Serum Folate Measurement by Chemiluminescence in Patients with Paraproteinaemia

Introduction: Paraproteins cause interference in many assays systems due to increased viscosity and/or by non-specific binding to either analytes or reagents which may variably affect the results. There is abundant evidence to indicate that performance characteristics of automated, competitive protein-binding assays for folate assays are normally sound, but there is a paucity of data comparing folate assays in patients with paraproteinaemia compared to normals.

Method: Serum samples were prepared from venous blood taken into Greiner Vacuette containers (code: 456018) in fifty patients with paraproteinaemia (IgG Kappa, n = 20: IgG Lambda, n = 9: IgM Kappa, n = 6: IgM Lambda, n = 4: IgA Kappa, n = 5: IgA Lambda, n = 3: multiclonal, n = 3). Serum folate assays were measured according to manufacturer’s instructions on Beckman-Coulter Access and Abbott Architect haematinics analysers on the day of sample collection.

Results: The ratio of means of serum protein concentrations in the paraprotein sub-groups compared to the mean values of age/gender related reference ranges were: IgG = 1.57: IgM = 7.8: IgA = 6.3. Serum albumin concentration was normal in all patients. Mean serum folate results (±1SD) for paraprotein patients and normals controls (laboratory personnel) are shown in the table below:

When analysed using the paired-t-test serum folate was significantly different in normal subjects using the Access compared to the Architect. However, no such significant difference between the two systems was observed in the paraproteinaemia group. Although the mean serum folate in normals was higher compared to the paraprotein group as analysed by the Access, this difference was not significant when analysed using the two-sample t-test. Interestingly the mean serum folate when analysed in the two groups using the Architect gave identical mean values. There were no statistically significant differences between the two methods in any of the paraprotein sub-types (p>0.05 in each case). Conclusions: The complex and variable mixture of proteins present in paraproteinaemias pose potential erroneous measurement in immunoassay systems. This is because they may interfere with the associated antigen-antibody reactions which are dependant on complimentary matching shapes being assumed by the antibody variable regions of the immunoglobulin. Despite this, our data using two distinct chemiluminescence analysers for folate measurement indicate no statistically significant difference for this parameter between patients with paraproteinaemia compared to normal controls. However, it has yet to be determined whether differing results between these two groups are generated using different analytical techniques.