Novel use of the Friedewald formula to tackle anomalous HDL-C results in two cases of paraproteinaemia

Abstract

Objectives: We report here two cases of paraproteinaemia with one falsely low and the other dubiously high HDL-cholesterol (HDL-C) results. The spurious results seemed to be related to the nature (IgG or IgM) as well as tshe concentration of the paraproteins. Design and methods: We have been using an alternative approach to estimate the HDL-C concentration by incorporating into it the LDL-cholesterol (LDL-C) value obtained by direct measurements and by back-calculation based on the time-honored Friedewald equation in these atypical specimens as an interim measure, pending optimization of the Roche direct HDL-C plus assay currently in use in our laboratory. Results: This approach is convenient and does not require sophisticated instrumentation. What we are suggesting is to tackle this analytical problems on HDL-C assay due to paraprotein interference by back-calculating the HDL-C values from the measured LDL-C and triglyceride values using the Friedewald formula and is to be regarded as an alternative way to circumvent the interference issue without the need for more elaborative laboratory procedures. We do not intend to advocate screening every single HDL-C value obtained by the direct method for possible analytical errors using this approach. Conclusions: The back-calculation for HDL-C based on the Friedewald formula is conceived by the authors as an alternative and relatively simple way to estimate the HDL-C value in the presence of paraprotein interference, in particular when there is a minus HDL-C value or when the result is dubiously high. By the same token, when the measured HDL-C and the calculated HDL-C do not match further investigations would be warranted to safeguard the validity of the reported result. It is also, to the best of our knowledge, the first time extra bands due to the IgM and IgG paraproteins were demonstrated in the lipoprotein electrophoresis plate.

Introduction

Since the introduction of the homogeneous HDL-cholesterol (HDL-C) assays, there have been continuing reports, mostly anecdotal though, of discrepant HDL-C values in atypical specimens generated by the direct measurements and the conventional precipitation methods that might confound treatment decisions [1]. The presence of paraproteins may cause dubious HDL-C results in some plasma specimens due to paraproteins reacting nonspecifically with the polyethylene glycol (PEG)-modified enzymes and alpha-cyclodextrin sulfate (Roche direct HDL-C plus) reagents at different time points during the absorbance measurements [2], [3]. Reports of significant discrepancies deserve further studies. To our knowledge, no documented evidence, however, is available to date that homogeneous LDL-cholesterol (LDL-C) assays are also susceptible to interference by such paraproteins [4]. The Friedewald formula, published in 1972, serves as the basis for the conventional determination of LDL-C [5]. The relationship between different lipoprotein classes in fasting plasma samples are described by the Friedewald formula rearranged as HDL-C = total cholesterol − (triglyceride / 2.2 + LDL-C), with measurements expressed in mmol/l. We have been using this approach to estimate HDL-C concentration in these atypical plasma samples as an interim measure, pending optimization of the direct assay systems available on the local market. We report here two cases: one falsely low and the other dubiously high HDL-C results. The spurious results seemed to be related to the nature (IgG or IgM) as well as the concentration of the paraproteins.