What Does dFLC Mean in AL Amyloidosis Blood Tests?

In AL amyloidosis, dFLC stands for the “difference in free light chains.” “Free” light chains are proteins that circulate in your blood without being attached to larger antibody molecules. The dFLC is a calculated number that estimates the excess amyloid-forming light chains produced by abnormal plasma cells in your bone marrow ^[1][2]. To find this number, your lab subtracts your “uninvolved” light chains from your “involved” light chains ^[1]. Tracking your dFLC over time is one of the most useful ways for you and your care team to measure if your plasma-cell-directed treatment (such as chemotherapy or targeted therapy) is successfully suppressing the disease ^[3][4].

“Involved” vs. “Uninvolved” Light Chains

The immune system naturally produces light chains in two types: kappa and lambda. In AL amyloidosis, abnormal cells in your bone marrow overproduce one of these types.

• Involved Light Chain: This is the specific type (kappa or lambda) that the abnormal cells are mass-producing. It is considered “involved” because it is the protein that misfolds, builds up in organs, and drives the disease ^[1]. Your doctor will determine which type is involved based on your initial diagnostic tests—not just by looking at which number is higher.
• Uninvolved Light Chain: This is the other type of light chain measured in your blood ^[1]. Its level can also fluctuate based on factors like kidney function, inflammation, or the treatments you are receiving.

How to Calculate Your dFLC

The free light chain assay on your lab report will typically list three main numbers: your kappa levels, your lambda levels, and a ratio between the two. The dFLC might not be explicitly written on the report.

You can calculate it using this formula:

For example, if your involved light chain is 150 mg/L and your uninvolved light chain is 20 mg/L, your dFLC is 130 mg/L. This calculation helps account for background fluctuations and provides a clearer estimate of the excess amyloid-forming protein ^[1][2].

Important Note on Units and Labs: Labs report these numbers in either milligrams per liter (mg/L) or milligrams per deciliter (mg/dL) ^[2]. Be very careful when comparing numbers or looking at response targets, because 40 mg/L is exactly the same as 4.0 mg/dL. Always verify your units and confirm your math with your care team. Furthermore, because different testing laboratories use different methods that can produce varying results, it is best to consistently use the same lab and assay for all your tracking ^[5].

Why Tracking dFLC Matters During Treatment

The primary goal of treatment is to stop the abnormal plasma cells from producing the involved light chains. While healing organ damage takes time, your blood light chain levels will often respond to treatment much faster ^[6].

Watching your dFLC drop is a reliable, early sign that the treatment is suppressing the abnormal cells ^[3]. A fast and deep reduction is closely linked to better survival and a higher chance of eventual organ recovery ^[4].

Doctors use specific milestones, known as “hematologic responses,” to measure this success:

• Partial Response (PR): A drop of at least 50% from your starting dFLC (if your starting number was high enough to measure) ^[3].
• Very Good Partial Response (VGPR): A dFLC that drops below 40 mg/L ^[7][3].
• Low-dFLC Target: For patients who start with lower levels, or as a very deep response target, getting the dFLC below 10 mg/L is associated with excellent long-term outcomes and higher organ response rates ^[1][8].
• Complete Response (CR): This is the formal category for a complete hematologic response. It requires more than just a low dFLC; it requires negative serum and urine immunofixation tests (meaning no abnormal proteins are detected) and a normal ratio between your kappa and lambda light chains ^[9].

What dFLC Can and Cannot Tell You

• It CAN show blood response: It helps estimate how well the treatment is suppressing the abnormal plasma cells ^[3].
• It CANNOT measure organ damage directly: dFLC does not measure the amount of amyloid already deposited in your organs ^[6].
• It CANNOT prove organ recovery: Your heart or kidney markers (organ response) will be tracked separately and may improve on a different, slower timeline than your blood markers ^[6][10].
• It CAN be skewed by kidney function: Reduced kidney function can cause free light chains to build up in the blood, making the numbers harder to interpret ^[11]. Your doctor will look at your dFLC trend alongside your kidney function (eGFR) and other tests, rather than relying on a single isolated number ^[12].