Is Hemolysis Guaranteed? A Case Based Discussion

An octogenarian woman with past medical history of coronary artery disease on aspirin and Plavix presents from an assisted living facility with complaints of blood in her diaper. Hemoglobin is 5.8 g/dL, and a unit of O negative blood is given on emergency release, meaning the blood was transfused before a full type and screen and crossmatch could be performed. Two hours following transfusion, information is obtained from an outside hospital significant for a history of anti-E. Upon completion of testing, the emergency release unit was found to be incompatible on crossmatch, and antigen typing of the unit was significant for the presence of E antigen.

What are you concerned about? Hemolysis!

Hemolytic transfusion reactions occur when a patient antibody binds to antigen on a transfused red blood cell (RBC). The bound antibody may then fix complement to the surface of the transfused RBC and initiate lysis, which releases the contents of the RBC into the blood stream. Contents of the RBC include lactate dehydrogenase (LDH) and bilirubin, and monitoring for elevations in these analytes helps determine a diagnosis of hemolysis. Similarly, hemoglobin is released from the lysed RBC which is then bound by haptoglobin. The haptoglobin-hemoglobin complex is then taken up and digested by macrophages, which prevents accumulation of hemoglobin in the kidneys and reduces the potential for renal injury. Thus, hemolysis involves a decrease in circulating haptoglobin, and reduced haptoglobin levels help determine a diagnosis of hemolysis.

Hemolysis can occur in two compartments in the body. Intravascular hemolysis occurs within blood vessels and generally causes a more severe syndrome. Because the contents of RBCs are being released directly into the blood stream, the classic triad of elevated LDH and bilirubin, and reduced haptoglobin, is commonly seen. The body’s natural defenses against hemolysis – namely haptoglobin – can be easily overwhelmed, and excess circulating hemoglobin can accumulate in the kidneys, causing both renal injury and hemoglobinuria. This is an important way to distinguish intravascular hemolysis from extravascular hemolysis. Extravascular hemolysis occurs when antibody-coated RBCs are taken up by macrophages in the reticuloendothelial system (i.e. the spleen), which then digests the RBC and its contents. Some of the RBC contents may spill out of macrophages, but because RBC destruction is largely occurring inside macrophages, extravascular hemolysis tends to feature less severe elevations in LDH and bilirubin, less severe reductions in haptoglobin, and less severe symptoms all around. It’s important to note that there are exceptions, and severe cases of extravascular hemolysis have been reported.

Hemolytic transfusion reactions can be acute, meaning they occur within 24 hours of transfusion, or delayed, meaning they occur between 1- and 28-days following transfusion. Acute hemolytic transfusion reactions involve a pre-existing antibody, and are typically more severe, whereas delayed transfusion reactions can involve either a pre-existing or newly made antibody and tend to be less severe. Hemolytic transfusion reactions can also be divided into two categories based on the type of antibody involved. ABO antibodies are naturally occurring largely IgM antibodies. IgM antibodies are very good at fixing complement, and ABO-incompatibility tends to cause severe, intravascular hemolytic transfusion reactions. Non-ABO antibodies typically develop in response to antigen exposure and are largely IgG. They may or may not fix compliment, may cause either intravascular or extravascular hemolysis, and tend to cause less severe reactions than ABO antibodies. Again, there are exceptions to this, with non-ABO antibodies causing severe hemolysis.

Prior to 1985, ABO-incompatibility was responsible for ~15% of all deaths from hemolytic transfusion reactions [1]. Investigations showed that erroneous patient identification on blood samples for type and screen was a common underlying culprit, and the term ‘wrong blood in tube’ (WBIT) was coined. Between 1985 and 2005 a bevy of initiatives to increase blood safety were launched, including two-factor patient identification, bar code scanning of sample labels and patient wrist bands, bar code scanning on blood product labels, and administrative systems for tracking of patient blood type and antibody history [1]. These efforts had good effect, and from 2005-2008 ABO-incompatibility accounted for only 5.5% of deaths from hemolytic transfusion reactions [1].

In line with innovations to prevent hemolysis came innovations in treating hemolysis. First line treatment remains supportive, such as maintaining normokalemia, normotension, and urine pH > 6.5. The options for second line treatment have expanded, and now include steroids, plasma exchange, and continuous dialysis [2]. There are now even third line options for refractory cases of hemolysis. Ruxolotinib is a JAK-STAT inhibitor that serves to inhibit downstream cytokine activity in severe cases of hemolysis, and eculizumab is a monoclonal antibody against C5 in the complement cascade that can inhibit formation of the membrane attack complex and destruction of RBCs [2].

Now that we know so much about hemolysis, let’s come back to our patient with a history of anti-E who was transfused an E positive unit on emergency release. She was briefly managed in the ICU where it was determined she was bleeding from her bladder. Aspirin and Plavix were held, and the patient underwent bladder irrigation. Notably, she developed no signs or symptoms of acute or delayed hemolysis during her hospital stay. She did well and was discharged to follow up outpatient.

So why didn’t this patient have hemolysis? Because hemolysis is influenced by a number of factors: how much antigen the patient is exposed to, how antigenic the antigen is, the titer of the alloantibody, and whether the antibody fixes complement can all influence whether a hemolytic transfusion reaction develops. In this case, anti-E is described as producing mild to moderate hemolysis, so is less antigenic than other antigens, and the patient’s DAT was negative for C3d, which means her anti-E antibody was not very good at fixing complement. These are both possible explanations for why this patient escaped hemolysis even though she was exposed to E antigen. Hemolysis does not occur 100% of the time, and is not 100% fatal.

Take away points:

1. Hemolytic transfusion reactions can be acute or delayed.

2. ABO incompatibility is generally more severe than non-ABO incompatibility.

3. Treatment for hemolytic reactions is largely supportive, but there are also fancier options.

4. Hemolysis does not happen 100% of the time, and is not 100% fatal.

5. A DAT helps you determine what type of antibody is on the patient’s RBC, and whether it fixes complement.

 References:

1.       Vamvakas, E. C., & Blajchman, M. A. (2010). Blood still kills: six strategies to further reduce allogeneic blood transfusion-related mortality. Transfusion medicine reviews, 24(2), 77-124.

2.       Ackfeld, T., Schmutz, T., Guechi, Y., & Le Terrier, C. (2022). Blood transfusion reactions—a comprehensive review of the literature including a swiss perspective. Journal of Clinical Medicine, 11(10), 2859.