Diagnostic Thoracentesis and Pleural Effusion Analysis

Pleural effusion is a
common yet complex medical condition that requires tailored and multidisciplinary treatment. Despite being a common disease, its cause remains unknown in at least 20% of patients even after diagnostic workups. Fortunately, pleural fluid tests have improved leading to an increased understanding of its pathophysiology, diagnosis, and use of imaging.

New and advanced medical developments, diagnostic methods, and patient-centered treatment proposals and options have also emerged. These advancements enable better diagnosis and treatment of the condition and enhance patient health outcomes.

In this article, we dive into the latest developments and protocols in pleural effusion analysis to help healthcare providers understand the new therapeutic options available. But first, we tell you what pleural fluid analysis is and how the test is conducted.

What Is Pleural Fluid Analysis?

Pleural fluid analysis is a diagnostic test that studies a fluid sample taken from the pleural cavity. This is the space between the chest wall and the pleura (the lining outside the lungs). A pleural effusion occurs when excess fluid accumulates in this area. When this happens, pleural effusion symptoms such as the following occur:

  • Pleuritic chest pain
  • Dyspnea (shortness of breath or labored breathing)
  • Orthopnea (breathlessness when lying down, which is relieved when the person sits up straight or stands erect)
  • A dry, nonproductive cough

Since chest pain and dyspnea are nonspecific symptoms, a detailed clinical history, a careful physical exam, and imaging studies are necessary to narrow the differential diagnosis.

How a Pleural Fluid Test Is Performed

Pleural Fluid Analysis Healthcare providers usually conduct a medical procedure called a diagnostic thoracentesis to get a pleural fluid sample. They examine the sample to check for:

  • Malignant or cancerous cells
  • Other cell types such as white and red blood cells
  • Levels of protein, glucose, LDH and other special tests like NT-pro-BNP
  • Bacteria, viruses, fungi, mycobacteria, and other infection-causing germs
  • ADA and PCR probes for certain infections

These factors help medical professionals determine the cause of pleural effusion and devise an appropriate treatment plan.

Determining Pleural Effusion Etiology and Differentiating Between Exudates and Transudates

Over 60 causes of pleural effusions exist, but the following four etiologies contribute to around 75% of cases:

  • Heart failure
  • Cancer
  • Pneumonia
  • Tuberculosis

Note that effusions can sometimes form due to two or more concomitant medical conditions in certain patients.

The primary step in diagnosing the causes of pleural effusions is distinguishing between exudates and transudates. Despite recent medical advances, healthcare professionals still use Light’s criteria as the default reference test to differentiate between exudates and transudates in clinical settings.

The criteria has a 98% sensitivity when classifying exudates. But they also incorrectly classify around 25% to 30% of transudates as exudates. It is especially common among patients who take diuretics or have bloody pleural fluid. About 80% of cardiac transudates that have been incorrectly identified as exudates exhibit the following:

  • Protein gradient higher than 2.5 g/dL
  • Albumin gradient of more than 1.2 g/dL
  • Concentrations of the pleural NT-proBNP measuring more than 1500 pg/mL

A medical professional has a higher chance of accurately identifying that an effusion is a transudate once the following conditions are met:

  • The patient is over 75 years old
  • Confirmation of both gradient levels 
  • The effusion is bilateral

When hepatic hydrothorax is miscategorized as an exudate — which happens often — a ratio between the pleural fluid and serum greater than 0.6 would be a better parameter than the albumin gradient. It would help in properly reclassifying the condition as a transudate.

Other viable options include:

  • PF cholesterol of more than 55 mg/dL
  • Lactate dehydrogenase (LDH) greater than 67% of the upper range of the normal serum LDH

Both parameters identify exudates with a 97% sensitivity.

Usually, effusions subside in 89% of cases after 14 days of diuretic treatment, but it can also be intractable. In these cases, therapeutic thoracentesis may be necessary. However, if the required monthly procedures are more than one or two, healthcare professionals must consider definitive options like an indwelling pleural catheter (IPC) or talc pleurodesis.

