Multiple Myeloma: This is the most common cause of rouleaux formation on peripheral blood smear (PBS), due to the high levels of monoclonal proteins (M-proteins) that increase blood viscosity and lead to the stacking of red blood cells.

Waldenström's Macroglobulinemia: Similar to multiple myeloma, this condition involves the production of high levels of monoclonal IgM proteins, which can cause rouleaux formation due to increased blood viscosity.
Chronic Inflammatory Conditions: Conditions like chronic infections, autoimmune diseases, or chronic liver disease can lead to the production of acute phase reactants, which may cause rouleaux formation.
Polycythemia Vera: Although not as common, this myeloproliferative disorder can lead to increased red blood cell mass and viscosity, potentially causing rouleaux.

Hyperviscosity Syndrome: This is a condition characterized by increased blood viscosity due to various causes, including monoclonal gammopathies, polycythemia, or other conditions. It's crucial to identify and treat promptly to avoid serious complications.
Sickle Cell Disease: In some cases, sickle cell disease can present with rouleaux formation, especially during crises. Missing this diagnosis could lead to delayed treatment and poor outcomes.

Cryoglobulinemia: This condition involves the presence of cryoglobulins (proteins that precipitate at low temperatures) in the blood, which can increase viscosity and lead to rouleaux formation.
Paraproteinemias: Other paraproteinemias, such as heavy chain disease or primary amyloidosis, can also cause rouleaux formation, although these are relatively rare conditions.