The invention provides a method for characterising a plasma cell associated disease in a patient comprising: (i)providing at least one sample from the patient; (ii)determining in the sample(s) two or more of; (a) the kappa:lambda free light chain (FLC) ratio; (b) the ratio of kappa light chains bound to a class of heavy chain: lambda light chain bound to the same class of heavy chain (HLCkappa : HLC lambda ratio); (c) the total amount of FLC in the samples and (d) the total amount of kappa light chains bound to the heavy chain class plus lambda light chains bound to the same heavy chain class (total HLC); (iii)comparing each ratio or amount from (a) (b), (c) and/or (d) to predetermined values and assigning a score to each amount or ratio; and (iv)using the scores to characterise the plasma cell associated disease. Apparatus configured to carry out the method of the invention are also provided. The invention also provides a kit comprising, in combination, (i) anti-kappa FLC specific and anti-lambda FLC specific antibodies or fragments thereof and (ii) anti-kappa heavy chain class specific and anti-lambda heavy chain class specific antibodies or fragments thereof, optionally mixed together.