There is provided a method of reducing blood free light chain concentration in a subject, the method comprising the step of subjecting the subject's blood to haemodialysis, haemodiafiltration or haemofiltration. There is also provided the use of a dialysis membrane which is a membrane that allows passage of molecules having a molecular weight of up to 45 kDa in presence of whole blood, and has a molecular weight exclusion limit in water of about 200 kDa in the preparation of a haemodialysis unit for conducting haemodialysis, haemodiafiltration or haemofiltration on a subject to reduce blood free light chain concentration in the patient.