A patient with low titer cold agglutinin disease is reported. A 66 years-old Japanese man was admitted because of general fatigue and palpitation. Laboratory data showed anemia with reticulocytosis, positive direct Coomb's test against anti-C3b, elevated level of lactic dehydrogenase, undetectable level of serum haptoglobin, and low titer of cold agglutinin. When the titer of cold agglutinin was measured by the 22% bovine serum albumin solution method, it became 1: 4096 from 1: 64 at 4°C, 1: 16 from 0 at 30°C, and 1: 4 from 0 at 37°C, respectively. Immunoglobulin subclass of cold agglutinin in this patient was IgM and its recognizing antigen was Lud or Lud-related antigen containing N-acetylneuraminic acid.
