Evaluating diagnostic tests in the pleural space. Differentiating transudates from exudates as a model

Pleural Fluid Analysis: Are Light's Criteria Still Relevant After Half a Century?

Fifty years from their initial description, Light's criteria are still unhesitatingly accepted as the default reference test for separating pleural transudates and exudates. Efforts should be focused not so much on trying to find an even more reliable technique for categorizing PEs but on improving the misclassification rate of transudates that characterize Light's criteria. Despite their shortcomings, Light's criteria may well continue their reign for another 50 years. Long live the Light's criteria!

Integration of voltammetric analysis, protein electrophoresis and pH measurement for diagnosis of pleural effusions: a non-conventional diagnostic approach

Pleural effusion is very common, but an etiologic diagnosis is often difficult. We used three unconventional diagnostic techniques (voltammetric analysis, protein electrophoresis and pH measurement) performed on pleural effusion to do a preliminary distinction between a neoplastic and a non-neoplastic origin. Pleural fluid samples were collected through thoracentesis, thoracoscopy, or post-surgery pleural drainage of 116 patients admitted to acute care wards. Samples were analyzed with the three unconventional techniques: voltammetric analysis using the BIONOTE system, capillary electrophoresis and pH measurement using a potentiometric method. The BIONOTE system is an innovative system that performs a cyclic voltammetric analysis of a biological liquid sample. The final output of the electrochemical analysis is an electrical pattern that represents a fingerprint of the analyzed sample and each sample has a different fingerprint. Data from the three unconventional diagnostic techniques were analyzed using partial least squares discriminant analysis to discriminate neoplastic from non-neoplastic effusions; we also evaluated sensitivity, specificity and percentage of correct classification. The mean age was 68 years (SD: 12); 78 (67.24%) participants were men. Results obtained from all the unconventional techniques employed showed that neoplastic and non-neoplastic pleural effusions were correctly classified in 80.2% of cases, with a sensitivity of 77% and specificity of 83%. The combined use of voltammetric analysis, protein electrophoresis and pH measurement of pleural fluid can easily and quickly distinguish a neoplastic from a non-neoplastic pleural effusion with reliable accuracy and represents an innovative diagnostic approach. In fact, this protocol can be executed in just few minutes directly in the patient's bed and it holds great promise to improve the prognosis and therapeutic chances.

Concordant and Discordant Exudates and Their Effect on the Accuracy of Light's Criteria to Diagnose Exudative Pleural Effusions

INTRODUCTION

To describe the incidence of discordant exudate (DE) effusions, their underlying etiologies and their effect on the accuracy of the Light's criteria to diagnose exudate effusions.

METHODS

A retrospective review of pleural fluid analysis (PFA) from a cohort of patients with pleural effusion (PE) who underwent thoracentesis. PEs were defined as exudative based on the Light's criteria. The effusions were further classified in concordant or DE.

RESULTS

From 847 PE samples, 611 (72.1%) were diagnosed as an exudate and 236 (27.9%) as a transudate. In 10.3% of cases (n = 87), there was discordancy between the final pleural fluid diagnosis and the PFA defined by Light's criteria. 281 (33.2%) of the 632 effusions classified as an exudate by Light's criteria were DE (52 transudates and 229 exudates). 182 (65%) of the 281 DE were found to be protein discordant (37 transudates and 145 exudates), and 99 (35.2%) were lactate dehydrogenase discordant (15 transudates and 84 exudates). The positive predictive value and positive likelihood ratio of Light's criteria for the diagnosis of an exudate effusion decreased from 99.4% and 67.4%, respectively, when the exudates were concordant to 81.5% and 1.7, respectively, if they were discordant.

CONCLUSIONS

In a significant percentage of patients, there is discordancy between the results of the PFA and the final clinical diagnosis. DE decreased the accuracy of Light's criteria to identify exudate PE, increasing the risk of misclassifying a transudate as an exudate. Concordant exudates almost universally established the presence of an exudative PE.

