Monoclonal gammapathy <5 g/L: the interest of T/O ratio.

There is currently no consensus for the quantification and identification of weak gammaglobulin monoclonal spike. We conducted a study of 12 months including 410 serum protein electrophoresis (SPE) with suspected weak monoclonal spike in gammaglobulin area without anteriority, which led to the realization of an immunofixation (IF); 276 SPE has a weak spike in gammaglobulins (defined with perpendicular drop method (orthogonal (O) quantification <5 g/L) but only 157 (57%) has monoclonal immunoglobulin by immunofixation of serum. The monitoring of 50 true positive monoclonal immunoglobulins showed the stability of the spike at more than half of the patients and its disappearance in 28% of cases. The number of true positives spike raise when using the tangent skimming method quantification (T) rather than the orthogonal method (O). The distribution of true positives based on T/O ratio propose a 6% threshold below which a monoclonal immunoglobulin is rarely found (VPNR>6%=73.7%), and a 10% threshold above more than 80% of true positives are detected (VPPR>10%=94.4%). The use of the T/O ratio could i) help to predict the presence of an IM when low intensity spike is detected, and help to choose between the realization of an immunofixation or a control of SPE later; ii) be a key tool for an inter-laboratory harmonization of IM follow up.