No agglutination (i.e., the absence of clumping of cells) is normal.

A positive direct Coombs' test indicates antibodies against the red blood cells, which may indicate one of the following conditions:

• Autoimmune hemolytic anemia without another underlying cause
• Drug-induced hemolytic anemia (many drugs have been associated with this complication)
• Erythroblastosis fetalis (hemolytic disease of the newborn
• Infectious mononucleosis
• Mycoplasmal infection
• Syphilis
• Chronic lymphocytic leukemia or other lymphoproliferative disorder
• Systemic lupus erythematosus or another rheumatologic condition
• Transfusion reaction, such as one due to improperly matched units of blood

The test is also abnormal in some people without any clear cause, especially among the elderly. Up to 3% of people who are in the hospital without a known blood disorder will have an abnormal direct Coombs' test.