Identifying transudates misclassified by Light's criteria

PURPOSE OF REVIEW

Light's criteria combine three dichotomous tests into a decision rule that is considered positive if any one of the tests is positive. This strategy clearly maximizes sensitivity, although at the expense of specificity. Although Light's criteria identify 98% of pleural exudates, they misclassify about 25% of transudates as exudates. The way to overcome this limitation is discussed in this review.

RECENT FINDINGS

Traditionally, measurement of the protein gradient between the serum and pleural fluid has been recommended to decrease the misclassification rate of Light's criteria. A recent study demonstrated that a gradient between the albumin levels in the serum and the pleural fluid more than 1.2 g/dl performs significantly better than a protein gradient more than 3.1 g/dl to correctly categorize mislabeled cardiac effusions (83 vs. 55%). On the other hand, the accuracy of a pleural fluid to serum albumin ratio less than 0.6 excelled when compared with albumin and protein gradients in patients with miscategorized hepatic hydrothoraces (77 vs. 62 vs. 61%).

SUMMARY

The simplest strategy to reveal the true transudative nature of heart failure-related effusions, labeled as exudates by Light's criteria, is to calculate the serum to pleural fluid albumin gradient. Conversely, for misclassified hepatic hydrothoraces, measurement of the pleural to serum albumin ratio is recommended. The serum to pleural fluid protein gradient should no longer be considered the preferred test for this purpose.

Pleural effusion: diagnosis, treatment, and management

A pleural effusion is an excessive accumulation of fluid in the pleural space. It can pose a diagnostic dilemma to the treating physician because it may be related to disorders of the lung or pleura, or to a systemic disorder. Patients most commonly present with dyspnea, initially on exertion, predominantly dry cough, and pleuritic chest pain. To treat pleural effusion appropriately, it is important to determine its etiology. However, the etiology of pleural effusion remains unclear in nearly 20% of cases. Thoracocentesis should be performed for new and unexplained pleural effusions. Laboratory testing helps to distinguish pleural fluid transudate from an exudate. The diagnostic evaluation of pleural effusion includes chemical and microbiological studies, as well as cytological analysis, which can provide further information about the etiology of the disease process. Immunohistochemistry provides increased diagnostic accuracy. Transudative effusions are usually managed by treating the underlying medical disorder. However, a large, refractory pleural effusion, whether a transudate or exudate, must be drained to provide symptomatic relief. Management of exudative effusion depends on the underlying etiology of the effusion. Malignant effusions are usually drained to palliate symptoms and may require pleurodesis to prevent recurrence. Pleural biopsy is recommended for evaluation and exclusion of various etiologies, such as tuberculosis or malignant disease. Percutaneous closed pleural biopsy is easiest to perform, the least expensive, with minimal complications, and should be used routinely. Empyemas need to be treated with appropriate antibiotics and intercostal drainage. Surgery may be needed in selected cases where drainage procedure fails to produce improvement or to restore lung function and for closure of bronchopleural fistula.

A systematic approach to the investigation and diagnosis of a unilateral pleural effusion

Fluid in the pleural space is a common sequela of a wide range of diseases which may be pulmonary, pleural or extrapulmonary. As the differential diagnosis is wide, a systematic approach to investigation and diagnosis is recommended. This review highlights the important features and recommendations for the investigation of a unilateral pleural effusion, a common condition encountered by the general physician. The aim of this study was to assist with a speedy diagnosis of the underlying pathology, using appropriate investigative techniques, while minimising the use of invasive procedures.

Diagnostic work-up of pleural effusions

A wide range of diseases may be the cause of an accumulation of fluid in the pleural space. Pleural effusion is a major diagnostic problem, since the pleura is an inner cavity with no direct access. The aim of this review is to provide a practical approach to the investigation of the patient presenting with pleural effusion. This should help to accurately diagnose pleural effusion and keep time-consuming, but necessary, invasive investigations to the minimum.

