Serum from patients with abnormally high concentrations of serum proteins, who have altered serum protein ratios or who have received plasma expanders of high molecular weight can cause reagent RBCs to appear agglutinated. Some of these samples cause rouleaux to occur.
Rouleaux formation can be easily recognized microscopically if the RBCs aggregate in what have been described as "stacks of coins." More often, such sera cause aggregates that appear as irregularly shaped clumps that closely resemble antibody mediated agglutinates.
Agglutination versus Rouleaux (Source)