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Pleural Disease
Table 1: Laboratory Investigations in Pleural Infection Diagnosis
Confirmation of pleural effusion is usually made on plain chest
Standard Investigations and Findings in Pleural Infection radiography. Further imaging is often utilised to delineate the extent
pH* of the effusion, to define the optimal site for thoracentesis or chest
Acidic pH <7.2
drain placement and to look for other abnormalities. Thoracic
Biochemistry ultrasound is a useful bedside tool to confirm the presence and
Exudate Fluid total protein > 0.5 x [serum total protein]
location of fluid. It is helpful in demonstrating pleural fluid septations,
Fluid LDH >1000IU/l
pleural thickening and underlying lung consolidation. Realtime
Low glucose Glucose <35mg/dl
thoracentesis or drain placement under ultrasound guidance has
Microbiology (often negative)
a success rate of over 95%.
16,17
Thoracic computed tomography (CT)
Standard Gram stain, culture and sensitivity (also consider inoculating blood
provides additional information about parenchymal abnormalities,
culture bottles)
including consolidation or lung abscess. It also delineates effusion
Mycobacterial staining and culture
Fungal staining and culture
location and pleural thickening and may be used to guide chest
drain insertion.
Additional Investigations
Biochemistry
Pleural fluid ADA Raised in tuberculous effusions (false-positives with
Pleural fluid should always be sampled as soon as pleural infection is
for tuberculosis empyema, malignancy and rheumatoid pleurisy)
suspected, prior to antibiotic administration if the clinical condition
Pleural fluid Salivary amylase iso-enzyme raised in oesophageal-
allows. Approximately 60% of patients with pleural infection will have
amylase rupture-associated infection
bacteria identified by standard microbiological techniques.
11
However,
Microbiology
culture takes several days, and 40% of pleural infections will be
Parasitic investigations (where appropriate) culture-negative; there is therefore the need for a surrogate marker of
*Measured using point-of-care blood gas machine, with pleural fluid in standard
infection. Bacterial metabolism and neutrophil phagocytic activity
heparinised syringe.
lead to increased lactic acid production and a fall in pleural fluid pH
ADA = adenosine deaminase; IU = international units; LDH = lactic acid dehydrogenase.
and glucose. Thus, a pleural fluid pH <7.2, in the correct clinical
with a prevalence of 52% (S. milleri group: 24%). Staphylococcus context, is used as a surrogate marker to define a high likelihood of
aureus and anaerobes are isolated in 8 and 20% of cases, pleural infection and therefore the requirement for effusion drainage.
respectively.
11
Such anaerobe prevalence is an important treatment A low glucose level (<35mg/dl) can be used when pH is unavailable.
consideration: any empirical antibiotic regime must afford Other pleural fluid investigations are shown in Table 1.
adequate coverage for anaerobes. Hospital-acquired infection
shows a much greater predominance of Gram-negative organisms The precise degree of pleural fluid acidity is a matter of debate, but
and methicillin-resistant S. aureus, with a respective prevalence most studies support a cut-off value around pH 7.2.
18
Clearly, such a
of 23 and 25%. value should not be absolute; borderline pH values should be tempered
by clinical context. Use of pH in the wrong clinical context can give
Prevention false-positives: heavily inflammatory effusions (e.g. some malignancies)
Risk factors for pleural infection include alcoholism and diabetes (in can bring about a low pH and glucose, while rheumatoid-associated
which the prevalence of Gram-negative organisms increases). Poor effusions almost always have such a biochemical profile. Consideration
oral hygiene and risk factors for aspiration (such as gastro- should also be given to multiloculated effusions, in which pH values
oesophageal reflux disease and seizures) are associated with an vary in each locule.
19
increasing prevalence of infection associated with anaerobes.
12
Nutrition is an often overlooked risk factor for poor outcome; a Tuberculous effusions are often pleural-fluid-culture-negative.
low admission albumin is associated with poorer outcome.
6
Histological examination of either blind pleural biopsy or
Immunosuppression, especially with HIV, increases the frequency of thoracoscopic/CT-guided biopsy usually reveals characteristic
pleural infection, with an increasing incidence of fungal, tuberculous caseating granulomas, occasionally with the presence of acid and
and Pneumocystis jirovecii infections, among others. alcohol-fast bacilli. A higher diagnostic rate (both culture and
histology) has been shown using thoracoscopic rather than blind
In spite of the associated risk factors of pleural infection, there has pleural biopsy.
20
been little work looking directly at the prevention of community-
acquired pleural infection. Considering that S. pneumoniae is isolated Treatment
in 21% of pleural infections,
11
vaccination is a theoretical modifiable The optimal treatment of pleural infection is debated and is currently the
factor in preventing empyema development. However, studies subject of large, multicentre, randomised, controlled trials. Commonly
investigating the seven-valent pneumococcal conjugate vaccine accepted modalities of treatment include drainage of fluid and tailored
(PCV7) found up to a five-fold increase in the rates of pneumonia and antibiotics. The role of surgery as primary treatment is contentious.
empyema, probably due to selection of virulent non-PCV7
pneumococcal serotypes.
13–15
Infected pleural fluid should be drained using a pleural catheter.
The size of the drain and the method of drain insertion are debated;
It would seem reasonable to assume that control of factors previous common practice was to insert large-bore drains, but
associated with tuberculosis, such as poverty, lack of national favourable outcomes can be achieved with smaller (<14F), less
tuberculosis control policies, prison settings and HIV infection, would traumatic Seldinger-type catheters.
9
Ultrasound or CT are
decrease the rate of tuberculous pleural infection; however, this has increasingly being used to optimise the site and safety of drain
never been subjected to formal investigation. placement, which is often a consideration in multiloculated effusions.
50 EUROPEAN RESPIRATORY DISEASE
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