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PubMed This is a summary of 13 peer-reviewed journal articles Updated
Pulmonology

Diagnosing Empyema: Imaging and Fluid Analysis

At a Glance

Pleural empyema is diagnosed using imaging scans and pleural fluid analysis. If your fluid test shows a low pH (below 7.2), low glucose (below 60 mg/dL), or visible pus, it confirms a severe infection that typically requires physical drainage with a chest tube or surgery.

Diagnosing a pleural empyema is like putting together a puzzle. Your doctors use a combination of high-tech imaging and detailed chemical analysis to determine if the fluid around your lungs is a simple byproduct of inflammation or a serious infection that needs aggressive drainage [1][2].

Seeing the Problem: Ultrasound vs. CT

Imaging is the first step in understanding what is happening in the pleural space. Each type of scan provides a different “view” of the infection.

  • Thoracic Ultrasound (TUS): This is often the most important tool for the daily management of empyema. Ultrasound is actually superior to CT scans at finding septations—the tiny “walls” or “strands” that start to form in Stage II empyema (as described in Understanding Pleural Empyema) [3][4]. These strands create pockets (loculations) that can trap infection, making it harder to drain with a single tube [5].
  • CT Chest (with contrast): While ultrasound sees the fluid best, a CT scan sees the “big picture.” It provides a comprehensive map of your chest anatomy [6]. It is critical for surgical planning because it can show the thickness of the pleural peel and help doctors decide if you need a minimally invasive procedure or a more involved surgery [7].

Cracking the Code: Light’s Criteria

When doctors drain a sample of the fluid (thoracentesis), they first need to know if it is a transudate or an exudate.

  • Transudate: Usually caused by pressure imbalances in the body (like heart or kidney failure) [8].
  • Exudate: Caused by local inflammation or infection, like pneumonia or cancer [9].

To tell them apart, doctors use a set of rules called Light’s Criteria, which compares the protein and enzymes in the fluid to your blood [8][10]. While all empyemas are exudates, not all exudates are empyemas. If the fluid is an exudate, doctors will then use further tests (like the checklist below) to look for a specific infection.

The “Completeness Checklist” for Fluid Analysis

If your fluid is an exudate, the doctors will look for specific “red flags” that prove it is a complicated parapneumonic effusion or a full-blown empyema [11]. Use this checklist to review your results with your team:

Test What it measures The “Red Flag” for Empyema
Appearance Physical look of fluid Thick, cloudy, or “frank pus” (looks like milkshake) [1]
pH Level Acidity Below 7.2 (low pH indicates bacteria are active) [11]
Glucose Sugar levels Below 60 mg/dL (bacteria and white cells are “eating” the sugar) [11]
LDH Cell damage High (usually over 1,000 units/L) [11]
Cultures Bacteria growth Positive (identifies the specific germ causing infection) [2]

Why This Matters

If your fluid has a low pH, low sugar, or visible pus, it is unlikely to clear up with antibiotics alone [12]. These markers tell the doctor that the infection is “locked in” and that you need to move to the next steps outlined in Treatment Strategies—such as a chest tube or surgery—to physically remove the infected material so you can begin to heal [1][13].

Common questions in this guide

Why do doctors use both ultrasound and CT scans to diagnose pleural empyema?
Ultrasound is highly effective at finding tiny strands or pockets of fluid called septations, which helps guide daily management. A CT scan provides a broader map of your chest anatomy, allowing doctors to see the thickness of the pleural lining and plan for potential surgery.
What is the difference between an exudate and a transudate?
A transudate is fluid buildup caused by pressure imbalances, such as heart failure. An exudate is fluid caused by local inflammation or infection, like pneumonia or an empyema. Doctors use a set of rules called Light's Criteria to test the protein in the fluid to tell them apart.
What does a low pH in my pleural fluid mean?
A pH level below 7.2 in your pleural fluid is a strong indicator that bacteria are actively causing an infection. When this happens, the infection is unlikely to clear up with antibiotics alone and usually requires a chest tube or surgery to physically drain the infected fluid.
What does it mean if the doctor finds "frank pus" during drainage?
Frank pus refers to pleural fluid that is thick and cloudy, often resembling the consistency of a milkshake. Finding frank pus during a drainage procedure is a definitive sign of a severe infection that requires immediate intervention to remove the infected material.
Can taking water pills affect my pleural fluid test results?
Diuretics, or water pills, can alter the concentration of proteins and enzymes in your bodily fluids. This can sometimes affect the results of Light's Criteria, making it harder for doctors to accurately classify the fluid as an exudate or transudate.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Does my pleural fluid analysis show a pH below 7.2 or low glucose levels?
  2. 2.What did the thoracic ultrasound reveal about septations or loculations in the fluid?
  3. 3.Does the CT scan show any signs of a 'pleural peel' or an abscess?
  4. 4.Have my fluid cultures grown any specific bacteria, and is my current antibiotic effective against it?
  5. 5.Based on Light's Criteria, is this definitely an exudative fluid?

