Accurate determination of the immunoglobulin (Ig) M paraprotein concentration is crucial to evaluating response in patients with Waldenström macroglobulinemia (WM). In most clinical laboratories, M-spike quantitation is performed by serum protein electrophoresis, which is the same method used to quantitate IgG and IgA paraproteins in patients with multiple myeloma (MM). However, the migration pattern and propensity of IgM paraproteins to form higher-order complexes in serum makes laboratory evaluation of samples from patients with WM especially challenging. We review examples of patients whose IgM paraprotein is particularly ill-suited to M-spike quantitation by serum protein electrophoresis: a case of “sticky M,” a case of IgM multimers that cannot be resolved, and a case of an IgM in the β region. In these and similar cases, a method other than M-spike quantitation, such as IgM heavy chain nephelometry, should be considered in laboratory evaluation of paraprotein concentration.