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

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.