Deirdre B Fitzgerald1,2,3, Su Lyn Leong1,2,3, Charley A Budgeon4, Kevin Murray5, Andrew Rosenstengal6, Nicola A Smith7, Silvia Bielsa8, Amelia O Clive9, Nick A Maskell9, José M Porcel8, Y C Gary Lee1,2,3. 1. Pleural Medicine Unit, Institute for Respiratory Health, Perth, Australia. 2. School of Medicine & Pharmacology, University of Western Australia, WA, Australia. 3. Respiratory Department, Sir Charles Gairdner Hospital, Perth, Australia. 4. Centre for Applied Statistics, University of Western Australia, WA, Australia. 5. School of Population and Global Health, University of Western Australia, WA, Australia. 6. Holy Spirit Northside Hospital, Brisbane, QLD, Australia. 7. Wellington Regional Hospital, Wellington, New Zealand. 8. Pleural Medicine Unit, Arnau de Vilanova University Hospital, Lleida, Spain. 9. North Bristol Lung Centre, Bristol, UK.
Abstract
BACKGROUND: Pleural fluid pH and glucose levels are both recommended in the workup of pleural effusions. Whether their levels correlate and predict each other or contribute independent knowledge is unclear. We aimed to investigate the pH/glucose relationship, assess their concordance and ascertain whether performing both tests provides additional information to performing either test alone. METHODS: The pH and glucose measurements from 2,971 pleural fluid samples, from three centers in Spain, UK and Australia, were categorized into Cancer (n=1,045), Infection (n=544), Tuberculosis (n=249) and Others (n=1,133) groups. The relationship between pH and glucose values and their concordance at clinically relevant cutoffs (pH 7.2 and glucose 3.3 mmol/L) were assessed. RESULTS: The mean pH of the cohort was 7.38 (SD 0.22) and median glucose 5.99 (range, 0.00-29.36) mmol/L. A regression model of the relationship between glucose (log-transformed) and pH with a restricted cubic spline showed linear (P<0.01) and nonlinear effects (P<0.01). The relationship was strong with a narrow confidence interval but the prediction interval was wide. Most (91.9%) samples were concordant using pH and glucose levels at cutoffs of 7.20 and 3.30 mmol/L respectively. Using pH alone, without glucose, captured 95.0% of the infection-related effusions with either pH or glucose below cutoff and glucose alone identified 91.7%. CONCLUSIONS: Pleural fluid pH and glucose have a strong non-linear relationship but, in most situations, the level of one cannot accurately predict the other. Concordance rates were high and either test is sufficient in the majority of cases.
BACKGROUND: Pleural fluid pH and glucose levels are both recommended in the workup of pleural effusions. Whether their levels correlate and predict each other or contribute independent knowledge is unclear. We aimed to investigate the pH/glucose relationship, assess their concordance and ascertain whether performing both tests provides additional information to performing either test alone. METHODS: The pH and glucose measurements from 2,971 pleural fluid samples, from three centers in Spain, UK and Australia, were categorized into Cancer (n=1,045), Infection (n=544), Tuberculosis (n=249) and Others (n=1,133) groups. The relationship between pH and glucose values and their concordance at clinically relevant cutoffs (pH 7.2 and glucose 3.3 mmol/L) were assessed. RESULTS: The mean pH of the cohort was 7.38 (SD 0.22) and median glucose 5.99 (range, 0.00-29.36) mmol/L. A regression model of the relationship between glucose (log-transformed) and pH with a restricted cubic spline showed linear (P<0.01) and nonlinear effects (P<0.01). The relationship was strong with a narrow confidence interval but the prediction interval was wide. Most (91.9%) samples were concordant using pH and glucose levels at cutoffs of 7.20 and 3.30 mmol/L respectively. Using pH alone, without glucose, captured 95.0% of the infection-related effusions with either pH or glucose below cutoff and glucose alone identified 91.7%. CONCLUSIONS: Pleural fluid pH and glucose have a strong non-linear relationship but, in most situations, the level of one cannot accurately predict the other. Concordance rates were high and either test is sufficient in the majority of cases.
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