Positive findings in repeated serum protein electrophoresis tests after an initial negative result in patients without prior history of plasma cell disorders: a pilot retrospective database study

Background: Serum protein electrophoresis is a routine pathology test that detects monoclonal immunoglobulins. It is primarily used to screen for plasma cell disorders. There is currently limited evidence concerning the positive rate of repeat serum protein electrophoresis following an initial negative result in patients without prior history of plasma cell disorders. In this pilot retrospective observational study, we described the utility of repeat serum protein electrophoresis testing for new findings of monoclonal proteins at a large tertiary health network. Methods: All serum protein electrophoresis performed between 8 January 2019 and 17 November 2020 at Northern Pathology Victoria were retrospectively reviewed. For each serum protein electrophoresis request, the date of collection, test result, patient demographics and clinical indication were extracted from the laboratory information system and clinical records. For patients whose serum protein electrophoresis findings changed from negative to positive monoclonal band(s), the medical records were reviewed to determine whether there was any new diagnosis of plasma cell disorders. Results: In the 23-month study period, there were 4,101 serum protein electrophoresis tests performed in 2,730 patients (median age, 69 years; range, 18–98 years). Repeat testing was performed on 566 (20.7%) of 2,730 patients, accounting for 1,371 (33.4%) of the total tests requested. After excluding patients with a history of plasma cell and/or lymphoproliferative disorders, a total of 160 of the 2,730 patients (5.9%) had two or more serum protein electrophoresis requests. Only three (1.9%) out of the 160 patients with an initial negative test had positive results on subsequent requests. All three patients had low level of monoclonal protein band (≤2 g/L), which is just above the lower limit of serum monoclonal protein quantitation (1 g/L) for the protein electrophoresis method. Conclusions: In this pilot study, repeating serum protein electrophoresis within six months after a negative result generally returns a negative result in patients without prior history of plasma cell disorders. A larger cohort with longer follow-up period is required to better document the clinical progression of this group of patients.