Classification tree analysis for the discrimination of pleural exudates and transudates

BACKGROUND

Classification and regression tree (CART) analysis is a non-parametric technique suitable for the generation of clinical decision rules. We have studied the performance of CART analysis in the separation of pleural exudates and transudates.

METHODS

Basic demographic, radiologic and laboratory data were retrospectively evaluated in 1257 pleural effusions (204 transudates and 1053 exudates, according to standard clinical criteria) and submitted for CART analysis. The model's discriminative ability was compared with that of Light's criteria, in both the original formulation and an abbreviated version, i.e., deleting the pleural fluid (PF)/serum lactate dehydrogenase (LDH) ratio from the triad.

RESULTS

A first CART model built starting from all available data identified PF/serum protein ratio and PF LDH ratios as the two best discriminatory parameters. This algorithm achieved a sensitivity of 96.8%, slightly lower than that of classical Light's criteria (98.5%) and comparable to that of the abbreviated Light's criteria (97.0%), and significantly better specificity (85.3%) compared to both classical (74.0%) and abbreviated (79.4%) Light's criteria. A second CART model developed after excluding serum measurements selected PF protein and PF LDH as the most discriminatory variables, and correctly classified 97.2% of exudates and 77.0% of transudates.

CONCLUSIONS

CART-based algorithms can efficiently discriminate between pleural exudates and transudates.

Discriminating Between Transudates and Exudates

The dichotomous classification of pleural fluid as a transudate or an exudate simplifies diagnostic efforts in determining the cause of pleural effusions. Multiple pleural fluid tests are available to discriminate between these two classes of effusions. Tests commonly used in clinical practice depend on the detection in pleural fluid of large-molecular-weight chemicals that enter the pleural space to greater degrees in conditions associated with exudative compared with transudative effusions. Considerable misclassifications can occur with all available testing strategies, so clinicians benefit from adopting a nondichotomous, bayesian approach for interpreting test results.

A systematic review finds that diagnostic reviews fail to incorporate quality despite available tools

BACKGROUND AND OBJECTIVE

To review existing quality assessment tools for diagnostic accuracy studies and to examine to what extent quality was assessed and incorporated in diagnostic systematic reviews.

METHODS

Electronic databases were searched for tools to assess the quality of studies of diagnostic accuracy or guides for conducting, reporting or interpreting such studies. The Database of Abstracts of Reviews of Effects (DARE; 1995-2001) was used to identify systematic reviews of diagnostic studies to examine the practice of quality assessment of primary studies.

RESULTS

Ninety-one quality assessment tools were identified. Only two provided details of tool development, and only a small proportion provided any indication of the aspects of quality they aimed to assess. None of the tools had been systematically evaluated. We identified 114 systematic reviews, of which 58 (51%) had performed an explicit quality assessment and were further examined. The majority of reviews used more than one method of incorporating quality.

CONCLUSION

Most tools to assess the quality of diagnostic accuracy studies do not start from a well-defined definition of quality. None has been systematically evaluated. The majority of existing systematic reviews fail to take differences in quality into account. Reviewers should consider quality as a possible source of heterogeneity.

Multilevel likelihood ratios for identifying exudative pleural effusions(*)

STUDY OBJECTIVES

To determine multilevel likelihood ratios for pleural fluid tests that are commonly used to discriminate between exudative and transudative pleural effusions.

DESIGN

Meta-analysis of patient-level data.

PATIENT DATA

Selected studies included patients with diagnoses of exudative or transudative pleural effusions who underwent thoracentesis and laboratory analysis of their pleural fluid.

MEASUREMENTS AND METHODS

Studies were identified by searching MEDLINE and related bibliographies. Data were obtained for 1,448 patients from seven primary investigators or extracted from dot plots in published reports. Likelihood ratios were calculated from extracted data stratified across ranges of test result values.

