Hemoglobin is salvaged by haptoglobin, and the heme moiety is broken down first to bilirubin and then to urobilinogen, which is excreted in the urine.2 Bilirubin produced from the breakdown of heme is not conjugated, but rather is delivered to the liver, where it is conjugated and excreted into the bile. In hemolysis, the concentration of unconjugated bilirubin (indirect bilirubin) is increased, while in liver disease the level of conjugated bilirubin (direct bilirubin) is increased. However, if the patient has concomitant liver disease with an increased direct bilirubin level, the serum bilirubin test is not reliable.
Haptoglobin binds free serum hemoglobin and is taken up by the liver. Haptoglobin usually falls to very low levels in hemolysis. A confounder is that haptoglobin is an acute phase reactant and can rise with systemic disease or inflammation. However, patients with advanced liver disease will have low haptoglobin levels due to lack of synthesis, and up to 2% of the population may congenitally lack haptoglobin.1
If the hemolysis is very rapid, the amount of free hemoglobin released will overwhelm the binding capacity of haptoglobin and lead to free hemoglobin in the plasma. This can be crudely quantified by examining the plasma color. Even minute amounts of free hemoglobin will turn the plasma pink. In fulminant hemolysis, the plasma will be cola-colored.
In most patients with hemolysis, the destruction of red cells is accompanied by an increase in the reticulocyte count. Reticulocytes are red cells that still contain RNA and are a marker of red cells that are approximately 24 hours old or less. Traditionally, reticulocytes were measured manually by staining the blood smear with vital blue and counting the percentage of cells that absorb the stain; this percentage needs to be adjusted for the hematocrit. Usually a percentage above 1.5% is considered indicative of an elevated reticulocyte count. Recently, automated complete blood count machines have taken advantage of the fact that reticulocytes will absorb certain stains; these machines can directly measure the reticulocyte count via flow cytometry, which results in an “absolute” reticulocyte count. The reticulocyte count obtained using this method does not have to be corrected for hematocrit, and levels of approximately 90,000/μL are considered raised. However, the reticulocyte count can also be raised in blood loss or in patients who have other causes of anemia (eg, iron deficiency) under treatment. In addition, as many as 25% of patients with AIHA will never have raised counts for various reasons, such as nutritional deficiency, autoimmune destruction of red cell precursors, or lack of erythropoietin.
The blood smear provides vital information. The hallmark laboratory parameter of AIHA is spherocytes seen on the blood smear. In AIHA, antibodies and/or complement attach to the red cells, and when the antibodies or complement are taken up by macrophages in the spleen some of the red blood cell mem-brane is removed as well, decreasing the surface area of the cell. As the surface area of the red cell decreases with each pass through the spleen, the cell's shape changes from a biconcave disk to a sphere before the cell is destroyed, reflecting the fact that a sphere has the smallest surface area for a given volume. The vast majority of patients with AIHA will have spherocytes on the blood smear. However, spherocytes are not specific to AIHA, as they can be seen in hereditary spherocytosis, Wilson’s disease, clostridial sepsis, and severe burns.
Patients with cold agglutinins will often have red cell agglutination on the blood smear. In addition, patients with AIHA will often have a raised mean corpuscular volume (MCV) for 2 reasons. In patients with brisk reticulocytosis, the MCV will be raised due to the large size of the reticulocyte. In patients with cold agglutinin disease, the MCV may be falsely raised due to clumping of the red blood cells.