DL test.^12,13 This test is diagnostic of PCH and can be performed with patient serum or whole blood although serum is preferred. Testing requires receipt of sample at 37°C to prevent binding of antibodies to patient cells. Three sets of 3 tubes are prepared as follows: (1) patient serum, (2) patient serum and normal serum, and (3) normal serum only. Normal serum is added to patient serum to supplement complement factors, and normal serum alone should act as a negative control. P-antigen–positive reagent cells are added to each tube. The first tube of each set is incubated in ice at ∼0°C and then brought to 37°C, the second tube of each set is incubated in ice only, and the third tube is incubated at 37°C only. Hemolysis in the first tube of sets 1 and 2 is a positive test.