Approach to anemia (Proceedings)


Approach to anemia (Proceedings)

Aug 01, 2010

Anemia: Anemia is most simply characterized by reduced numbers of erythrocytes and/or decreased erythrocyte hemoglobin content.
Dog: PCV < 38%
Cat: PCV < 24%

The nature (blood loss, hemolytic, or non-regenerative), duration and severity of anemia can defined by a few simple tests. History, physical and results of these tests will determine if further tests are required.

1. Packed Cell Volume (PCV):
Dog: 38% to 57%
Cat: 24% to 45%

More complete evaluation of red cell parameters, including hemoglobin, red cell count, mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH) and mean corpuscular hemoglobin concentration (MCHC) can be obtained by submitting blood to a diagnostic laboratory with an impedance (Coulter) type hematology analyzer. Recently, impedance type analyzers have also become available for in house use in veterinary practices. Ideally, the analyzer should be calibrated differently for cats and dogs, since feline red cells are significantly smaller than canine red cells. Otherwise, small feline erythrocytes may be below the analyzer range for red cells, and may mistakenly be miscounted as platelets. Some but not all of the above parameters can also be obtained from centrifuge-based hematology analyzers such as the Idexx QBC.

2. Total Serum Protein (TSP):
Dog: 5.8 to 7.6 g/dl
Cat: 6.0 to 8.0 g/dl

3. Examination of Direct Smear

Examination of air-dried, stained blood smear under high power.

4. Reticulocyte Count

Reticulocyte count quantitates erythrocyte regenerative response.

Automated analyzers that provide a RBC count (RBC x 106/µl) enable an absolute reticulocyte count to be calculated (reticulocytes/µl: normal is usually less than 60,000/µl).

Based on the results of the above tests, anemia can usually be separated into one of three simple classifications: hemorrhagic, hemolytic, or non-regenerative.

1. Hemorrhagic

(i) Acute Blood Loss

Acute, severe blood loss causes hypovolemic shock rather than anemia. Proportional loss of all major blood components means that initially both PCV and TSP remain normal. Volume expansion during recovery from hypovolemia progressively dilutes both PCV and TSP. Reflex splenic contraction, however, initially boosts red cell numbers, therefore TSP tends to drop before PCV. Decreased TSP occurs 1-4 hours after blood loss, and decreased PCV occurs after 12-24 hours. Loss of over 30% of blood volume in a single episode of hemorrhage can cause death due to hypovolemic shock. Patients that survive are therefore unlikely to have lost more than 30% of circulating red cells, and will not be severely anemic unless bleeding continues at a slower rate.

Erythrocyte regeneration (rising reticulocyte count) is not evident for 3-4 days. Acute blood loss can therefore initially mimic non-regenerative anemia. The subsequent regenerative response peaks at 5-7 days, although PCV may take up to 2-3 weeks to return to normal levels. TSP returns to normal levels more rapidly (1 week). Persistent anemia and hypoproteinemia suggest ongoing blood loss.

Diagnostic Features

Anemia will be regenerative after 3-5 days, featuring anisocytosis, polychromasia and sometimes nucleated red cells on examination of stained blood smears, with an increased corrected reticulocyte count. For the first week after a bleeding episode, there may also be hypoproteinemia