The utility of pleural fluid lactate dehydrogenase to adenosine deaminase ratio in pleural tuberculosis

<b>Background:</b> In high burden settings, the diagnosis of pleural tuberculosis is frequently inferred in patients who present with lymphocytic exudative effusions and high adenosine deaminase (ADA) levels. Recent retrospective evidence suggests that the lactate dehydrogenase (LDH)/ADA ratio is significantly lower in TB than in non-TB pleural effusions. <b>Objectives:</b> We aimed to investigate the utility of this ratio in patients with a high pre-test probability for TB. <b>Methods:</b> We compared the pleural LDH/ADA ratios, ADA levels and lymphocyte predominance of a prospectively collected cohort of patients with proven pleural TB (<i>n</i>=160) to those with a definitive alternative diagnosis (<i>n</i>=68). <b>Results:</b> The mean pleural fluid LDH/ADA ratio was lower in patients with pleural TB than alternative diagnoses (6.2 vs 34.3, p&lt;0.001). The area under the receiver operating characteristic curve was 0.92 (p&lt;0.001). A ratio of ≤12.5 had the best overall diagnostic efficiency (sensitivity 86%; specificity 88%). An LDH/ADA ≤10 and ≤7.5 had specificities of 90% and 96% respectively and positive predictive values (PPVs) of 90% and 97% respectively. A pleural fluid ADA cut-off of ≥40IU had a specificity and PPV of 78% and 91%, whereas the specificity and PPV of ADA ≥40IU in the context of lymphocyte predominance was 95% and 98% respectively. <b>Conclusions:</b> In a large cohort of patients we found that the LDH/ADA ratio of the effusion to be a valuable adjunct to support the diagnosis of pleural TB, particularly if lymphocyte predominance cannot be assessed.