The invention relates to the following methods [1] to [3] and to a kit for carrying out the methods: [1] a method for detecting heart disease, characterized in that the method comprises determining a free immunoglobulin kappa light chain level and/or a free immunoglobulin lambda light chain level of a specimen and comparing at least one value of the kappa chain level, the lambda chain level, and a kappa/lambda ratio with that of a healthy subject; [2] a method for detecting a complication of viral infection in a patient with heart disease, characterized in that the method comprises determining a free immunoglobulin kappa light chain level and/or a free immunoglobulin lambda light chain level of a specimen and comparing at least one value of the kappa chain level, the lambda chain level, and a kappa/lambda ratio with that of a healthy subject; [3] A method for detecting severity of heart disease, characterized in that the method comprises determining a free immunoglobulin kappa light chain level and/or a free immunoglobulin lambda light chain level of a specimen and comparing at least one value of the kappa chain level, the lambda chain level, and a kappa/lambda ratio with that of a healthy subject or a patient suffering slight heart disease.