Finding the right tests…
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Finding the right tests…
Biochemistry
When fluid builds up somewhere it shouldn't — around a lung, inside the abdomen, in a joint — a doctor often drains a sample and sends it to biochemistry along with a blood sample taken close to the same time. Protein is one of the first things checked in that fluid, and it's read against your blood protein level, not on its own. A fluid protein level close to your blood level usually points toward fluid pushed out by raised pressure, such as heart failure. A fluid protein level much higher than that gap would predict usually points toward inflammation, infection, or a growth near the fluid space. Which body cavity the fluid came from changes what the pattern means.
The test reports total protein in the fluid sample, in grams per decilitre. This number is compared with your blood protein level from a sample taken around the same time, usually within a couple of hours — without that paired blood value, the fluid result is hard to read. For fluid drained from around a lung, a fluid-to-blood protein ratio above roughly 0.5 leans toward an inflammatory or infectious cause rather than a pressure-related leak. For fluid from the abdomen, doctors more often turn to a related calculation, the albumin gradient, instead of protein alone.
Doctors sort body fluid into two rough categories: transudate, which leaks out because of raised pressure somewhere in the circulation, and exudate, which leaks out because something local — infection, inflammation, cancer — is making nearby vessel walls leaky. For fluid drained from around a lung, this protein test is one part of a small set of comparisons, known together as Light's criteria, that also weighs a fluid enzyme called LDH against the same enzyme in blood. A fluid-to-blood protein ratio above roughly 0.5 is one of three separate thresholds used to call a fluid an exudate; meeting even one of the three is usually enough.
For fluid drained from the abdomen, the same protein-ratio approach turns out to be less reliable, particularly once infection is also present, so doctors more often calculate the serum-ascites albumin gradient instead — a related but separate test that uses albumin rather than total protein.
The result only means something next to a paired blood sample, so the fluid draw and the blood draw are usually timed close together. No special preparation applies to the protein test itself; anything you're asked to do beforehand relates to the drainage procedure, and that's worth asking whoever is performing it.
Because the fluid protein number only makes sense compared with your blood protein level from around the same time. Without that comparison, a lab can't tell whether the fluid came from a pressure leak or something more local, like infection.
A transudate leaks out because of raised pressure somewhere in your circulation, such as heart failure. An exudate leaks out because something local — infection, inflammation, or a growth — is making nearby vessel walls leaky, and it tends to carry more protein.
Not quite. Both use a paired blood sample, but SAAG is calculated from albumin specifically and is used mainly for fluid drained from the abdomen. Fluid protein and Light's criteria are used more for fluid around the lungs.
This page is general information about the test, not medical advice. Your own report's reference ranges apply — please discuss your results with your doctor.
Not in the usual sense. There isn't one fixed normal fluid protein level, since the number is read against your blood protein level and the ratio between the two. Cutoffs vary by lab and by which body space the fluid came from.
A doctor drains it directly from the affected space using a needle or catheter, usually under local anaesthesia. Ask your doctor or the centre about what to expect and any preparation involved.
No. It narrows down the likely category — pressure-related versus inflammatory or infectious — but the actual cause usually needs more tests on the same fluid sample, such as cell counts or cultures.