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Case study: The hook effect in serum free ligh chain assay

Madihah Abbas, Specialist Biochemistry Team Manager at the Christie Pathology Partnership in Manchester, presents a case study.

This case study features a turbidimetric assay used for the quantitation of kappa and lambda free light chains (FLC) in serum. Most commonly, measurement of free light chains in serum aids in the diagnosis and monitoring of multiple myeloma, lymphocytic neoplasms, Waldenström’s macroglobulinaemia, light chain amyloidosis (AL) and light chain deposition disease, in conjunction with other laboratory and clinical findings. 

Serum immunofixation electrophoresis (IFE), serum protein electrophoresis (SPE), and turbidimetric assay are recommended for screening at diagnosis. For the purpose of screening for monoclonal proteins (for all diagnoses except AL), the turbidimetric assay can replace the 24-hour urine IFE, which often presents a difficult sample collection process for the patient.

Antibodies are produced by normal plasma cells to defend the body against infection. Antibodies usually comprise two heavy chains and two light chains. There are five types of heavy chain (IgG, IgA, IgM, IgD and IgE) and two types of light chain (kappa and lambda). In myeloma, all the abnormal plasma cells make the same antibody. Therefore, the disease can be classified by the type of light and heavy chains produced, such as IgM kappa. There is a phenomenon that can occur with the turbidimetric assay called antigen excess. Also known 
as hook or prozone effect, this occurs when very high levels of FLC in a sample give a falsely low result. The assay works to overcome this by initially running all samples at a 1/10 dilution as default and flagging a sample that requires further dilution, by marking with a P. Undetected antigen excess (i.e. no P warning) is rare, but can occur on serum free light chain assays. 

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Picture Credit | Science Photo Library

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