Clinical conditions, such as multiple myeloma, Waldenstrom macroglobulinemia, and certain leukemias, can impact ABO testing. For example, there is an increase in the amount of circulating immunoglobulin in multiple myeloma, which results in an overall increase in total circulating protein. This increase in protein can cause interference in the forward ABO typing (Table 10). Red blood cells coated with protein have a tendency to clump together and demonstrate a "stacked coins" appearance. This is called rouleaux. Rouleaux can macroscopically look like agglutination; however, with vigorous shaking, the reaction will dissipate.
In vitro, rouleaux can be misinterpreted as agglutination, causing discrepancy between the patient's forward and reverse testing. Although this can be a significant discrepancy, it is one of the easiest to resolve.
Table 10. Example of Initial Testing from a Patient with Multiple Myeloma. Note that the forward type results as AB while the reverse type results as O.
If the gammopathy is known, the antibody screen is negative, and no antibodies are noted in the patient's historical record, it can be hypothesized that the patient's red blood cells are coated with excess protein. This excess protein is causing rouleaux formation, which is what we are seeing in this forward typing.
A simple three-fold wash with physiological saline should be enough to remove the excess protein (antibody) and allow proper antigen detection in the forward type (Table 11). Further washing could be required if the reactions are not eliminated in their entirety.
Table 11. Example of Results after Washing with Saline. | Anti-A | Anti-B | A1 cells | B cells |
| Patient Results | 0 | 0 | 4+ | 4+ |
