Approach
Identification of a monoclonal protein in the serum or urine requires one or more laboratory tests. Its presence is often unaccompanied by symptoms. Detection of a monoclonal protein typically occurs:
In the context of routine assessment where an elevated total protein or elevated sedimentation rate leads to additional specific testing
In the presence of a set of symptoms and signs suggestive of a monoclonal protein-associated condition such as myeloma, where a specific search is undertaken for the monoclonal protein.
Monoclonal gammopathy of undetermined significance (MGUS) is the most common monoclonal gammopathy. It is an asymptomatic pre-malignant disorder associated with relatively low risk (on average 1% per year) of progression to multiple myeloma or related lymphoproliferative malignancies. However, there are other conditions where a monoclonal protein may be detected in the serum and/or urine. These include:
Lymphoproliferative diseases where clonal B lineage cells secrete a monoclonal protein (chronic lymphocytic leukaemia, non-Hodgkin's lymphoma, post-transplant monoclonal gammopathies)[26]
Conditions associated with or predisposing to a higher prevalence of monoclonal gammopathy (hepatitis C virus infection, HIV infection)
Infectious or inflammatory conditions associated with a transient development of several clones of reactive B cell/plasma cell populations (systemic lupus erythematosus [SLE], rheumatoid arthritis, psoriatic arthritis, Sjogren syndrome, Schnitzler syndrome).
Smoldering multiple myeloma is a clinical entity that represents the transitional stage between MGUS and active or symptomatic multiple myeloma. While it is not a distinct biological entity, its recognition is critical to determining the optimal follow-up of these patients. Patients with smoldering multiple myeloma are defined by the presence of a higher plasma cell burden (M spike >3 g/dL or marrow with 10% to 60% plasma cells) but without any of the features consistent with active disease (CRAB: hypercalcaemia, renal insufficiency, anaemia, and bone lesions).[57] In addition, the criteria for diagnosis of myeloma requiring therapy have been redefined to include those patients with a serum free light chain (sFLC) ratio ≥100, bone marrow plasma cell content ≥60%, and those with more than one lesion on MRI.[58] Biologically, smoldering multiple myeloma consists of a mix of patients with MGUS-like biology and patients who have undergone the transformation to myeloma at a molecular level but have not developed manifestations of active disease. Long-term follow up of these patients indicate a substantial risk of progression to myeloma (10% per year) during the first 5 years, which then decreases to some extent and approaches that of MGUS after 10 years. Hence, patients with smoldering myeloma should be observed closely for the development of any of the myeloma-defining events.[59]
Light-chain MGUS is a clinical entity that has been described as a pre-malignant lesion for light-chain multiple myeloma corresponding to MGUS.[60] Patients with light-chain MGUS have been shown to have a lower progression rate (0.3% per year) than those patients with conventional MGUS (1% per year).[60]
Clinical manifestations
Most patients are asymptomatic. Patients with monoclonal protein-associated disorders such as symptomatic myeloma or amyloidosis may present with the following signs and symptoms:
Weakness and fatigue (secondary to anaemia, hypercalcaemia)
Pains (due to lytic bone lesions with or without fractures, vertebral compression fractures)
Confusion, altered mental status (hypercalcaemia, hyperviscosity, uraemia)
Numbness and tingling (peripheral neuropathy, cord compression, hypercalcaemia)[61]
Paraplegic or quadriplegic symptoms (spinal cord compression due to plasmacytoma or vertebral compression fracture)
Visual symptoms, headaches (hyperviscosity)
Recurrent infections (hypogammaglobulinaemia due to suppression of normal immunoglobulins)
Polyuria, polydipsia, constipation (hypercalcaemia)
Increased bleeding and bruising (thrombocytopenia, hyperviscosity, uraemia, acquired clotting factor inhibitors, non-specific interference with bleeding cascade by M-protein)
Peripheral oedema (nephrotic syndrome, hypoalbuminaemia, renal failure)
Loss of height, kyphosis (vertebral compression fractures)
Dysphagia, jaw claudication, diarrhoea, and macroglossia (in patients with amyloidosis).
A complete physical examination should be performed with particular attention to the presence of macroglossia, peripheral neuropathy, oedema, spine tenderness, hepatosplenomegaly, and skin lesions, including periorbital purpura and petechiae.
