•Clumping (agglutination) of red blood cells is frequently caused by cold agglutinins.  Cold agglutinins are IgM antibodies that may arise following viral or Mycoplasma infections, or in the setting of plasma cell or lymphoid neoplasms.  Agglutination of red cells can interfere with red blood cell indices.  The red blood cell count may be falsely decreased, and the MCV falsely increased, as clumps of red cells are measured as single cells.  The hemoglobin level will be accurate, as this parameter is measured after lysing the red cells.