PLAB Practice Exam 2026 – The Complete Guide to Mastering the Professional and Linguistic Assessments Board Test!

The criterion for classifying a pleural effusion as exudative is based on the levels of lactate dehydrogenase (LDH) in the pleural fluid compared to serum LDH levels. Specifically, if the pleural fluid LDH is greater than two-thirds of the upper limit of normal for serum LDH, it indicates that the effusion is exudative. This elevated level suggests an underlying pathological process, such as infection, malignancy, or inflammation, which is causing the pleural space to produce fluid with higher levels of LDH.

In clinical practice, the normal upper limit for serum LDH is typically around 200-250 U/L, though this can vary slightly based on the laboratory reference ranges. Therefore, calculating two-thirds of the upper limit provides a threshold that helps in identifying exudative effusions, essential for appropriate management and further investigation of the underlying cause.

This understanding is crucial for practitioners as it guides diagnostic procedures and treatment decisions in patients presenting with pleural effusion. The other criteria regarding LDH levels do not accurately reflect the definitions used in standard clinical practice for distinguishing exudative from transudative pleural effusions.