Finding the right tests…
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Finding the right tests…
Biochemistry
When fluid collects around the lungs, LDH measured in that fluid — not in your blood — is one of three numbers used to decide whether the effusion is a transudate or an exudate, a distinction called Light's criteria. A transudate usually means fluid has leaked through from a systemic problem like heart failure, while an exudate points to something local going on in the lung or pleura itself, from infection to cancer to inflammation. The rule doesn't work from the fluid LDH alone: it compares your pleural fluid LDH to your blood LDH at the same time, and also checks the fluid-to-blood protein ratio. Getting this classification right shapes what gets investigated next.
This test measures the amount of LDH in fluid drawn from the pleural space around your lungs, and compares it against the LDH level in a blood sample taken at the same time. The ratio between the two, together with a similar protein ratio, is what classifies the effusion as a transudate or an exudate under Light's criteria. Fluid LDH is also read as a rough marker of how much inflammation or cell turnover is happening within the fluid itself.
Pleural fluid is classed as an exudate if any one of three conditions is met: the fluid-to-blood protein ratio is above 0.5, the fluid-to-blood LDH ratio is above 0.6, or the fluid LDH itself is more than two-thirds of the upper limit of normal blood LDH. If none of these apply, the fluid is a transudate.
A transudate generally points toward a systemic cause — heart failure, liver disease, or kidney disease pushing fluid out through otherwise normal pleura. An exudate suggests the pleura or lung itself is involved, which opens up a different, usually longer list of causes: infection, cancer, pulmonary embolism, or inflammatory disease. The two categories send the workup in different directions.
Light's criteria aren't perfect. Research has found they can misclassify a meaningful share of transudates — particularly in heart failure — as exudates, especially in patients already given diuretics, which concentrates the fluid's protein and LDH. When the picture doesn't fit the clinical story, doctors sometimes turn to additional markers alongside LDH rather than relying on it alone.
Beyond the transudate/exudate split, a fluid LDH above roughly 1000 U/L has specifically been associated with tuberculosis, lymphoma, and empyema, so a markedly high result can help point your doctor toward this narrower group of causes rather than just confirming an exudate in general.
Pleural fluid LDH is compared against a blood LDH taken at the same time, and that ratio is one of the criteria used to classify pleural fluid as a transudate or an exudate. Blood LDH alone can't make that distinction.
It's a set of three ratios comparing pleural fluid to blood — protein ratio, LDH ratio, and fluid LDH against the upper limit of normal blood LDH — used to sort a pleural effusion into transudate or exudate. Meeting even one of the three classifies the fluid as an exudate.
A transudate usually comes from a systemic problem, like heart failure, pushing fluid out through normal pleura. An exudate suggests the pleura or lung itself is involved, from causes such as infection, cancer, or inflammation, and usually needs more investigation.
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.
No. Studies have found it can misclassify some transudates, particularly heart-failure-related ones, as exudates, especially after diuretic treatment. Your doctor may use additional tests if the result doesn't match the clinical picture.
Pleural fluid protein is tested at the same time to complete Light's criteria, and depending on the suspected cause, glucose, cell counts, cytology, or ADA may also be ordered on the same sample.
No. A high fluid LDH mainly points toward an exudate rather than a transudate, and an exudate has a wide range of possible causes, including infection, cancer, and inflammatory disease. Further tests are needed to narrow it down.