Diagnosis of myeloma and related plasma cell disorders

Morphology Central to the diagnosis of myeloma is the demonstration of bone marrow infiltration by monoclonal plasma cells. The extent of infiltration typically exceeds 10% although it is well recognized that marrow disease can be patchy and a minority of patients with symptomatic myeloma will have <10% bone marrow plasma cells (BMPC)[1]. The 10% threshold chosen to distinguish myeloma from MGUS is somewhat arbitrary as both disorders form a continuous spectrum and it is therefore essential that the extent of marrow infiltration is correlated with the clinical features before a definitive diagnosis is made. It is recommended that a trephine biopsy be obtained in most patients as it provides a better assessment of the extent of marrow infiltration than even the best quality aspirate smears[2]. It is well recognized that bone marrow aspirate plasma cell counts consistently underestimate the overall level of infiltration[3–6]. An adequate trephine also ensures that a diagnosis can be made when the bone marrow aspirate specimen is of poor quality. Plasma cell cytomorphology varies considerably from patient to patient and a number of groups have proposed classification schema and have also demonstrated that cases with so-called blastic cytology have an inferior outcome[7–9]. However, such morphological classification schemes are poorly reproducible and have been largely superseded as prognostic tools by cytogenetic abnormalities and clinical parameters such as the International Staging System (ISS).