Diagnosis of certainty, or of high diagnostic probability, of a pleural effusion established through the pleural fluid analysis or the pleural fluid/serum ratio, as well as the mechanisms by which it occurs
| Parameter (in PF, unless otherwise specified) | Diagnostic characteristics | Most likely diagnosis | Pathophysiology |
|---|---|---|---|
| Adenosine deaminase | >45 U·L−1 and lymphocytes >50% | Tuberculosis | Rupture of a subpleural caseous focus and mycobacterial antigens interact with CD4+ T-lymphocytes producing a hypersensitivity reaction It is released by macrophages stimulated by the living microorganisms inside them |
| Amylase PF/serum | >2 | Acute pancreatitis | Acute inflammation of the pancreas produces an exudative fluid that is transferred through interconnected lymphatic vessels on both sides of the diaphragm into the pleural space |
| Amylase | >100 000 U·L−1 | Pleuro-pancreatic fistula | Due to obstruction of the pancreatic duct |
| β2 transferrin | Present in PF | Duropleural fistula | Communication between the subarachnoid space (positive pressure) and the pleural space (negative pressure) The cerebrospinal fluid flows through a pressure gradient from the space with the highest pressure to the one with the lowest pressure |
| Bilirubin PF/serum | >1 (greenish appearance) | Biliopleural fistula | Complete biliary obstruction or prolonged drainage (>7 days) favours fistula formation |
| C-reactive protein | ≥100 mg·L−1 and neutrophils >50% | Pleural infection | Acute phase reactant released by the liver that is elevated as a nonspecific response to infection and non-infectious inflammatory situations PE is produced by increased pleural capillary permeability |
| Cholesterol | ≥250 mg·dL−1 | Pseudochylothorax | A pleural thickening blocks the drainage of PF to the lymphatic system; the lysis of red blood cells and neutrophils trapped in the pleural space causes an increase in cholesterol released in the PF |
| Creatinine PF/serum | >1 (colour and smell of urine) | Urinothorax | Obstructive urinary disease with urine passing from the abdominal or retroperitoneal cavity to the pleural space due to a pressure gradient |
| Culture | Positive | Pleural infection | Presence of the microorganism in pleural fluid |
| Cytology | Positive | Neoplasia | Implantation of the tumour in the subserous layer |
| Glucose PF/serum | >1 (if a glucose solution received) and anomalous position of the catheter | Extravascular migration of the central venous catheter | Erosion of the superior vena cava due to a catheter of insufficient length |
| Haematocrit PF/serum | >0.5 | Haemothorax | Presence of blood in the pleural space |
| Interferon-γ | >140 pg·mL−1 | Tuberculosis | Rupture of a subpleural caseous focus and mycobacterial antigens interact with CD4+ T-lymphocytes producing a hypersensitivity reaction Cytokine released by CD4+ T-lymphocytes to increase the mycobactericidal activity of macrophages |
| LE cells | Positive | SLE | Localised immune inflammation process with activation of the complement system and production of immunocomplexes |
| Löwenstein culture | Positive | Tuberculosis | Presence of Mycobacterium tuberculosis in PF |
| Mesothelin serum | >2.00 nmol·L−1 | Malignant pleural mesothelioma | Implantation of the tumour in the subserous layer Mesothelin is expressed in normal mesothelial cells and overexpressed in mesothelioma, lung, ovarian and pancreatic cancer |
| NT-proBNP | ≥1500 pg·mL−1 | Heart failure | Molecule secreted by the cardiac ventricles in response to their acute distension |
| Total proteins | <1 g·dL−1 | Peritoneal dialysis | Dialysate may migrate from the peritoneal cavity to the pleural space through a pleuroperitoneal leak |
| Triglycerides | ≥110 mg·dL−1 | Chylothorax | Obstruction/rupture of the thoracic duct causes lymph to accumulate retrogradely in the pleural space |
| Tumour markers (e.g. carcinoembryonic antigen) | Elevated | Neoplasia | Tumour cells implanted in the pleura can express a greater amount of a certain protein in PF The marker varies depending on the type of tumour |
PF: pleural fluid; PE: pleural effusion; LE: lupus erythematosus; SLE: systemic lupus erythematosus; NT-proBNP: N-terminal pro-brain natriuretic peptide.