Remember Light’s Criteria is SENSITIVE for picking up EXUDATES but mis-classifies ~25% transudative effusions as exudative!
-You only need to have ONE of Light’s criteria to call it an Exudate!

TRANSUDATE
Remember the top 3 but there is an extensive list…

–CHF (most common)
-Hepatic hydro-thorax from cirrhosis
-Nephrotic syndrome

EXUDATE 
Remember the top 3 but also has an extensive list…

-Malignancy
-Pneumonia
-Tb

What percentage of patients with bacterial PNA get an associated effusion?
>40 % and up to 60 % with pneumococcal PNA, and can be either uncomplicated, complicated, or empyema

Uncomplicated parapneumonic effusion

-Resolves with antibiotic treatment of PNA
-On CXR, must be free-flowing and <10 mm
-Very low risk, does not need drainage 

Complicated parapneumonic effusion

-Small/Mod  in size, >10 mm but <1/2 hemithorax
-Negative gram stain/culture, pH>7.20
-May need drainage based on clinical status

Complicated parapneumonic effusion (second type)

-Large >1/2 hemithorax, LOCULATED, thickened parietal pleura (high risk if thickened parietal pleura, suspect Empyema) 
-Positive culture or gram stain OR
-pH<7.20
-MUST drain effusion, moderate risk

EMPYEMA!

-defined as PURULENT appearance of pleural fluid OR
–pH <7.20 (don’t forget to order pH and keep it on ice!)
-If ether of these exist, MUST drain effusion, highest risk, usually with tube thoracostomy (chest tube)

Treatment

-Empiric therapy should cover Gram + (eg: Staph, Strep), and Anaerobes (eg: Fusobacterium, Bacteriodes)
-Empyema is treated with 4-6 weeks of antibiotic treatment