What Does Cystic Fluid Reveal About a Cyst?

Fluid drawn from a cyst carries traces of the lining that made it, so it works like a sample of the cyst itself. Its most useful job is sorting cysts into harmless types and types that could one day turn cancerous, rather than simply saying yes or no to cancer. How much the fluid can tell you varies a great deal from one organ to another, and a reassuring result is usually only one piece of the decision.

The Three Things Doctors Look For in Cyst Fluid

For a pancreatic cyst, the needle passes through the stomach wall during an endoscopy with an ultrasound probe, and you are sedated. For ovarian, kidney, liver or thyroid cysts, a thin needle is usually guided through the skin by ultrasound or CT. Only a few millilitres may come out, so the doctor may have to choose which tests to run. A lab has to divide that small sample among the cell check, chemistry and DNA tests, and when fluid is scarce, doctors usually prioritize the tests most likely to change the plan for that organ and cyst. Cell and chemistry results usually take days; DNA tests can take longer.

The first test is usually a look under the microscope for abnormal cells floating in the fluid. Cyst fluid often holds very few cells, so this cell check misses many cancers. In pancreatic cysts, the cell check alone correctly classified only about half of cysts.¹

A report reading “no malignant cells” or “insufficient cellularity” describes what was in the tiny sample, not the whole cyst. Cells shed into fluid are often degraded, and a cyst’s worrying area may be a nodule on the wall that the needle never touched. So a negative cell check is read alongside the scan appearance and the chemistry, not on its own.

Labs also measure substances the lining releases, such as the protein CEA, sugar, and salts.²³⁴ Fluid can also carry DNA shed by abnormal cells, which can be tested for cancer-linked gene changes.⁵⁶ Doctors also judge thickness: mucus-rich fluid is sticky and stretches into a string between two fingers.⁷ The key distinction is whether a cyst is mucinous, meaning its lining makes mucus, since those types can become cancer.

In pancreatic cysts, combining CEA, the cell check and the string test reached about 94% accuracy.⁸ Your report may list several values, and no single one decides the plan.

Pancreatic Cysts Reveal the Most

CEA, a protein made by mucus-producing lining, is the main number on a pancreatic cyst report. It is much higher in mucus-making cysts: a typical value is around 420 ng/mL, versus about 7 in non-mucinous cysts.⁹ The common cutoff of 192–200 ng/mL is right about 9 in 10 times when it points to mucus-making, but it misses about 4 in 10 mucus-making cysts.¹⁰ Some labs use a lower cutoff of 30 ng/mL, which catches about 9 in 10 mucus-making cysts but more often mislabels harmless ones.¹¹¹² Ask which cutoff your lab uses, since it shifts how a borderline value reads.

A CEA below 5 ng/mL points toward a harmless serous cyst or pseudocyst, and above 800 strongly suggests a mucus-making one.¹³ Values in between are the hardest to read, because both cyst types can land there. In that range the other clues carry more weight.

Fluid sugar is one of those clues. It is very low in mucus-making cysts, a median of 1 mg/dL versus about 101 in non-mucinous ones.¹⁴ A glucose of 58 mg/dL or lower pointed to a mucus-making cyst about 93% of the time, and this test is cheap and quick.¹⁵

Amylase, a digestive enzyme, below 250 U/L virtually rules out a pseudocyst, the fluid pocket left after pancreas inflammation.¹⁶ A high amylase does not prove a pseudocyst, though, because mucus-making cysts that connect to the pancreatic ducts can also have high amylase. A positive string sign was right about 99% of the time when it pointed to mucus-making, but a negative one was wrong about half the time.¹⁷

Gene tests are the most accurate fluid test for one common cyst type. Changes in two genes, KRAS and GNAS, mark mucus-making cysts and almost never appear in harmless ones.¹⁸ Together they identified IPMNs, the most common mucus-making pancreatic cyst, about 94% of the time, better than CEA, though they can’t tell how dangerous a cyst is on their own.¹⁹ Adding changes in genes such as TP53 helped flag cysts that already had advanced precancer or cancer in about 9 of 10 cases.²⁰ Gene tests often miss the other mucus-making type, MCN, finding it in only about 3 in 10.²¹

A high CEA means the cyst makes mucus, not that it is cancer; CEA doesn’t measure how dangerous a cyst is. A mucus-making result usually means scheduled scans, with surgery only if worrying features appear. A non-mucinous result often means far less follow-up, or none. Panels combining fluid proteins and tiny RNA molecules are being tested to predict cancer risk, but they are not standard care yet.

What Ovarian Cyst Fluid Can Show

Of about 460 ovarian cyst samples, roughly 9 in 10 were read as benign.²² When the cell check called a cyst cancer, it was always right, but it found only about half of the cancers that were there.²³ So a benign cell result doesn’t settle the question for a cyst that looks worrying on ultrasound.

