IMHA and Regenerative Anaemia

Question 1

List mechanisms that cause a regenerative anaemia

Answer 1

1. Haemolysis 2. Haemorrhage

Question 2

Describe the regenerative response as it occurs with acute anaemia

Answer 2

* Anaemia causes renal cortical hypoxia and release of EPO * EPO stimulates stem cells in the bone marrow (+ liver and spleen) to proliferate and differentiate towards RBC * Red blood cells mature through to reticulocytes over 5-7 days. * In-health reticulocytes remain in the bone marrow for 2-3 days. * The mature reticulocytes within the bone marrow can be released early during acute haemolysis under the influence of EPO. * Reticulocytosis develops within 2-5 days. * Reticulocytes lose their residual RNA over 24-48 hours in the circulation to form mature RBCs

Question 3

List the tests available to assess for regenerative anaemia

Answer 3

* CBC and blood smear * Osmotic Fragility Test * Assess for spherocytes * In-saline agglutination test * Direct agglutination test

Question 4

Describe the rapid osmotic fragility test

Answer 4

* The test assesses propensity for red blood cells to lyse under physiological stress * Spherocytes or RBC with membrane defects are less resistant to a hypotonic environment * ROFT uses 5 drops of blood in 0.9% saline (5 ml) and 5 drops of blood in 0.55% saline (3 ml saline, 2 ml distilled water). * Two tubes are incubated for 5 minutes at room temperature * Centrifuged at 2431 g for 5 minutes * Positive result has a more red supernatant in the hypotonic solution. * Can be positive with hereditary cell membrane defects and other diseases affecting cell membrane stability.

Question 5

Discuss the blood smear changes that can be seen in the various causes of regenerative anaemia

Answer 5

* Macrocytes * Polychromasia * RBC precursors - normoblast / nucleated red blood cells * +/- spherocytes * +/- Heinz bodies * +/- fragmented red blood cells (eg. schistocytes) * Intracellular parasites - babesia, mycoplasma

Question 6

List non-immune causes for acquired haemolytic anaemia

Answer 6

1. Red blood cell toxins: onions, zinc, acetaminophen (cats), methylene blue toxicosis 2. Hypophosphataemia: cats with diabetes, cats with hepatic lipidosis, with enteral feeding 3. Red blood cell infection: Babesia, Haemotropic mycoplasma 4. Red blood cell fragmentation syndromes: Microangiopathic injury - eg haemangiosarcoma, heart failure, glomerulonephritis, myelofibrosis. Haemolytic uraemic syndrome, DIC 5. Hereditary haemolytic syndromes: pyruvate kinase deficiency, phosphofructokinase deficiency, 6. Haemophagocytic anaemia: haemophagocytic HS

Question 7

Discuss major pathophysiological causes of IMHA

Answer 7

* Primarily cause by activation of humoral immune response causing proliferation of B lymphocytes * Increased secretion of auto-antibodies directed again red blood cell surface antigens. * Antibodies induce opsinisation of the target red blood cells which are subsequently removed via complement activation or the reticuloendothelial system

Question 8

Discuss major pathophysiological consequences of IMHA

Answer 8

* Systemic inflammation triggered * Systemic inflammation causes hypercoagulable state * Microthrombi formation and DIC are common * Anaemia causes significant reduction in oxygen transport and hypoxaemia induced cell damage (note: clinical hypoxia is not often present) * Hypoxic necrosis around hepatic central veins - associated with an increase WCC * Systemic inflammation causes increased WCC * Icterus occurs with rapid haemolysis and decreased hepatic function - inability to process free haemoglobin.

Question 9

Discuss laboratory testing results in dogs with IMHA - CBC

Answer 9

* Anaemia, spherocytosis, variable reticulocytosis * Inflammatory leukogram: neutrophilia +/- left shift, monocytosis. * Thrombocytopenia: present in up to 70% of dogs, severe (<50,000) in ~ 25% of dogs.

Question 10

Discuss laboratory testing results in dogs with IMHA - Coagulation testing

Answer 10

• PT increased in ~ 50%
• APTT increased in 50-60%
• Low fibrinogen in ~ 20%
  
  • Increased fibrinogen in 30-90% of dogs as it is secreted during the acute phase inflammatory response
• TEG tracing generally suggest hypercoagulability - may not be reliable with reduced red blood cell mass.
• May see:
  
  • Decreased AT
  • Decreased coagulation factor activity
  • Increased D-dimers
  • Increased FDPs.

Question 11

Discuss laboratory testing results in dogs with IMHA - specific tests

Answer 11

1. Direct agglutination test (Coombs test)
   
   • Detects erythrocyte bound immunoglobulins and complement
   • Sensitivity of 50-89%
2. Detection of auto-agglutination
3. Osmotic fragility test
   
   • Confirms haemolysis, but not immune mediated cause
   • Positive result may be due to spherocytes or complement mediated cell membrane damage.