A case of monoclonal gammopathy of undetermined significance with abnormal low levels of plasma glycated albumin by M protein

It is known that an immunoglobulin abnormality affects various clinical laboratory measurements and leads to abnormal values. We experienced a case of monoclonal gammopathy of undetermined significance (MGUS) showing a falsely low plasma glycated albumin (GA) level. The patient was a 75-y-old male who visited our hospital for thrombocytosis identified during a medical checkup. Based on further examinations, he was diagnosed with MGUS (IgM-κ type). Laboratory examinations revealed that the plasma GA level was significantly low at −1.3% but the serum GA level was reasonable at 15.5%. We investigated the cause of the falsely low plasma GA level. The patient's plasma became turbid after mixing with the first reagent for GA measurement. The plasma GA level was increased by dilution of the plasma. The plasma GA level was falsely decreased only at the time of measurement on a sample collected using a blood-collecting tube with heparin sodium. The GA level was decreased by adding heparin sodium to the patient's serum, whereas the GA level was increased by neutralization of the patient's plasma with protamine sulfate. The GA level was increased after adding polyethylene glycol to the patient's plasma. Serum GA levels in healthy controls were decreased by adding purified M protein from the patient's serum. We report a patient with MGUS whose plasma GA concentration was falsely decreased by M protein when blood was drawn in a heparin sodium-containing tube.