Final answer:
To differentiate between rouleaux formation and a cold agglutinin when a positive auto control is observed, a saline replacement technique should be performed. This technique helps to determine if agglutination disperses in saline (indicating rouleaux) or remains (indicating cold agglutinins). Additionally, a direct Coombs' test can confirm if the agglutination is due to immune processes.
Step-by-step explanation:
To determine if the positive auto control is due to rouleaux formation or a cold agglutinin, one would perform a saline replacement technique (option b). This involves washing the red blood cells and subsequently resuspending them in saline. The agglutination caused by rouleaux will typically disperse in saline, whereas agglutination due to cold agglutinins will not. A direct Coombs' test (DAT) or direct antiglobulin test can also be useful to verify if the agglutination is immune-mediated. The presence of nonagglutinating antibodies or complement proteins on red blood cells in vivo is detected by DAT.
Rouleaux formation involves the stacking of red blood cells due to increased plasma proteins and typically disperses when saline is added. Cold agglutinins refer to antibodies that bind red blood cells at colder temperatures and cause agglutination; these remain even after the saline wash.