Chapter 13: Diagnostic tests
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Related theory
Pleural fluid is generated from pleural vessels as a result of negative intrapleural pressure. It exits via the parietal pleural lymphatic system. Pleural effusions occur when there is an accumulation of fluid within the pleural cavity, which implies an increased production of fluid that exceeds the capacity of lymphatic removal and/or obstruction of drainage of the pleural space (Chernecky and Berger [32], Pagana and Pagana [177], PHE [192]).
Analysis of pleural effusion fluid is obtained via thoracocentesis, which determines the cause of the effusion. Effusion occurs when factors that influence the formation and absorption of fluid are altered either systematically, for example in cirrhosis or congestive heart failure (transudate effusions), or locally, for example in infection or malignancy (exudate effusions). Pleural fluid in a patient with undiagnosed exudative effusions should be cultured for bacteria (aerobic and anaerobic), mycobacteria and fungi (Chernecky and Berger [32], PHE [192]).
Infection of pleural fluid caused by bacteria, viruses or fungi may originate in the pleura or translocate from another site. Micro‐organisms can infiltrate the pleural space by various routes: directly in cases of empyema or a primary tuberculous focus rupturing into the pleural cavity; from an adjacent area of pneumonia, thoracic surgery or drainage; from chest trauma; or from transdiaphragmatic spread from intra‐abdominal infection (PHE [192]).
Pleural fluid is clear and straw‐coloured, and contains a small number of white blood cells but no red blood cells or micro‐organsims (PHE [192]). In cases of suspected infection, it appears cloudy due to the presence of micro‐organisms and/or increased white blood cells (Chernecky and Berger [32], Pagana and Pagana [177]). Differential white blood cell counts assist further in the diagnosis of infection because increased numbers of neutrophils indicate bacterial infection. Other changes include decreased glucose levels, increased protein or albumin levels, and disturbances in lactate level.
