Background: Cold agglutinin disease is a rare autoimmune hemolytic disease causing the agglutination of erythrocytes by the activation of antibodies at low temperatures. Cold agglutinin disease should be kept in mind in terms of getting the right diagnosis and treatment because it may result in incorrect detection of blood groups, or may cause intravascular hemolysis during surgery or, if blool is given not pre-heated.

Material and Methods: In this study, a patient who faced difficulty during blood drawing and had an apparent blood clot in the hemogram tube. Samples were drawn into K₃-EDTA, Na-citrate and K₃-EDTA tubes bathed at 37ºC.

Results: Complete blood count was performed and the results were a follows; In K₃-EDTA tube erythrocytes were 0.35 10⁶/mm³, MCH and MCHC were +++++, hemoglobin level was 11.5 g/dl, and platelets were 212 10³/mm³. In K₃-EDTA tubes bathed at 37ºC, erythrocytes were 1.3 10⁶/mm³, MCH and MCHC were 39.7 pg ve 32.5 g/dl respectively, hemoglobin level was 11.9 g/dl, platelets were 249 10³/mm³. The patient's serum showed a monoclonal Ig M Kappa band in the immunofixation electrophoresis.

Conclusions: Cold agglutinin disease should be considered in patients with abnormal blood counts with visible agglutination in the tubes. Misdiagnosis of patients should be prevented.