Why Order a Coombs Test for Hereditary Spherocytosis?

If your doctor suspects you have hereditary spherocytosis (HS), it can be confusing and alarming to see them order a Coombs test — also known as a Direct Antiglobulin Test or DAT ^[1]. The Coombs test looks for antibodies that attack your red blood cells, which is the hallmark of an autoimmune disorder ^[2]. You might wonder: If they think I have a genetic condition, why are they testing me for an autoimmune disease?

The answer comes down to what your red blood cells look like under a microscope.

The Case of the Identical Spherocytes

When a hematologist looks at your blood smear, they are looking for spherocytes. These are red blood cells that have lost their typical donut-like shape and instead look like small, perfectly round spheres.

While spherocytes are the defining feature of hereditary spherocytosis, HS is not the only condition that causes them. Spherocytes are also a classic sign of Autoimmune Hemolytic Anemia (AIHA) ^[1].

In AIHA, your immune system mistakenly produces antibodies that attach to healthy red blood cells ^[3]. As these antibody-coated cells travel through your spleen, the spleen “bites” off portions of the cell membrane, turning them into spheres. In contrast, in HS, the spherocytes are caused by an inherited genetic defect in the structural proteins of the cell membrane itself ^[4][5], not by an immune system attack.

Because the spherocytes from HS and AIHA look completely identical under a microscope, your doctor needs a way to tell them apart.

What the Coombs Test Tells Your Doctor

The Coombs test (DAT) is a standard blood test used to definitively distinguish between these two very different conditions ^[1][6]:

• A Positive Coombs Test: This means antibodies were detected on your red blood cells. This points to Autoimmune Hemolytic Anemia (AIHA) as the cause of your spherocytes and anemia ^[3][1].
• A Negative Coombs Test: This means no antibodies were detected on your red blood cells. A negative result usually rules out AIHA and strongly supports the diagnosis of a genetic structural issue, like hereditary spherocytosis ^[7].

Once a negative Coombs test helps rule out an autoimmune cause, your doctor will likely proceed with specific tests to officially confirm hereditary spherocytosis, such as an EMA binding test or an osmotic fragility test ^[7][8].

Why Getting the Diagnosis Right Matters

Even though HS and AIHA both cause spherocytes and hemolytic anemia (the premature destruction of red blood cells), they are treated very differently. AIHA is typically treated with medications that suppress the immune system, such as steroids ^[9][10]. Hereditary spherocytosis, being a genetic structural issue, does not respond to steroids or immunosuppressants. Instead, HS is managed through monitoring, folic acid supplementation, and in some cases, surgical removal of the spleen (splenectomy) ^[11][12].

By running a Coombs test, your doctor is simply being thorough. A negative result provides the necessary confirmation that your immune system is working normally, allowing your care team to safely proceed with confirming and treating hereditary spherocytosis ^[7].