It involves the cooling of the patients serum to 4°C to allow the absorption of anti-P autoantibodies to the red cells, followed by warming to 37°C to activate complement fixation and hemolysis. Indirect DL test with addition of ABO-compatible P antigen-positive blood can be performed in case the direct DL test is negative, since the complement in the original serum may be consumed and result in false negative.The hematopathological findings can reflect both the presence of intravascular hemolysis and the underlying immunological process. The complete blood count usually shows normocytic anemia. Reticulocytosis may be subtle in the acute phase. Peripheral blood smear may show corresponding polychromasia. Neutrophil erythrophagocytosis is suggestive of PCH, while the absence of red cell agglutination as in CAD or microspherocytosis in WAIHA should also be noted. Classification
AIHA can be classified as warm autoimmune hemolytic anemia  or cold autoimmune hemolytic anemia, which includes cold agglutinin disease and paroxysmal cold hemoglobinuria. These classifications are based on the characteristics of the autoantibodies involved in the pathogenesis of the disease. Each has a different underlying cause, management, and prognosis, making classification important when treating a patient with AIHA. Autoimmune hemolytic anemiaWarm-antibody typePrimary
Secondary  (lymphoproliferative disorders, autoimmune disorders): 259 Cold-antibody type (anemia)Primary cold agglutinin disease
Secondary cold agglutinin syndromeAssociated with malignant disease
Acute, transient, infection-associated  (acute cold antibody mediated AIHA complicating Mycoplasma pneumoniae or viral infections)
Chronic (lymphoproliferative disorders): 259 Paroxysmal cold hemoglobinuria: 259 Idiopathic
SecondaryAcute, transient (Infections other than syphilis): 259 
Chronic (syphilis): 259 Mixed cold- and warm-antibody typeIdiopathic
Secondary (lymphoproliferative disorders, autoimmune disorders)Drug-induced immune hemolytic anemia: 259 Autoimmune type
Drug absorption type
Neoantigen type

Management
Acute PCH is usually transient and self-limiting.
The hallmark feature is the formation of polyclonal IgG autoantibody against the P antigen, which is a polysaccharide surface antigen on red cells in most humans. As a weak, biphasic antibody, it absorbs to the P antigen in the cold temperature as in the periphery in the primary phase, and fixes complement on recirculation to the core temperature in the secondary phase, resulting in intravascular hemolysis. Diagnosis
The purposes of laboratory investigations are to: 

Confirm the presence of intravascular hemolysis in terms of metabolic products of red cells and hematopoietic response. Establish the diagnosis of autoimmune hemolytic anemia and differentiate from other AIHA. Identify other supporting features as in hematopathological findings.For intravascular hemolysis, the laboratory parameters include increased serum free hemoglobin, lactate dehydrogenase, unconjugated bilirubin, and reduced haptoglobin. Urine tests may show elevated hemoglobinuria and hemosiderinuria in chronic cases. Reticulocytosis may not be apparent in the acute phase or when there is viral-induced myelosuppression.Once the clinical suspicion of autoimmune hemolytic anemia is made, direct antiglobulin test (DAT) or direct Coombs test is the first line of investigation to confirm the presence of warm autoantibodies. Testing with polyspecific and IgG-specific antiglobulin agents is usually negative, and that with C3-specific agent may be positive. On excluding warm autoimmune hemolytic anemia (WAIHA), the cold agglutinin titer should be examined for cold agglutinin disease (CAD). The diagnosis of PCH is suspected when both WAIHA and CAD are excluded. The complement level is usually low. Donath-Landsteiner test is the confirmatory test for PCH.