Evidence of temperature-dependent interference in an immunonephelometric assay by monoclonal IgM.

To the Editor: Interference by monoclonal IgM has been described in the immunonephelometric assays of C-reactive protein, antistreptolysin O, ferritin, and complement C4 (1–6). This report describes the interference by monoclonal IgM (IgMk) in the immunonephelometric assays of apolipoproteins A-I and B, haptoglobin, and immunoglobulins A and G. The patient, a man 54 years of age, was admitted to a cardiac care unit with angina. His total plasma protein was 110 g/L, his serum cholesterol was 3.3 mmol/L, his apolipoprotein A-I was 1.5 g/L, and his apolipoprotein B was 2 g/L. Myeloma was suggested by the increase in total plasma proteins and verified by immunonephelometric assay of immunoglobulins and by immunofixation electrophoresis (Beckman reagents), which identified a k monoclonal IgM. This monoclonal IgM was not a cryoglobulin because after 8 days at 4 °C, no flocculation was observed in the serum sample. The apolipoproteins, immunoglobulins, and haptoglobin were assayed initially by immunonephelometry with a BNA and a BN II (Behring Nephelometer; Dade-Behring) and on a second sample with a BN 100 and again with a BN II. The BN II is operated at 37 °C, the others at room temperature (;23 °C). The results are shown in Table 1. The BNA (room temperature) appeared to overestimate all proteins except IgM (2.94 g/L), which was underestimated in the crude serum; after the crude serum was diluted, the IgM concentration was 60 g/L. The serum cholesterol value was consistent for apolipoproteins A-I and B with the BN II (37 °C) procedures but not with the room temperature methods. To determine the frequency of this interference by monoclonal immunoglobulins, a double-blind study was conducted that involved determining the proteins with both a BN 100 and a BN II. Among 70 sera with monoclonal immunoglobulins (38 with monoclonal IgM, 12 with monoclonal IgA, 20 with monoclonal IgG), comparable interference occurred only for the index patient (Table 1B). We therefore conclude that the monoclonal IgM could influence the BNA and BN 100 procedures. We (5 ) and others (1–4, 6) have reported interference between monoclonal IgM and latex particles in the immunonephelometric assay of C-reactive protein, antistreptolysin O, and ferritin. However, latex particles and supplement reagents for precipitation were not used in the immunoglobulin and haptoglobin assays by the BNA, BN 100, or BN II procedures. This rules out any dependent unselective precipitation of the reagents. Thus, after excluding a cryoglobulin and the reagents as causal factors of the discrepancy in the presented data, we must conclude that the monoclonal IgMk was the primary reason for the present unselective precipitation during the immunonephelometric assays of proteins, which did not occur when the temperature was increased to 37 °C. We point out the difficulty in taking into account this rare interference because it occurred only once among 38 samples containing monoclonal IgM checked in our laboratory.