The Biologic and Analytic Variability of Serum Protein Electrophoresis M-Spike, Nephelometric Ig Quantitation, Serum FLC Quantitation, and Urine M-Spike in Monoclonal Gammopathies.

Plasma cell proliferative disorders are monitored by a variety of methods. Serum protein electrophoresis (SPEP) and/or urine PEP M-spike quantitation are commonly assessed in patients with monoclonal gammopathy of undetermined significance (MGUS), smoldering multiple myeloma (SMM), and multiple myeloma (MM) to determine disease progression, response, or relapse. Serum immunoglobin (Ig) concentrations can be quantitated when the M-spike is large or if the migration is obscured within the SPEP beta fraction. Serum FLC quantitation provides a rapid indicator of response, will detect the rare occurrence of FLC escape, and will allow disease monitoring in the absence of a measurable serum or urine M-spike. The International Myeloma Working Group (IMWG) has recommended that the serum and urine M-spike should be used to monitor disease, and that FLC quantitation should be used only if there is no measurable disease by electrophoresis and if the monoclonal FLC concentration is greater than 10 mg/dL in the context of an abnormal FLC K/L ratio. We have studied serial samples in clinically stable patients in order to assess the total variability (analytic and biologic) of these assays and to confirm the IMWG recent recommendations.

Serial data from stable MGUS patients (n=35) were identified by the availability of all 3 serum test results (M-spike, Ig, FLC) in at least 4 serial samples that were obtained 9 months to 5 years apart and whose serum M-spikes varied by less than 25%. For MM (n=60) and SMM (n=48) the samples were within 9-15 months and serum M-spikes varied by less than 0.5 g/dL. Among the 60 MM, 48 SMM, and 35 MGUS patients, there were 23, 41, and 18 patients with measurable disease by serum M-spike (i.e. M-spike >1 g/dL); 19, 10, and 10 patients with an evaluable FLC (i.e. monoclonal FLC > 10 mg/dL and an abnormal FLC ratio); and 5, 5, and 1 patient with an evaluable urine (i.e. M-spike > 200mg/24 hr). The FLC data was analyzed as the involved FLC concentration (iFLC), the difference between the involved and uninvolved FLC concentration (dFLC), and the FLC K/L ratio (rFLC). The coefficients of variability (CV) were determined for each methodology in each patient sample set, and the average CVs were determined. Igs were quantitated by immunonephelometry using a Siemens BNII and Siemens reagent sets; kappa and lambda FLC were quantitated using a Siemens BNII and Freelite reagent sets from The Binding Site; M-spikes were quantitated using Helena SPIFE SPE and reagent sets.

The CVs for the Ig quantitation, SPEP M-spike, FLC quantitation, and urine M-spike in each of the patient groups are listed in the table:

Our laboratory's interassay analytic CV for replicate samples are 4-5% for Ig quantitation, 6-8% for SPEP M-spikes, 6-7% for FLC quantitation, and 5-7% for urine M-spikes. The analytic CVs of the methods are similar, but the total (analytic + biologic) CVs are very different. The samples have been selected by restricting the variability of serum M-spike values; when we apply the same criteria to the IgG quantitation, the IgG total CV comes closer to the serum M-spike CVs. The remaining differences, however, may be due to biologic variability contributed by polyclonal Ig. The total CV for iFLC is similar to the urine M-spike CV and suggests a previously unrecognized large biologic CV for serum FLC. The iFLC and dFLC CVs were comparable but were smaller than the rFLC CV.

The variability of the serum and urine M-spike, Ig, and FLC measurements confirm the IMWG recommendations for patient monitoring. If a patient has a measurable M-spike >1 g/dL, then the serum M-spike should be followed. If there is no measurable disease, then the iFLC can be monitored, provided that the rFLC is abnormal and the iFLC concentration is >10 mg/dL. Although the number of patients with evaluable urine M-spikes in this study is small, larger studies may confirm the utility of serum FLC compared to urine M-spike for monitoring patients with monoclonal gammopathies.

No relevant conflicts of interest to declare.