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

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).

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

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.

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.

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.