Questions For You

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References

References (13)
  1. 1

    Evaluation and management of pleural sepsis.

    Lui JK, Billatos E, Schembri F

    Respiratory medicine 2021; (187()):106553 doi:10.1016/j.rmed.2021.106553.

    PMID: 34340174
  2. 2

    Factors influencing pleural drainage in parapneumonic effusions.

    Porcel JM, Valencia H, Bielsa S

    Revista clinica espanola 2016; (216(7)):361-366 doi:10.1016/j.rce.2016.04.004.

    PMID: 27161381
  3. 3

    Chest ultrasound is better than CT in identifying septated effusion of patients with pleural disease.

    Yang L, Wang K, Li W, Liu D

    Scientific reports 2024; (14(1)):11964 doi:10.1038/s41598-024-62807-4.

    PMID: 38796511
  4. 4

    Sonographic septation: a useful diagnostic predictor of complicated parapneumonic effusion.

    Chang SY, Chen YC, Tsai CL, et al.

    Journal of investigative medicine : the official publication of the American Federation for Clinical Research 2021; (69(8)):1447-1452 doi:10.1136/jim-2020-001770.

    PMID: 34282046
  5. 5

    Clinical Characteristics of Community-Acquired Viridans Streptococcal Pneumonia.

    Choi SH, Cha SI, Choi KJ, et al.

    Tuberculosis and respiratory diseases 2015; (78(3)):196-202 doi:10.4046/trd.2015.78.3.196.

    PMID: 26175772
  6. 6

    Bochdalek hernia masquerading as pleural effusion in a young adult.

    Wong C, Lam CT, Yam LYC

    Respirology case reports 2023; (11(3)):e01104 doi:10.1002/rcr2.1104.

    PMID: 36818459
  7. 7

    Open window thoracostomy for a chronic calcified pleural empyema in the setting of a closed chest trauma.

    Harmouchi H, Hamraoui Y, Lakranbi M, et al.

    SAGE open medical case reports 2021; (9()):2050313X211025437 doi:10.1177/2050313X211025437.

    PMID: 34178354
  8. 8

    Pleural fluid biochemical analysis: the past, present and future.

    Zheng WQ, Hu ZD

    Clinical chemistry and laboratory medicine 2023; (61(5)):921-934 doi:10.1515/cclm-2022-0844.

    PMID: 36383033
  9. 9

    Approaching an Undiagnosed Pleural Effusion.

    Laniado I, DeMaio A, Feller-Kopman D

    Clinics in chest medicine 2025; (46(2)):217-226 doi:10.1016/j.ccm.2025.02.001.

    PMID: 40484497
  10. 10

    Diagnostic value of Light's criteria and albumin gradient in classifying the pathophysiology of pleural effusion formation in cats.

    Zoia A, Drigo M

    Journal of feline medicine and surgery 2016; (18(8)):666-72 doi:10.1177/1098612X15592170.

    PMID: 26116619
  11. 11

    Video-Assisted Thoracoscopic Surgery in Community-Acquired Thoracic Empyema: Analysis of Risk Factors for Mortality.

    Lin CW, Huang KY, Lin CH, et al.

    Surgical infections 2022; (23(2)):191-198 doi:10.1089/sur.2021.191.

    PMID: 35085460
  12. 12

    Loculated empyema in a neonate successfully treated with chest tube thoracostomy and antibiotics.

    Diez JRV, Perez MLM, Malayan GV, Cenabre MVL

    Respiratory medicine case reports 2020; (31()):101274 doi:10.1016/j.rmcr.2020.101274.

    PMID: 33209575
  13. 13

    Does the time to diagnosis and treatment influence outcome in adults with pleural infections.

    Klausen MB, Laursen C, Bendixen M, et al.

    European clinical respiratory journal 2023; (10(1)):2174645 doi:10.1080/20018525.2023.2174645.

    PMID: 36743828

This page explains diagnostic tests and fluid analysis for pleural empyema for educational purposes. Always review your specific lab and imaging results with your pulmonologist or surgical care team.

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