Alternative Tests to Differentiate Pleural Effusion Exudates from Transudates

by Joseph Esherick, M.D., FAAFP, FHM

How to Differentiate Pleural Effusion Exudates from Transudates

 

Historically, Light’s criteria have been used to classify pleural effusions into transudates or exudates.  Light’s criteria were established by Dr. Richard Light in 1972.[1]  These criteria maximize sensitivity but have a lower specificity.  Light’s criteria identify pleural exudates with 98% accuracy, but they misclassify transudates as exudates 15-25% of the time.[2]  A diagnostic thoracentesis or a therapeutic thoracentesis can obtain fluid for pleural fluid analysis.

 

Light’s criteria

     

Category

Transudates

Exudates

Pleural fluid LDH

<2/3 ULN serum LDH

≥2/3 ULN serum LDH

Pleural fluid/serum LDH ratio

<0.6

≥0.6

Pleural fluid/serum protein ratio

<0.5

≥0.5

ULN = upper limit of normal

 

A new study adds another pleural fluid test that can help to distinguish pleural fluid transudates from exudates.[3]  This meta-analysis analyzed 20 studies and nearly 3,500 patients.  The study found that, on the average, a pleural cholesterol level of 55 mg/L or more had an 88% sensitivity, 96% specificity and positive likelihood ratio for an exudate of 20.3.  On the other hand, a pleural cholesterol level below 55 mg/L has a negative likelihood ratio of 0.12 suggesting a transudate.  In addition a pleural fluid/serum cholesterol ratio of 0.3 or more has 94% sensitivity and 87% specificity for a pleural exudate.  A P/S cholesterol ratio less than 0.3 has a negative likelihood ratio of 0.07.3

 

Two common etiologies of pleural transudates that are misclassified as exudates are CHF and hepatic hydrothorax.  Typically, pleural effusions related to heart failure that are misclassified as exudates occur in patients who take chronic diuretics.  Porcel determined that pleural effusions from CHF that are misclassified as exudates are best reclassified using a serum-effusion albumin gradient (serum album minus pleural effusion albumin).2  On the other hand, pleural effusions from hepatic hydrothorax that are misclassified as exudates are best reclassified using a pleural effusion/serum albumin ratio.2

 

Transudates have a serum-effusion albumin gradient of more than 1.2 gm/dL or a pleural effusion/serum albumin gradient less than 0.6.2  Another pleural fluid test that is accurate for pleural effusions secondary to CHF is a pleural fluid NT-pro-BNP. 

A pleural fluid NT-pro-BNP level of 1,300 pg/mL or more has 94% sensitivity and 94% specificity for heart failure-related pleural effusions.[4]

 

All of this is summarized in the table below:

Category

Transudates

Exudates

Pleural fluid LDH

<2/3 ULN serum LDH

≥2/3 ULN serum LDH

Pleural fluid/serum LDH ratio

<0.6

≥0.6

Pleural fluid/serum protein ratio

<0.5

≥0.5

Serum-effusion albumin gradient

>1.2 gm/dL

≤1.2 gm/dL

Effusion/serum albumin ratio

<0.6

≥0.6

Pleural fluid NT-pro-BNP

≥1,300 pg/mL

<1,300 pg/mL

Pleural fluid cholesterol

<55 mg/L

≥55 mg/L

Effusion/serum cholesterol ratio

<0.3

≥0.3

ULN = upper limit of normal

 

I would still start with Light’s criteria and if a transudate is diagnosed no additional studies are necessary.   However, if Light’s criteria diagnoses an exudate, I would consider checking a serum-effusion albumin gradient, an effusion/serum albumin ratio, a pleural fluid cholesterol and an effusion/serum cholesterol ratio to differentiate a transudate from an exudate.

Pleural effusion chest X-ray