Improving the Diagnostic Value of Pleural Effusion Analysis

To ensure an accurate pleural effusion analysis, the correct evaluation and interpretation of data gathered from the following sources are necessary:

  • Medical and work history
  • Physical examinations
  • Radiological studies
  • Pleural fluid laboratory tests

These help determine an effusion’s etiology, especially when it is caused by rare diseases.

Medical professionals should also perform thoracentesis and pleural biopsy under image guidance to avoid complications and improve the diagnostic yield.

Note that thoracentesis is unnecessary for diagnosing effusions arising from heart failure. Healthcare providers should perform this procedure only in the presence of atypical clinical signs, imaging studies, or when another cause is suspected.

Pleural tissue cultures, nucleic acid amplification tests, and collecting pleural fluid in blood culture bottles enhance pleural fluid cultures’ diagnostic sensitivity and performance. Since undiagnosed effusions are usually benign and lead to a favorable prognosis, we recommend a follow-up to avoid the growth of malignant effusion.

If a malignant effusion is suspected, cytology tests can help rule out its incidence. Note that the cytology yield depends on the following factors:

  • Tumor type
  • Tumor burden
  • Sample quality
  • The pathologist’s experience

Generally, cytology testing has a diagnostic yield of 49% to 91%. If a second cytology test is necessary, biopsy sampling is highly ideal.


Pleural effusion is a rapidly evolving medical condition that can now be diagnosed and treated in many ways. Learning about thoracentesis and pleural fluid analysis is essential so that healthcare providers can expertly diagnose and treat the condition, and help patients heal and recover.

At Hospital Procedures Consultants, we offer various related courses to help medical professionals keep their knowledge and skills updated. You can check out our Thoracentesis Course and other programs to learn more.


Bhatnagar, R. Maskel, N. Developing a ‘pleural team’ to run a reactive pleural service. Clin Med. 13 (5) 452- 456
Karkhanis, V. Joshi, J. Pleural effusion: diagnosis, treatment, and management. Open Access Emerg Med. 31- 52
Porcel, J. Esqueda, A. Vives, M. Bielsa, S. Etiology of pleural effusions: analysis of more than 3,000 consecutive thoracenteses. Arch Bronconeuomol. 2014. 50 (5): 161- 5
Bintcliffe, O. Hooper, C. Rider, I. Finn, R. Morley, A. Zahan, N. Harvey, J. Skyrme, A. Maskell, N. Unilateral Pleural Effusions with More Than One Apparent Etiology. A Prospective Observational Study. Observational Study. 2016. 13(7) 1050 – 6
Pachon, G. Navas, P. Pleural effusion: criteria for distinguishing between transudates and exudates. Med Interna. 1996 13(2) 91- 4
Porcel ,J. Light, R. Pleural Fluid Analysis: Are Light’s Criteria Still Relevant After Half a Century?. Clin Chest Med. 2021. 42 (4) 599- 609
Light, R. Macgregor, P. Luchsinger, W. Pleural effusions: the diagnostic separation of transudates and exudates. Intern Med. 77 (4): 507 – 13
Rial, M. Pallarés, J. Viedma, E. Lopez Gonzalez, F. Porcen, J. Rodriguez, M. Romero, B. Valdéz, L. Villena, V. Cordovilla, R. Diagnosis and Treatment of Pleural Effusion. Recommendations of the Spanish Society of Pulmonology and Thoracic Surgery. Update 2022 . Arch Bronconeumol. 2023. 59 (1) 27- 35
Porcel, J. Ferreiro, L. Civit, C. Valdés, L. Esquerda, A. Light, R. Bielsa, S. Development and validation of a scoring system for the identification of pleural exudates of cardiac origin. Eur J Intern Med. 2018 50:60-64.
Bielsa, S. Porcel, J. Castellote, J, Mas, E. Esquerda, A. Light, Richard. Solving the Light’s criteria misclassification rate of cardiac and hepatic transudates. Respirology. 2012 May;17(4):721-6

Read all articles in Featured, Thoracentesis
Tags: cause of pleural effusion, pleural effusion, Pleural Effusion Etiology

Live Courses

Online Courses

Get Live & Online Courses Info

Please send me information on Live and Online Courses

* indicates required