A pfADA level <30 IU/l makes a diagnosis of TB highly unlikely in the South London population. Its high sensitivity and negative predictive values make pfADA a valuable screening test for excluding suspected pleural TB.

Introduction and objectivesPleural fluid adenosine deaminase (pfADA) is a validated diagnostic marker for pleural tuberculosis (TB) in high prevalence areas, with good sensitivity and specificity reported at a threshold of 40 U/L. However, in north-west Europe pfADA is not routinely measured, due to a lack of evidence as to its diagnostic utility in areas of low TB prevalence. The aim of this study is to assess the sensitivity and specificity of pfADA in a low-prevalence area, evaluating its diagnostic value for pleural TB.MethodsA retrospective analysis considered all pfADA-tested suspected pleural TB patients within one hospital trust from 2009–2015. This cohort was then divided into two groups: those with a confirmed diagnosis of pleural TB and those without pleural TB. Those without pleural TB were used as a control group, to determine the sensitivity and specificity of pfADA at various thresholds.ResultsOf 156 patients tested for pfADA, 25 had confirmed pleural TB and 131 did not, with mean pfADA levels of 71.7 (±25.2) and 19.8 (±22.4), respectively. On a Receiver Operating Characteristic (ROC) curve (Figure 1), pfADA of 30 U/L has a sensitivity of 100%, specificity 83%, positive and negative predictive values of 53% and 100% respectively. At a threshold of 40 U/L, sensitivity was 88% with a specificity of 88%. The calculated area under ROC curve is 0.949 (95% CI 0.91–0.982).Abstract P252 Figure 1 ConclusionAlthough the positive predictive value of pfADA may be lower in areas of low TB prevalence, its negative predictive value is unaffected, retaining its value as a worthy screening test to exclude pleural TB, allowing focus on obtaining adequate culture samples and biopsies in suspected pleural TB.