BACKGROUND: We examined whether the use of test result interval-specific likelihood ratios (LR) could improve the clinical interpretation of serum FLC kappa/lambda ratio for the diagnosis of malignant plasma cell disorders. METHODS: We calculated LRs for different FLC kappa/lambda intervals using sera from patients diagnosed with intact multiple myeloma (MM), light chain MM (LCMM), non-secretory MM (NSMM) and light chain amyloidosis (AL-A). Consecutive patients with a clinical suspicion of a monoclonal B-cell disorder that were diagnosed with MGUS or no B-cell monoclonal disorder served as the disease control group. RESULTS: Using LRs for different test result intervals, a distinction can be made between FLC kappa/lambda ratios that are within the normal diagnostic range, ratios that are inconclusive (1.66-5.0, LR+/-1), ratios that indicate the possible presence of a malignant plasma cell disorder (0.05-0.25 and >5.0-10, LR+/-10) and ratios that were suggestive of a malignant plasma cell disorder (<0.05 or >10; LR+/-50). A FLC kappa/lambda ratio within the normal diagnostic range virtually excluded LCMM and AL-A, but not intact MM or NSMM. CONCLUSIONS: Interpreting serum FLC kappa/lambda ratios using LRs for different result intervals improves the clinical interpretation for the diagnosis of malignant plasma cell disorders excluding plasmacytoma.