Differentiating transudative and exudative pleural effusions

In this video, we'll discuss some components that can be used to differentiate between transudative and exudative pleural effusions, why you need to be careful with heart failure patients, and what your patient's serum glucose can tell you about their risk of malignancy.

Amer Wahed, MD FRCPath
Amer Wahed, MD FRCPath
24th Jun 2021 • 3m read
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In this video, we'll discuss some components that can be used to differentiate between transudative and exudative pleural effusions, why you need to be careful with heart failure patients, and what your patient's serum glucose can tell you about their risk of malignancy.

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Video transcript

In this Medmastery lesson we'll discuss some components that can be used to differentiate between transudative and exudative pleural effusions. First is pH. The pH of normal pleural fluid is 7.6. Generally transudative pleural effusions have a pH ranging from 7.4 to 7.55. Most exudates have a pH ranging from 7.3 to 7.45.

The level of pleural fluid lactate dehydrogenase, is also one of the key criteria for differentiating transudates and exudates. The normal serum lactate dehydrogenase is 200 international units per liter. Several specific disease associations have been noted with pleural fluid protein and lactate dehydrogenase levels.

Pleural fluid lactate dehydrogenase levels above 1000 international unit per liter, are characteristically found in empyema, rheumatoid pleurisy and pleural paragonimiasis, and are sometimes observed with malignancy. Pleural fluid, secondary to pneumocystis jiroveci pneumonia has the characteristic finding of a pleural fluid lactate dehydrogenase to serum lactate dehydrogenase ratio greater than one, and a pleural fluid protein to serum protein ratio of less than 0.5.

Such a pattern may also be suggestive of malignancy. Urinothorax is another cause of elevated pleural fluid lactate dehydrogenase, associated with low pleural fluid protein levels. Protein concentration is another way to differentiate transudates and exudates. Transudates have a protein concentration below three grams per deciliter.

Exudates have a protein concentration above three grams per deciliter. Acute diuresis in heart failure can elevate protein levels into the exudative range. However, heart failure patients have a serum albumin to pleural fluid albumin ratio greater than 1.2 grams per deciliter, or a protein ratio greater than 3.1 grams per deciliter, which correctly categorizes the effusion as a transudate.

Tuberculous pleural effusions always have total protein concentrations above four grams per deciliter. When pleural fluid protein concentrations are in the seven to eight grams per deciliter range, Waldenstrom's macroglobulinemia, and multiple myeloma should be considered. Exudates and transudates may have glucose levels similar to that of blood.

However, a pleural fluid glucose concentration less than 60 milligrams per deciliter, or a pleural fluid glucose to serum glucose ratio less than 0.5, narrows the differential diagnosis of the exudate to the following possibilities. Rheumatoid pleurisy, complicated parapneumonic effusions, empyema. Malignant effusion, tuberculous pleurisy, lupus, pleuritis, or esophageal rupture.