Did you know that health care providers use multiple tests to diagnose autoimmune hemolytic anemia (AIHA)? These tests can detect anemia (low red blood cell levels) and determine why you don’t have enough of these cells. While the exact cause of AIHA is not always clear, health care providers have a variety of diagnostic tests available to help identify the condition and determine the best course of treatment.

The direct antiglobulin test (DAT) can help identify one potential cause of anemia. Specifically, it can tell you whether your immune system is attacking and killing your red blood cells (RBCs). The DAT, also called the Coombs test or direct Coombs test, can help you understand whether you have AIHA or another related condition.

Read more about what to expect from Coombs testing, what your results mean, and how it helps doctors diagnose or rule out AIHA.

Why Immune System Testing Is Important for Diagnosing AIHA

Normally, your immune system keeps you healthy by destroying disease-causing microbes like bacteria and viruses.

In AIHA, your immune system mistakenly attacks your RBCs. It does this by making antibodies (immune system proteins) that attach to the outside of RBCs and labeling them as harmful substances that the body needs to get rid of. Your immune cells attack the labeled RBCs, breaking them open in a process known as hemolysis.

Diagnostic tests such as the DAT can determine whether your blood contains antibodies that recognize RBCs. This information tells you whether an autoimmune disease like AIHA is responsible for your anemia or if another factor may be involved.

How Does DAT Work?

To perform DAT, your health care team will take a sample of blood from your arm. This sample will then be sent to a laboratory.

In the lab, your blood sample will be mixed with a chemical called anti-human globulin or Coombs reagent. When this chemical is combined with cells that are covered with antibodies, it makes the cells clump together. Laboratory technicians can use various methods to determine whether your RBCs form clumps and, therefore, are being attacked by your immune system.

When the test is complete, your health care team will tell you the results. Ask your doctor how long it will take to get your results, and make sure you know how to access your results once they arrive.

What Do DAT Results Mean?

Your DAT results will say either “negative” or “positive,” which indicates that your RBCs either lack or contain antibodies, respectively.

A Negative Result

A negative finding indicates that the DAT didn’t find evidence of antibodies on your RBCs. This most likely means that you don’t have AIHA. A negative DAT result is considered normal.

If you have low levels of RBCs, a negative result means that your anemia has other causes not related to the immune system.

There is a small chance you may still have AIHA if you receive a negative DAT result. In rare cases, this test may produce a false-negative result, which means it may indicate that there are no antibodies against RBCs even if they are present.

Additionally, the DAT looks for a type of antibody called IgG (another word for antibody is immunoglobulin, or Ig), but some people have AIHA caused by other types of antibodies like IgA or IgM antibodies. In this case, your doctor can use other tests to figure out the cause of your AIHA. Talk to your health care team if you want to learn more about what is causing your condition.

A Positive Result

A positive finding on your DAT means that your immune system is producing antibodies that target your RBCs. Your DAT results may show a number between 1+ and 4+. Getting a 1+ on this test means that your RBCs contain a very small amount of antibodies, and a 4+ means that there are many antibodies against RBCs in your system.

A positive DAT test result may mean that you have AIHA. However, it can also be a sign of other blood disorders, including:

• Hemolytic transfusion reaction — A condition that occurs when you undergo a blood transfusion (receive donated blood cells) and either your immune system attacks the donated RBCs or the immune cells in the donated blood attack your own RBCs
• Drug-induced hemolytic anemia — A disease that forms when certain medications cause your immune cells to start attacking your RBCs
• Passive antibody transfer — A condition that develops when you use a treatment such as plasma or immunoglobulin made from someone else’s blood and their antibodies destroy your RBCs
• Passenger lymphocyte syndrome — A rare condition in which you receive someone else’s immune cells during an organ transplant and these cells target your RBCs
• Hemolytic disease of the newborn — A disorder that develops when a mom and her baby have different blood types and the mother’s immune cells attack the baby’s RBCs

You may also receive a positive result if you have an unrelated condition like systemic lupus erythematosus (also simply called lupus), chronic lymphocytic leukemia (CLL), or an infection like mononucleosis or syphilis.

After a Positive Result: Next Steps

The DAT test probably isn’t the only test you will need. If your test results are positive, you’ll likely also undergo additional tests to check your levels of different types of blood cells and look for proteins that can signal RBC destruction.

Additionally, the DAT can’t distinguish between different AIHA types — you will get a positive result whether you have warm or cold AIHA. Other tests, such as a cold agglutinin titer, can help your doctor determine whether your antibodies become activated at particular temperatures.

AIHA is sometimes caused by other disorders, so your doctor may also perform tests to check for viral infections, autoimmune diseases, blood cancers, or other conditions that could be triggering AIHA.

Once your doctor has a full picture of your health, they can recommend AIHA treatment options that will best suit your needs.

What Should You Know Before Undergoing a DAT?

This test has minimal risks. You may feel a little bit of pain during the blood draw. Afterward, you may have bleeding or bruising in the affected area, or you may experience lightheadedness.

Certain medications can affect your test results. There is a chance that you will receive a false-positive result if you are taking:

• Penicillin or other antibiotics
• Anti-inflammatory drugs
• Certain chemotherapy medications

Make sure to tell your doctor about any medications or supplements, including natural or herbal products, that you are currently using.

Talk With Others Who Understand

AIHA is a rare disease, but you’re not alone. On myAIHAteam, people with AIHA and their loved ones come together to ask questions, give advice, and share their stories with others who understand life with this condition.

Have you had to get direct antiglobulin testing? What was your result? Share your experience in the comments below, or start a conversation by posting on myAIHAteam.