Incidence of Discordant Pleural Fluid Exudates and Diagnostic Patterns: A Retrospective Cohort Study

Our plain-English summary of Incidence of Discordant Pleural Fluid Exudates and Diagnostic Patterns: A Retrospective Cohort Study, by D.N. Addala, R. Mercer, A. Sundaralingam, et al (Chest, 2025).

The study

• Authors: D.N. Addala, R. Mercer, A. Sundaralingam, B. Iqbal, A. Elsheikh, E.O. Bedawi, J. Wrightson, R.J. Hallifax, N.M. Rahman
• Journal: Chest
• Published: 2025;168(6):1517-1527
• Cited by: 1 article, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Background: Light's criteria use pleural fluid protein and lactate dehydrogenase (LDH) to differentiate pleural effusions as exudative or transudative. In a subset of exudative pleural effusions, discordance occurs between LDH and protein (ie, protein high, LDH low, or vice versa).

Research Question: What incidence and diagnostic profile are associated with discordant pleural fluid biochemistry?

Study Design And Methods: We conducted a retrospective analysis of 995 pleural fluid samples between 2015 and 2017 from a UK tertiary center. Exudates were subdivided into concordant or discordant, with low protein defined as < 30 g/L and low LDH as < 170 IU/L. Demographic characteristics and diagnostic patterns were assessed in both groups. A χ2 test and ORs (± 95% CI) were calculated for each diagnosis between discordant and concordant pleural effusions, and adjusted ORs were calculated by using multivariable logistic regression.

Results: In 715 exudative pleural fluid samples, 229 (32%) were discordant. Eighty-five (37%) of these displayed low protein, with high LDH, and 144 (63%) displayed low LDH with high protein. The median age was higher in the discordant group than in the concordant group (75 years vs 70 years; P = .01). The proportion of patients with the following diagnoses were significantly higher in the discordant group compared with the concordant group: fluid overload (10% [24 of 229] discordant vs 2% [10 of 486] concordant; P < .0001), benign asbestos-related pleural effusion (14% [33 of 229] vs 9% [44 of 486]; P = .031), and ICU-associated effusion (9% [20 of 229] vs 3% [15 of 486]; P = .001). The following were less frequent in the discordant group: pleural infection (6% [14 of 229] vs 16% [79 of 486]; P < .0001) and malignant pleural effusion (34% [77 of 229] vs 42% [206 of 486]; P = .025). These patterns were maintained when adjusting for age and sex.

Interpretation: Our results indicate that discordant pleural effusions are common and represent a biologically distinct entity with different diagnostic patterns compared with concordant effusions. Clinicians should assess for discordance early and tailor investigations accordingly.

Copyright © 2025 The Author(s). Published by Elsevier Inc. All rights reserved.

