Have you heard the term "Free Light Chains"? Here is a great explanation from a MM patient

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This is a good short explanation from another MM patient: "Antibodies in blood are made of 4 protein chains, 2 heavy, and 2 light. The 2 light chains are different and are called kappa and lambda. In MM, some of the plasma cells that make one or the other of them start reproducing uncontrollably. That is cancer.
For most people who have MM, one of the immunoglobulins is also over produced (there are several, designated by letters, such as IgG or IgA). When the protein in your blood is subjected to electrophoresis, the proteins are separated by weight. The over-produced immunoglobulin shows up as an M-spike, and where the M-spike is indicates which immunoglobulin you are over-producing.
A minority of people with MM have what is called Light Chain disease, which means that they don't over-produce one of the immunoglobulins, and they don't usually have an M-spike.
In people without MM, the ratio between the Kappa and Lambda forms is about 1.65. If the ratio is much larger than 1.65, then

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continued: then the form of MM is Kappa. If the ratio is much smaller than 1.65, then the form of MM is Lambda. In general, trends are more important than individual values, and the ratio (usually reported as K/L ratio, but in the case of people with very high Lambda values, it can be reported as an L/K ratio) is more important than individual free light chain values. AFAIK, there is no difference in the treatment of the different forms. The genetic anomolies present in the myeloma plasma cells can determine how aggressive the MM is."