[IgM-Lambda Type Paraproteinemia Interferes with Haptoglobin Measurements].

Paraproteinemia is a condition induced by an increase in paraprotein. Paraprotein is a monoclonal immu- noglobulin produced by plasma cells as a result of aging or malignancy. Paraprotein often induces a variety of laboratory test abnormalities by interfering with laboratory test reagents. The haptoglobin (Hp) measurement in a 55-year-old woman with IgM-lambda type paraproteinemia associated with lymphoplasmacytic lymphoma was found to be falsely low because of white-turbidity caused by an abnormal reaction. An in- creased absorbance was observed at 596 nm after the addition of the buffer reagent and was reduced by dilution with saline. This result indicates the existence of interfering substances in the patient's sample. To identify the inhibitors, we obtained the white-turbidity pellet by centrifugation of a mixture of the patient's serum and the Hp buffer reagent. A high IgM concentration was observed in the white-turbidity pellet. Moreover, a correlation was observed in a time series between the IgM concentration in sera and the extent of the turbidity during the Hp measurement. These results indicated that IgM was the main component of the white-turbidity. Next, we showed that the addition of the white-turbidity pellet to normal serum caused an increase in absorbance and a false low Hp value. A correlation was also observed in a time series be- tween the IgM concentration in sera and the rate of the Hp reduction. These results suggest that the false low Hp measurements were due to IgM present in the white-turbidity. In conclusion, false low values of Hp may occur in patients with IgM-lambda type paraproteinemia. Therefore, the presence or absence of an increase in absorbance after the addition of the buffer reagent in a time-course reaction may be required.