That is one reason ovarian cysts are often removed rather than drained. Draining a suspicious one could spill cells if it is cancer, so many surgeons take it out whole. Needle sampling is more often used for simple-looking cysts or when surgery is risky, and simple-looking cysts often go away on their own and are watched with repeat ultrasound.

Markers and DNA in the fluid sort ovarian cysts further. Ordinary functional cysts, like follicle cysts from the monthly cycle, have very low CEA and CA125 in their fluid, which fits a cyst that often shrinks without treatment.²⁴ Serous growths, harmless or cancerous, have very high CA125, and mucus-making ones have high CEA above 600 ng/mL.²⁵ A very high CA125 narrows the cyst to the serous family but cannot tell harmless from cancerous, and a high CEA points to a type of growth, not a cancer diagnosis.

Fluid CA125 is a different measurement from the blood CA125 test that is common for ovarian cysts. Fluid CA125 can be enormously high even in benign serous cysts, so fluid values don’t read like blood values. Ask your doctor which sample a CA125 result came from.

Cancer gene changes showed up in the fluid of about 8 in 10 borderline tumors and all aggressive cancers, and in none of the benign cysts.²⁶ This DNA test is still being studied and isn’t standard yet.²⁷

Kidney, Liver, Thyroid and Breast Cysts

Complex kidney cysts are usually judged on CT or MRI rather than fluid. That makes sense, because in complex kidney cysts that proved cancerous, the cell check flagged only about 1 in 6.²⁸ Doctors usually follow the scan features and consider removal or close imaging. Most kidney cysts are simple and harmless, and imaging is usually enough to tell them from complex ones. The protein CA9 was high in the fluid of every cancerous kidney cyst and low in most benign ones in one small group, but it isn’t a routine test.²⁹

Liver cysts have the opposite problem. Every simple liver cyst in one group had at least one raised marker such as CEA or CA19-9.³⁰ A raised marker was a poor sign of a precancerous cyst, right only about 1 time in 8.³¹ A high number on a liver fluid report is common and not, by itself, alarming, and these values behave differently from blood tumor markers. A cyst with solid parts or thick walls on imaging is the more meaningful warning. For telling a parasite cyst from a simple one, fluid tests combined with MRI look useful.³²

Thyroid and breast fluid give narrower clues. In thyroid nodules, very high thyroglobulin, a thyroid-made protein, in the needle-rinse fluid leans toward benign.³³ It was right only about 3 times in 4, so it supports rather than replaces the cell check.³⁴

Breast cyst fluid with low sodium relative to potassium marks one type of cyst.³⁵³⁶ Women with those cysts had about four times the later breast cancer risk of women with the other type.³⁷ Most breast cysts, though, are drained to relieve a lump or pain, and clear fluid is typically discarded. Bloody fluid, or a lump that remains after drainage, is usually sent for testing or imaged. Cyst-type salt testing isn’t routine, so ask about your overall risk instead. A raised risk from cyst type is a reason to keep up regular screening, not a diagnosis.

Is Sampling Cyst Fluid Safe

The main fear is that a needle could spill cancer cells into the belly. In pancreatic IPMN cysts, spread to the belly lining was no more common after needle sampling (about 2%) than without it (about 4%).³⁸ That is why endoscopic sampling is widely used when the result would guide care.

Biopsy of cancerous kidney cysts followed by surgery didn’t seem to increase spread either.³⁹ Spread was reported in two patients after alcohol was injected to destroy pancreatic cysts, but that is a different procedure from sampling.⁴⁰

The more common side effects are milder. Pain or bloating for a day or two is typical. Pancreatic sampling carries a small risk of pancreatitis or infection, so antibiotics are often given. Call your doctor for fever, worsening belly pain or vomiting after the procedure.

Before the procedure, you can ask which fluid tests will be run if only a little fluid comes out, whether gene testing is included or costs extra, and how the results will change your follow-up plan.

Why Cysts Fill With Fluid and Why Draining Isn’t a Cure

Many cysts fill because their lining pushes salt into the cavity and water follows.⁴¹ In kidney cysts from polycystic kidney disease, the fluid itself triggered more fluid production.⁴² Some fluid pockets, such as lymph cysts after cancer surgery, form when leaking fluid outpaces the body’s ability to absorb it.⁴³ Drained cysts often refill because the lining is still working.

After a simple drain, the lump or pressure often eases right away, but refilling over weeks to months is common and doesn’t by itself mean something is wrong. A cyst that refills quickly or changes on scans should be rechecked.

Treatments that stop refilling target the lining. For simple liver cysts that cause symptoms, doctors may drain them and then inject alcohol into the empty cavity.⁴⁴ The alcohol damages the lining cells so they can’t keep making fluid, and the cavity can scar down.⁴⁵ Other agents like polidocanol work the same way.⁴⁶ If you’ve had a cyst drained and it came back, ask whether a lining-destroying treatment or surgery fits your case.