Inflammatory tuberculous effusions may occur in any of the serous cavities of the body, i.e. pleural,pericardial or peritoneal cavities.They are more common in HIV-positive than in HIV-negative adults, and also occur in school-aged children with or without HIV infection. Serous effusions are often indicative of primary disease or reinfection.
The presentation is usually with constitutional and local features. Microscopy of the aspirate from tuberculous serous effusions rarely shows AFB because the fluid forms as an inflammatory reaction to TB lesions in the serous membrane. TB culture, even if available, is of no immediate help. A culture result usually takes 4-6 weeks. The white cell content is variable, usually with predominant lymphocytes and monocytes. The aspirate is an exudate (i.e. protein content is more than 30 g/l).
A biochemistry laboratory is not essential to diagnose an exudate. Simply leave the aspirate standing: if "spider clots" develop in the specimen, it is an exudate.
In populations in sub-Saharan Africa with high HIV prevalence,TB is the commonest cause of an exudative serous effusion. The diagnosis is usually presumptive (i.e. without microbiological or histological confirmation). It is important to exclude other causes of an exudate.
Interpret with caution the laboratory result of protein concentration in any aspirated fluid. If there has been a delay in laboratory analysis, a protein clot may have formed in the sample.The laboratory result may then be falsely low.
The clinical and CXR diagnosis of a pleural effusion is straightforward. The typical clinical features are constitutional and local (chest pain; breathlessness; tracheal and mediastinal shift away from the side of the effusion; decreased chest movement, dull percussion note and decreased breath sounds on the side of the effusion). CXR shows unilateral, uniform white opacity, often with a concave upper border. If available, ultrasound can confirm the presence of fluid in the pleural space in case of doubt.
Always perform diagnostic pleural aspiration if a patient has a pleural effusion. The fluid is usually straw-coloured. The white cell count is usually high (about 1000-2500 per mm3) with predominant lymphocytes. Occasionally the fluid is blood-stained.The presence of pus on aspiration indicates an empyema (purulent effusion).
In a hospital with limited facilities serving a population with high TB prevalence, patients with a unilateral exudative pleural effusion that has not responded to a full course of antibiotics should be treated with anti-TB drugs.
If facilities are available, closed pleural biopsy using an Abrams needle is useful for histological diagnosis. Since the distribution of TB lesions in the pleura is patchy, the diagnostic yield of closed pleural biopsy is about 75%. Multiple biopsies increase the diagnostic yield. A small open pleural biopsy increases the yield even further but is not usually necessary.
The differential diagnosis of an exudative pleural effusion includes malignancy, post-pneumonic effusion, pulmonary embolism and amoebic liver abscess (extending on the right).
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