In plain English

This retrospective cohort study examined the first diagnostic pleural fluid sample from each patient at Oxford University Hospitals, a UK tertiary centre, from January 2015 to December 2017. Of 995 samples, 715 were exudates with complete results. Exudates are fluid the article says typically arises from local causes such as inflammation of the pleura. The authors called an exudate "discordant" when only one of two laboratory markers, protein or lactate dehydrogenase (LDH), was high. 229 of 715 (32%) were discordant. Compared with the 486 concordant exudates, discordant ones more often came from fluid overload (10% vs 2%), benign asbestos-related pleural effusion (14% vs 9%; odds ratio 1.69, 95% CI 1.06-2.72) and ICU-associated effusion (9% vs 3%). They less often came from pleural infection (6% vs 16%) or malignant pleural effusion (34% vs 42%). These differences held after adjusting for age and sex. The asbestos-related association weakened when missing results were all counted as concordant. The article did not report patients' asbestos exposure. The authors concluded that discordant effusions are common and show a distinct diagnostic pattern. They listed the retrospective, single-centre design among the limitations.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Retrospective cohort study (single centre)
• Who was studied: 995 pleural fluid samples (first diagnostic sample per patient) from Oxford University Hospitals, UK, January 1, 2015 to December 31, 2017. 89 samples lacked protein or LDH results and 191 were transudates. That left 715 exudates: 486 concordant and 229 discordant (144 high protein/low LDH, 85 low protein/high LDH).
• Exposure: The factor compared was discordant pleural fluid biochemistry among exudates. Discordant meant protein high with LDH low, or LDH high with protein low. Low protein was defined as below 30 and low LDH as below 170 IU/L. Benign asbestos-related pleural effusion (BAPE) was one of the final diagnoses compared. The article did not measure or describe asbestos exposure, fiber types or occupations.
• Main results:
  • Of 715 exudates, 229 (32%) were discordant. Of these, 144 (63%) had high protein with low LDH and 85 (37%) had high LDH with low protein.
  • Median age was 75 years in the discordant group vs 70 years in the concordant group. Older age (OR 1.02 per year; 95% CI 1.01-1.03) and female sex (OR 1.68; 95% CI 1.23-2.31) were independently associated with discordant effusions.
  • Benign asbestos-related pleural effusion: 33 of 229 (14%) discordant vs 44 of 486 (9%) concordant. Unadjusted OR 1.69 (95% CI 1.06-2.72); adjusted for age and sex, OR 1.88 (95% CI 1.13-3.13).
  • In the sensitivity analysis for missing biochemistry, the association between discordance and benign asbestos-related pleural effusion weakened when all missing values were treated as concordant.
  • Benign asbestos-related pleural effusion by subgroup: 23 of 144 (16%) high-protein/low-LDH discordant effusions (Table 3 OR 2.99, 95% CI 1.90-4.96, vs concordant). In the high-LDH/low-protein subgroup it was 10 of 85 (12%), with no significant difference from concordant effusions.
  • Fluid overload: 24 of 229 (10%) discordant vs 10 of 486 (2%) concordant. OR 5.57 (95% CI 2.67-11.37); adjusted OR 5.37 (95% CI 2.55-11.30).
  • ICU-associated effusion: 20 of 229 (9%) discordant vs 15 of 486 (3%) concordant. OR 3.01 (95% CI 1.53-5.85); adjusted OR 3.32 (95% CI 1.62-6.80).
  • Malignant pleural effusion: 77 of 229 (34%) discordant vs 206 of 486 (42%) concordant. OR 0.69 (95% CI 0.49-0.96); adjusted OR 0.57 (95% CI 0.41-0.80).
  • Mesothelioma was the primary tumour in 36 (17%) of malignant effusions in the concordant group and 11 (14%) in the discordant group. No statistical comparison was reported for this.
  • Pleural infection (complex parapneumonic effusion or empyema): 14 of 229 (6%) discordant vs 79 of 486 (16%) concordant. Unadjusted OR 0.34 (95% CI as printed 0.29-0.61); adjusted OR 0.39 (95% CI 0.21-0.71).
• The authors' conclusion: The authors concluded that discordant exudates are common, occurring in slightly less than one-third of exudative effusions. They concluded that these effusions appear to have a distinct diagnostic pattern that clinicians should be aware of.
• Limitations the authors state:
  • The retrospective design may have underestimated the rate of dual diagnoses.
  • Diuretic use in patients with fluid overload was hard to quantify retrospectively.
  • The single-centre design may limit wider application, especially to centres with more tuberculous or infective effusions. The results are more applicable to a Western/European context.
  • Serum protein and LDH were rarely measured, so the study could not assess the effect of serum values on discordance.
  • Some diagnoses, including tuberculous effusions, had numbers too low for meaningful comparison, and the centre is in a low-TB area.
  • Careful history, examination and other tests such as pleural biopsy may give diagnostic information early, reducing the usefulness of observing discordance.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Pleural plaques and thickening.

Medical terms in this study, explained in our Lexicon: Exudate, Cohort Study, Light's Criteria, Lactate Dehydrogenase (LDH), Pleural Effusion.

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