CXR
Earliest sign is blunting of costophrenic angle
Lateral decubitus with affected side down requires 50-75 cc of fluid for visualization
PA view requires 200-250 cc of fluid
Supine view may only show a generalized hazy appearance of affected hemithorax
Subpulmonic<unk> 
Fluid collects in isolation between lung base and diaphragm
May not cause blunting of costophrnic angle or meniscus appearance
Suspect if "hemidiaphragm" (actually fluid) is elevated and flattened
Lateral decubitus with suspected side down will show free-flowing<unk>  fluid
CT
US
Earliest sign is blunting of costophrenic angle
Lateral decubitus with affected side down requires 50-75 cc of fluid for visualization
PA view requires 200-250 cc of fluid
Supine view may only show a generalized hazy appearance of affected hemithorax
Subpulmonic<unk> 
Fluid collects in isolation between lung base and diaphragm
May not cause blunting of costophrnic angle or meniscus appearance
Suspect if "hemidiaphragm" (actually fluid) is elevated and flattened
Lateral decubitus with suspected side down will show free-flowing<unk>  fluid
PA view requires 200-250 cc of fluid
Supine view may only show a generalized hazy appearance of affected hemithorax
Fluid collects in isolation between lung base and diaphragm
May not cause blunting of costophrnic angle or meniscus appearance
Suspect if "hemidiaphragm" (actually fluid) is elevated and flattened
Lateral decubitus with suspected side down will show free-flowing<unk>  fluid
Thoracentesis
If one of the following is present the fluid is virtually always an exudate
If none is present the fluid is virtually always a transudate<unk>  fluid/serum protein ratio >0.5<unk>  fluid/serum LDH ratio >0.6<unk>  fluid LDH > two thirds of upper limit for serum LDH<unk>  fluid/serum protein ratio >0.5<unk>  fluid/serum LDH ratio >0.6<unk>  fluid LDH > two thirds of upper limit for serum LDH
Gram stain and culture (place 10cc into blood culture bottle at the bedside)
Cell count
RBC >100K: trauma, malignancy, pneumonia, or pulmonary infarction
Neutrophil predominance (>50%): parapneumonic, pulmonary embolism, pancreatitis
Lymphocytic predominance (>50%): malignancy, TB,<unk> , viral pleuritis
Glucose
Low glucose (<60) seen in parapneumonic, empyema, malignant, TB, and RA
ABG (pH)
May be left at room temperature for up to 1hr with out affecting results
Normal<unk>  fluid pH = 7.64;
In parapneumonic<unk> s, <7.10 predicts development of empyema or<unk> rsistence and indicates need for thoracostomy tube drainage
Amylase: >100 in<unk> s due to pancreatitis or esophageal rupture
TB (adenosine deaminase)
India ink
Cytology (requires 50cc)
RBC >100K: trauma, malignancy, pneumonia, or pulmonary infarction
Neutrophil predominance (>50%): parapneumonic, pulmonary embolism, pancreatitis
Lymphocytic predominance (>50%): malignancy, TB,<unk> , viral pleuritis
Low glucose (<60) seen in parapneumonic, empyema, malignant, TB, and RA
May be left at room temperature for up to 1hr with out affecting results
Normal<unk>  fluid pH = 7.64;
In parapneumonic<unk> s, <7.10 predicts development of empyema or<unk> rsistence and indicates need for thoracostomy tube drainage
Pleural effusion
Transudative
Pleuritis
CHF
Hemothorax
Exudative
Spontaneous pneumothorax
Cancer