The immunoglobulin free light chain assay (FREELITE, Binding Site, Birmingham, UK) is a very important assay that has been in use for nearly a decade. Its importance has been growing over time, and new uses for it have also been emerging.
What is an immunoglobulin?
Before defining what the immunoglobulin free light chain is, we first must understand what immunoglobulins are. Immunoglobulins are the proteins made by myeloma cells and their normal counterparts called plasma cells.
Their role in health is to help us fight infections. Another name for immunoglobulin is antibody, and as doctors follow patients with myeloma and related disorders, they may refer to these proteins as M-proteins, M-spikes, or monoclonal proteins. These immunoglobulin proteins can be handy markers to follow the status of disease since for a given patient, the amount of these immunoglobulin proteins found in the blood and/or urine typically reflects how much myeloma (or abnormal plasma cell population) there is, providing doctors and patients with a convenient means of knowing on a month-to-month basis what is going on in the bone marrow. Monthly blood and/or urine tests are far easier than monthly bone marrow biopsies!
What is the immunoglobulin free light chain?
As shown in the figure, an intact (whole) immunoglobulin is made up of two "heavy chains" and two "light chains." Most of the time, there are nearly equal amounts of the heavy and light chains produced and secreted into the blood stream, so there is little "free" or unattached immunoglobulin light chain. In some patients, however, almost all of the secreted immunoglobulin is light chain without accompanying heavy chain. Either little to no heavy chain is made. Since these light chains are very small, they do not stay in blood stream. To measure them, one has to look in the urine by doing the test called "24-hour urine protein electrophoresis," or one can measure them in blood using the specialized test called the "immunoglobulin free light chain assay," also known as FREELITE. This test can detect and quantify the amount of unbound or free immunoglobulin light chains in the blood stream, allowing the myeloma of certain patients to be followed with a simple blood test.
Any one plasma cell (or myeloma cell) makes one of two types of immunoglobulin light chain, either kappa or lambda light chain. The immunoglobulin free light chain assay, therefore, is capable of measuring both kappa and lambda free light chains. When the test is done, three results are provided: 1) the amount of kappa free light chain in the blood; 2) the amount of lambda free light chain in the blood; and 3) the ratio of the kappa and lambda free light chain in the blood. Everyone—even those people without myeloma and related disorders—have low levels of measureable serum immunoglobulin free light chains. There is a normal balance of kappa and lambda free light chains circulating in the bloodstream in health; the normal ratio of kappa and lambda in the blood is 0.26-1.65. The reason we bother with considering the ratio is that problems other than bone marrow conditions like myeloma can cause elevations in immunoglobulin free light chains, but both kappa and lambda are equally affected, such that the ratio of one to the other remains within the normal range. The two most common conditions that raise the absolute values of serum immunoglobulin free light chains—but not their ratio—are kidney disease and inflammatory conditions.
Which patients should have their serum immunoglobulin free light chains measured?
The test is important for baseline risk estimation among patients with a new diagnosis of a pre-cancerous condition called monoclonal gammopathy of undetermined significance (MGUS), immunoglobulin light chain amyloidosis, multiple myeloma, solitary plasmacytoma of bone, and diffuse large cell lymphoma.
Serial measurements of the serum immunoglobulin free light chain are most helpful for the following patients
- Patients who present with myeloma kidney.
The test can provide information about whether there is rapid killing of myeloma cells within weeks of starting therapy.
- Patients with "oligosecretory" (very low-secreting) light chain myeloma.
There are some patients whose myeloma cells make such a small amount of monoclonal protein that other standard myeloma blood and urine tests cannot measure it.
- Patients with "light-chain" or "Bence Jones" myeloma
About 20% of patients with multiple myeloma do not make any immunoglobulin heavy chain to go with their immunoglobulin light chain. Historically, the only way to follow these patients was to do a 24-hour urine protein electrophoresis. Most patients and laboratories do not like the bother of the collection and handling of a 24-hour urine protein. There are emerging data that the serum immunoglobulin free light chain may be able to replace the 24-hour urine protein electrophoresis for routine clinical practice, but the level of evidence is not sufficient that the 24-hour urine collection has not yet been dispensed with for clinical trials. Until these data are forthcoming, it is reasonable to do a baseline 24-hour urine collection for protein electrophoresis along with a serum immunoglobulin free light chain, but follow the patient with monthly or quarterly serum immunoglobulin free light chains, depending on the patient’s status. A 24-hour urine protein electrophoresis could be repeated at times of decision-making, that is at establishment of response or progression.
- Patients with immunoglobulin light-chain amyloidosis (primary amyloidosis)
The majority of these patients have elevated immunoglobulin free light chains, and this is the test of choice for the majority of these patients.
Since there are 3 numbers that are reported, which one should I pay attention to?
This is one of the most confusing aspects of the assay for patients and doctors alike. For most baseline risk assessment schemes, the ratio of the kappa to lambda is used. The ratio is also used to document clonality (a cancerous or pre-cancerous status of plasma cells). Because every patient’s myeloma makes either kappa or lambda light chains, the excess of either (but typically not both)kappa or lambda free light chain (FLC) will yield an abnormal ratio, and suggests clonality.
In contrast, if serial measurements are being done, the ratio loses its importance in most situations, with the major exception of establishing a very deep response to therapy called a "stringent complete response." When serial measurements are made to document disease improvement, stability, or worsening, the "involved" serum immunoglobulin FLC is most important. In an individual with "kappa" myeloma, the kappa FLC is the most important result; in a patient with "lambda" myeloma, the lambda FLC is the most important result. The FLC that reflects the amount of your myeloma is your "involved" FLC. Substantial (i.e. more than 25 to 50%) increases and decreases of the involved FLC suggest a change in disease status. Because kidney issues can increase both the "involved" and the "uninvolved" FLC, a handy trick for serial measurements is to subtract the "uninvolved" from the "involved" FLC. The result is called the "FLC difference," and it is the measurement used in calculating response for patients on clinical trials.
Important things to remember about the immunoglobulin free light chain assay
For the majority of patients with multiple myeloma, the assay needs only to be done at baseline, once every year or so to exclude a phenomenon called "light chain escape," and to document stringent complete response.
- The way serum immunoglobulin free light chains are reported is not standardized. Some laboratories report the result in mg/dL while others report mg/L. The latter value will be 10-times higher than the former. Not realizing this can result in needless anxiety and an inappropriate designation of disease status.
- The amount of free light chain in the blood stream can vary significantly from day to day due to degrees of dehydration or hydration as well as other factors. As in the case for 24-hour urine protein measurements, changes of involved FLC of less than 50% should be taken with a grain of salt. The test should be repeated to make sure that there is a clear trend.