RESULTS

Sufficient data were available to calculate multilevel likelihood ratios for the elements of Light's criteria (pleural fluid lactate dehydrogenase [LDH], ratio of pleural fluid to serum LDH, and ratio of pleural fluid to serum protein), pleural fluid protein, ratio of pleural fluid to serum cholesterol, pleural fluid cholesterol, and gradient of pleural fluid to serum albumin. Each of these tests provided levels of likelihood ratios through the most clinically relevant range (0 to 10).

CONCLUSION

Multilevel likelihood ratios combined with a clinician's estimation of the pretest probability of an exudative effusion improve the diagnostic accuracy of discriminating between exudative and transudative pleural effusions. Likelihood ratios avoid the use of confusing terms, such as "pseudoexudates," that derive from the use of single cutoff points for pleural fluid tests.

Pleural fluid pH as a predictor of pleurodesis failure: analysis of primary data

STUDY OBJECTIVES

To determine the predictive accuracy of pH for identifying patients with malignant pleural effusions who will fail pleurodesis.

DESIGN

Analysis of published and unpublished individual patient-level data retrieved from a MEDLINE search and correspondence with primary investigators.

STUDY SELECTION

Studies that reported pleural fluid pH values and outcomes of pleurodesis for patients with malignant pleural effusions.

DATA COLLECTION AND ANALYSIS

Primary investigators supplied data for 433 patients. Receiver operating characteristic analysis and logistic regression estimated the predictive accuracy, decision thresholds, and value of pleural fluid pH compared with other clinical factors. The primary investigations were graded for study design.

RESULTS

Pleural fluid pH was the only independent predictor of pleurodesis failure (odds ratio, 4.46; 95% confidence interval [CI], 2.69 to 7.45; p < 0.0001) and had an area under the receiver operating characteristic curve (decision threshold, < or = 7.28) of 0.671 (95% CI, 0.624 to 0.715). The pH model fit the data well (p = 0.48) with the probability of pleurodesis failure increasing as pH decreased; specificity and negative predictive values for pleurodesis failure exceeded 90% and 80%, respectively, with a positive predictive value of 45.7% at pH values < or = 7.15. The primary studies had several important design limitations.

CONCLUSIONS

Using patient-level data, this study showed that pleural fluid pH has only modest predictive value for predicting symptomatic failure and should be used with caution, if at all, in selecting patients for pleurodesis. The limitations of the primary studies and low predictive accuracy should be considered when using pleural fluid pH for patient care.

Pleural fluid pH as a predictor of survival for patients with malignant pleural effusions

STUDY OBJECTIVES

To assess the accuracy of pleural fluid (PF) pH in predicting duration of survival of patients with malignant pleural effusions.

DESIGN

Analysis of patient-level data from nine sources retrieved from a MEDLINE search and correspondence with primary investigators.

STUDY SELECTION

Published and unpublished studies that report PF pH values and duration of survival of patients with malignant pleural effusions.

DATA COLLECTION AND ANALYSIS

Primary investigators supplied patient-level data (n = 417), which was examined by receiver operating characteristic (ROC) analysis, logistic regression, and survival time modeling to determine the utility of PF pH for predicting survival compared with other clinical factors. The primary investigations were graded for study design.

MEASUREMENTS AND RESULTS

Median survival (n = 417) was 4.0 months: PF pH (p < 0.0039) was an independent predictor of survival duration. A PF pH test threshold < or = 7.28 had the highest accuracy for identifying poor 1-, 2-, and 3-month survivals. The predictive accuracies of PF pH (area under the ROC curve range, 0.571 to 0.662) and a PF pH-high-risk tumor (lung, soft tissues, renal, ovary, gastrointestinal, prostate, and oropharynx) model (odds ratio range, 2.91 to 6.67), however, were modest for predicting 1-, 2-, and 3-month survival. Only 54.4% and 62.7% of patients identified by PF pH < or = 7.28 or the PF pH-high-risk tumor model to die within 3 months were correctly classified. Weaknesses of the primary data were identified.

CONCLUSIONS

PF pH has insufficient predictive accuracy for selecting patients for pleurodesis on the basis of estimated survival.