Laboratory assessment
Antinuclear antibodies detection by the indirect immunofluorescence assay (IIFA) was performed using HEp-2 cells (Menarini, Florence, Italy) and antigen specificity was determined by ELISA test. Erythrocyte sedimentation rate (ESR), CRP, C3, uric acid, serum creatinine (sCr) and urinalysis were evaluated.
The serum and urine were collected for the determinations of FLC concentrations on OPTILITE instruments (The Binding Site Group Ltd, Birmingham, UK). The quantitative detection of FLC has been obtained using the latex particle enhanced Freelite® k and λ immunoassays (The Binding Site Group Ltd, Birmingham, UK) following the manufacturer’s instructions.
The lower limit of quantification was 0.1 mg/L. The immunoassay consisted of two separate measurements in serum, for free k (normal range: 3.3-19.4 mg/L) and free λ (normal range: 5.7-26.3 mg/L). A ratio of k/λ <0.26 or >1.65 is abnormal, according to the manufacturer’s recommendations. In urine, the normal value of free k is 0.39–15.1 mg/L and free λ is 0.81–10.1 mg/L. A ratio of k/λ uFLC >1.80 is considered abnormal. Samples were thawed only once and immediately assayed in a single batch, following the manufacturer’s instructions. All the determinations were performed by an operator without knowledge of the clinical information of the handled sample. Each sample was tested twice to minimize eventual discrepancies, and all tests were performed in the same laboratory with the same instruments.