenerative (a regenerative anaemia in the early stages of development, where bone marrow response is not yet seen in circulating erythrocytes).

In cats, the regenerative response takes longer than in dogs – typically three to five days, occasionally up to 11 days for maximum effect. This means a cat that appears non-regenerative today might be regenerative in a few days.

Measuring regeneration:

• Mean cell volume (MCV) and mean cell haemoglobin concentration (MCHC) are classic markers, with macrocytic, hypochromic cells suggesting regeneration
• Red blood cell distribution width (RDW) shows whether the size range of erythrocytes is expanding
• Reticulocyte count is the most direct measure. Anything over 80 × 10⁹/L generally indicates regeneration
• Corrected reticulocyte percentage is invaluable: multiply the reticulocyte percentage by (patient’s PCV ÷ 35). Anything above 0.4% confirms regeneration. This corrects for the degree of anaemia – where a very anaemic patient might have an absolute reticulocyte count that looks normal but represents a much larger percentage of total red cells.

3. In-Saline Agglutination and the Cat’s Normal Rouleaux

Cats normally display rouleaux – with red blood cells stacking like coins due to surface charge interactions. This is not agglutination. True agglutination appears as rounded clusters of red cells and indicates antibodies coating the erythrocytes.

Testing approach:

• Start with one drop of blood to four or five drops of saline (1:4 or 1:5 dilution)
• If agglutination persists at this level, it is strong evidence of immune-mediated disease
• Agglutination can be macroscopic (visible to the naked eye) or microscopic (visible only on a slide)

A positive in-saline agglutination test represents one sign of immune-mediated destruction, but it is not diagnostic alone. When combined with evidence of haemolysis (hyperbilirubinaemia, bilirubinuria, ghost cells on smear), it becomes supportive of immune-mediated haemolytic anaemia (IMHA). Adding a positive Coombs test moves the patient into a confirmed diagnosis.

4. The Coombs Test: A Diagnostic Game-Changer

The direct Coombs test (direct antiglobulin test) detects IgG and IgM antibodies bound to erythrocyte surfaces. It has approximately 80% sensitivity and 95% specificity. A positive result strongly supports IMHA.

Practical advantage: The test can be run on refrigerated blood up to seven days after collection. This means a sample taken on that Friday evening can be sent to the laboratory on Monday without loss of diagnostic accuracy.

The 2019 ACVIM consensus statement provides a helpful algorithm:

• Signs of immune-mediated destruction (e.g. positive agglutination) + signs of haemolysis (hyperbilirubinaemia, ghost cells) = supportive of IMHA
• Adding a positive Coombs test = confirmed diagnosis

This distinction matters for treatment decisions and client communication.

5. Mycoplasma, Heinz Bodies, and Oxidative Damage

Mycoplasma haemofelis and related haemoplasmas can cause haemolytic anaemia in cats. While Mycoplasma haemofelis is relatively uncommon (around 4% prevalence in some studies), less pathogenic species like Candidatus Mycoplasma haemominutum can affect up to 25% of cats. Flea transmission is implicated, making outdoor cats more at risk.

Clinical approach: When treating suspected IMHA, adding doxycycline provides coverage for mycoplasma while waiting for PCR results. This is particularly relevant because mycoplasma infection can cause false-positive Coombs tests, and the initial haemolysis may expose internal erythrocyte antigens, triggering secondary IMHA.

Heinz bodies indicate oxidative damage to haemoglobin. Cats are uniquely susceptible because:

• They have eight exposed sulphydryl groups on haemoglobin (compared to four in other species)
• They have reduced capacity for detoxification pathways like glucuronidation

Common causes of Heinz bodies include onion and garlic (including in baby food), paracetamol/acetaminophen, bee stings, and snake bites. Heinz bodies make red cells more rigid, leading to premature removal from circulation. On Diff-Quick stains, Heinz bodies appear as small blebs on the cell membrane; methylene blue staining makes them much more visible.

6. Anaemia of Chronic Disease: The Iron Sequestration Problem

Non-regenerative anaemia without evidence of haemolysis or haemorrhage should prompt investigation for underlying chronic disease. 

Common underlying causes in cats include chronic kidney disease, neoplasia (particularly lymphoma), inflammatory bowel disease, and pancreatitis. 

In chronic kidney disease, the old teaching that gastrointestinal ulceration contributes to anaemia has been revised. Histopathology shows fibrosis and mineralisation in the stomach rather than ulceration. The more relevant mechanisms are reduced erythropoietin, increased inflammatory cytokines, and reduced erythrocyte half-life.

Treatment for CKD-associated anaemia:

• Threshold for intervention is PCV below 20%
• Darbepoetin (erythropoietin analogue) is preferred due to longer duration and lower risk of antibody reactions
• Iron dextran supplementation at initiation helps overcome sequestration, though pre-treatment with maropitant is recommended due to vomiting risk
• Target PCV is above 25%

7. Precursor-Targeted Immune-Mediated Anaemia (PIMA): When Bone Marrow Is the Problem

In cats with marked non-regenerative anaemia and no abnormalities on imaging, biochemistry, or infectious disease testing, bone marrow sampling becomes the key diagnostic step. PIMA (precursor-targeted immune-mediated anaemia) occurs when the bone marrow produces erythrocyte precursors, but macrophages within the marrow phagocytose them before they reach circulation.

Key features:

• More common in young cats
• Bone marrow aspirate shows increased erythroblasts with evidence of erythrophagocytosis
• Treatment is immunosuppression (as for IMHA)
• Remission rate around 73%, though many patients require long-term low-dose therapy
• Average time to remission is six weeks—patients often need multiple blood transfusions during this period

Important distinction: Unlike IMHA, where haematocrit can drop critically within days, the decline in PIMA is slower due to the 65-day lifespan of feline erythrocytes. This slower progression gives clinicians time to establish treatment, but the extended time to remission means patience and repeated transfusion support are essential.

Final Framework for Anaemic Cats

When approaching any anaemic cat, ask these questions in sequence:

1. How severe is the anaemia? (PCV and total protein)
2. Is it regenerative? (reticulocyte count, corrected reticulocyte percentage, blood smear evaluation)
3. Is there evidence of haemolysis? (hyperbilirubinaemia, bilirubinuria, ghost cells, haemoglobinuria)
4. If regenerative and haemolytic, is it immune-mediated? (in-saline agglutination, Coombs test)
5. What additional information is needed? Prioritise based on clinical picture, such as imaging for suspected haemorrhage, biochemistry for chronic disease, bone marrow for unexplained non-regenerative anaemia

The key is not running every test on every patient, but identifying the single most informative next step. This approach saves time, reduces costs, and leads to faster, more accurate diagnoses—even on a Friday evening.