A fat pad biopsy is a simple procedure that removes a small sample of fat from just beneath the skin of your abdomen, typically near the belly button. It is primarily used to check for amyloidosis, a condition where abnormal proteins build up in your organs and tissues and gradually impair their function. The procedure is considered the first-choice screening biopsy for suspected systemic amyloidosis because it’s low-risk, inexpensive, and can be done in a clinic without sedation.
Why Fat Tissue Reveals Amyloidosis
Amyloidosis occurs when misfolded proteins, called amyloid, deposit throughout the body. These proteins don’t stay confined to a single organ. They circulate in the bloodstream and settle into tissues, including the thin layer of fat beneath your skin. That makes abdominal fat a convenient window into what’s happening deeper in the body. Rather than biopsying the heart, kidney, or liver (organs that amyloid commonly damages), doctors can sample belly fat to look for the same protein deposits with far less risk.
Once the sample reaches the lab, pathologists apply a special dye called Congo red to the tissue. Under a polarized light microscope, amyloid deposits stained with Congo red glow with a distinctive apple-green color. This green glow, known as birefringence, is the hallmark confirmation that amyloid is present. If amyloid is found, additional testing with antibody panels or mass spectrometry identifies the specific type. This subtyping step is critical because different types of amyloidosis require very different treatments. The most common form, AL amyloidosis, involves abnormal immune cells and is treated with chemotherapy, while other forms like ATTR amyloidosis have entirely separate treatment pathways.
What Happens During the Procedure
The biopsy targets the lower abdomen, off to one side of the belly button. A nurse or doctor cleans the skin and marks a small diamond-shaped area roughly two inches across. They numb this area with a local anesthetic injected just under the skin, similar to what you’d get at the dentist. The numbing takes a minute or two.
Once the area is numb, the doctor inserts a needle attached to a syringe into the fat layer beneath the skin. They pull back on the syringe to create suction and move the needle back and forth through the fat in different directions, collecting tiny fragments of tissue. The needle stays shallow, running parallel to the skin surface to avoid going too deep. The goal is to gather enough fatty tissue fragments (roughly up to 1 mL of material mixed with a small amount of blood) for the lab to work with. The whole process takes only a few minutes.
Because the needle stays in the superficial fat layer and no incision is made, this is classified as a fine needle aspiration rather than a surgical biopsy. There are no stitches. Firm pressure with gauze stops any bleeding at the needle site. Most people can return to normal activities right away, and the risks are minimal: mild bruising or soreness at the site is the most common aftereffect.
How Accurate Is It?
Fat pad biopsy is a screening tool, not a perfect one. In a large study of patients with cardiac AL amyloidosis (the form caused by abnormal immune proteins affecting the heart), Congo red staining of fat pad samples detected amyloid in 84% of cases. That’s a strong detection rate, but it means roughly 1 in 6 patients with confirmed disease had a negative fat pad result. When the lab tried to identify the specific amyloid subtype using antibody testing, it succeeded in only about 47% of cases, highlighting that subtyping from fat alone can be challenging.
A negative fat pad biopsy does not rule out amyloidosis. If clinical suspicion remains high, doctors may proceed to a biopsy of the affected organ itself, such as the heart or kidney, which is the gold standard for diagnosis. The trade-off is that organ biopsies carry more risk and require specialized facilities. Fat pad aspiration exists specifically to avoid those procedures when possible.
Where It Fits in the Diagnostic Process
The American Society of Hematology’s 2026 guidelines confirm that a reliable diagnosis of AL amyloidosis can be made through what are called surrogate biopsies: a combination of fat pad sampling and a bone marrow biopsy. The fat pad sample looks for amyloid deposits, while the bone marrow biopsy checks for the abnormal immune cells that produce the misfolded proteins. Together, these two tests often provide enough information to confirm the diagnosis and guide treatment without needing a biopsy of the damaged organ.
That said, organ biopsies are still preferred in certain situations, particularly when the surrogate biopsies come back inconclusive or when the clinical picture doesn’t match the results. The fat pad biopsy’s role is as a safe, accessible first step. Given its safety and ease, it is considered the first biopsy site of choice in the workup for systemic amyloidosis.
What Your Results Mean
If your fat pad biopsy is positive for amyloid, the next step is identifying the type. The lab will test the sample with antibodies targeting the most common amyloid-forming proteins, including light chains (from immune cells) and transthyretin (a protein made by the liver). Getting the type right matters enormously because it determines treatment. AL amyloidosis is treated aggressively with chemotherapy to eliminate the abnormal immune cells. ATTR amyloidosis is managed with medications that stabilize the liver protein or prevent it from misfolding.
If your biopsy is negative but your doctor still suspects amyloidosis based on imaging, blood tests, or symptoms, additional biopsies of the affected organ may follow. A negative result simply means amyloid wasn’t found in that particular fat sample, not that it isn’t present elsewhere in the body.