Investigations
Whether triggered by an incidental abnormality in a laboratory test, a finding that suggests one of the monoclonal gammopathies, or a clinical scenario suggestive of an associated disorder (e.g., SLE, rheumatoid arthritis, psoriatic arthritis, Sjogren syndrome, Schnitzler syndrome, hepatitis C virus infection, HIV infection, chronic lymphocytic leukaemia, non-Hodgkin's lymphoma), the initial screening tests should include a serum protein electrophoresis (SPEP) and a 24-hour urine collection, with immunofixation performed on both samples.[11][21][62] Studies have suggested that the 24-hour urine examination may be replaced by a serum free light chain (sFLC) assay when screening for a monoclonal protein.[63] This test identifies situations associated with light chain only production and secretion by the plasma cells.[64]
Once a monoclonal protein is confirmed in the serum or urine, subsequent tests will depend on the clinical situation. If the discovery was the result of an abnormal laboratory test and the patient is asymptomatic, limited additional testing is required. A comprehensive history covering the presence or absence of symptoms is essential. Most of these patients have MGUS. At this stage, the possibility of multiple myeloma, amyloidosis, or one of the other conditions that would require therapy should be ruled out.[26] The quantity of monoclonal protein indicates the need for additional testing:
If the M protein level is 15 g/L (1.5 g/dL) or lower and the patient is asymptomatic with no other laboratory or physical examination abnormalities, this likely represents a MGUS, and a repeat evaluation can be performed in 3 to 6 months to ensure stability.[65]
In patients with an M protein level of >15 g/L (>1.5 g/dL), the likelihood of myeloma is higher and additional assessment is warranted.
A bone marrow biopsy and a skeletal survey, including long bones, should be performed. Low-dose whole-body CT or the CT scan portion of a PET scan can also allow evaluation for lytic bone disease. Findings may include demonstration of monoclonal plasma cells on bone marrow: punched out lytic lesions, vertebral compression fractures, pathological fractures, or generalised osteopenia on skeletal films.
If any assessment raises concerns for myeloma, additional work-up may include an FBC, serum chemistry (including calcium, creatinine, beta 2 microglobulin, and lactate dehydrogenase [LDH]), and fluorescence in situ hybridisation (FISH) in bone marrow.
If there is suspicion of amyloidosis, a fat aspirate and bone marrow biopsy is recommended. These can be examined for amyloid deposits as well as (in the case of the bone marrow biopsy) showing the monoclonal plasma cell population.
Additional evaluations will depend on the clinical scenario with respect to the symptoms and findings upon physical examination. All patients should have an FBC, serum chemistry, including serum creatinine, serum calcium that should be corrected for albumin (albumin value can be obtained from the SPEP), bilirubin (total and direct), alkaline phosphatase, liver enzymes, and urinalysis. Common laboratory findings can include, in addition to the monoclonal protein: anaemia, thrombocytopenia, rouleaux formation (stacked coin appearance) on peripheral smear, elevated erythrocyte sedimentation rate (ESR), circulating clonal plasma cells, hypogammaglobulinaemia, hypercalcaemia, hyperuricaemia, elevated creatinine and urea, elevated LDH, beta 2 microglobulin, and C-reactive protein, elevated serum viscosity, or bone marrow plasmacytosis.
Detection, identification, and quantitation of monoclonal proteins
The most commonly used test is serum protein electrophoresis (SPE or SPEP), an inexpensive and easy procedure to perform screening.[66][67] It is typically performed using the agarose gel method or the less commonly used capillary zone electrophoresis. Using SPEP, the serum proteins can be separated into 5 groups based on their electrophoretic mobility (albumin, alpha-1, alpha-2, beta, and gamma), and the M protein, when present, can be quantitated by means of a densitometric tracing of the gel. The various immunoglobulin classes are usually of gamma mobility and are found in the gamma region, but occasionally they may also be found in the beta-gamma and beta regions as seen with IgA. [Figure caption and citation for the preceding image starts]: Serum protein electrophoresisFrom the personal collection of Dr Kumar [Citation ends].
Urine protein electrophoresis (UPEP) requires a 24-hour urine collection in order to estimate the total amount of protein excreted per day. The quantity of M-protein excreted can be determined by estimating the M-protein as a proportion of the total protein in the densitometer tracing, and then multiplying it by the total 24-hour urinary protein excretion. This approach helps to overcome the wide variability in the daily urinary volume.
SPEP and UPEP techniques have several limitations. They fail to detect small monoclonal proteins, and to provide information on the type of M-protein.
Immunofixation involves electrophoretic separation of the proteins followed by staining with a set of antibodies specific for each of the heavy chains and light chains, enabling the immunoglobulin type to be characterised. Serum or urine immunofixation as a subsequent step can detect small amounts of M-protein and form an essential part of screening in conjunction with SPEP or UPEP. Use of mass spectrometry-based detection and follow-up of the monoclonal protein has also afforded a more sensitive approach to detecting a very small amount of monoclonal protein.[68]
Quantitation of immunoglobulins can be achieved by nephelometry, which involves measurement of the degree of turbidity produced by antigen-antibody interaction. This is particularly useful for detecting hypogammaglobulinaemia, in serial estimations of IgA monoclonal protein that may be difficult to detect on SPEP, and in situations with high levels of monoclonal protein, where SPEP may underestimate the actual degree of monoclonal protein elevation. However, it does not provide any information on the monoclonal nature of the protein.
In some patients, only small amounts of light chains are produced without an accompanying heavy chain and this may be difficult to detect using SPEP. Immunoassays are now available for detecting low concentrations of monoclonal free light chains in serum, using antibodies that recognise epitopes that are usually hidden when the light chain is bound to the heavy chain.[64][69] This assay is useful in the diagnosis, prognosis, and response to treatment in several monoclonal plasma cell disorders, including MGUS, primary amyloidosis, and multiple myeloma.[70]
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