Evaluation of Diagnostic Value of Urinary Deep Stick in Differentiating Different Types of Exudative Pleural Effusions and Differentiating Exudative Pleural Effusions From Transudates

Pleural effusion is caused by the accumulation of fluid in the pleural cavity, which can occur by itself or due to the disease of the surrounding parenchyma, such as heart failure, infection, malignancy, pulmonary embolism, and inflammatory conditions [1]. Pleural effusion is one of the main causes of mortality and pulmonary morbidity [2,3].

Pleural effusion is divided into two types, transudative and exudative, and the first step in the etiological diagnosis of pleural effusion is the distinction between exudative and transudative types [4] which are distinguished in the clinical context using Light’s criteria [5].

Based on this criterion, there is at least one of the following findings in exudative fluids: the ratio of pleural fluid protein to serum protein is more than 0.5, the ratio of pleural fluid lactate dehydrogenase (LDH) to serum LDH is more than 0.6, the level of fluid LDH pleural more than two thirds of the highest normal limit of serum LDH; None of these criteria exist in transudative fluids [6,7].

Pleural effusion can occur in a wide variety of complex conditions and there are more than 60 known causes for it [1]. Also, many cases of pleural effusion do not have a specific cause, so in addition to physical examination and radiological evaluations, a detailed analysis of pleural fluid is necessary to diagnose the cause and treat it [1,8], such as the evaluation of protein level [8,9], pH and glucose [10], LDH, cytology and pleural fluid microbiology are included in common clinical guidelines for the diagnosis and treatment of pleural effusion [1,8].

Delay in the diagnosis of pleural effusion can be associated with significant complications and mortality [2]. Therefore, rapid diagnosis is a key factor in the management of pleural effusion. Rapid tests help in early diagnosis, immediate treatment, prevention of complications and reduction of transmission of infectious diseases.

Dipstick or leukocyte esterase reagent (LER) strip for use in other body fluids, including ascites and cerebrospinal fluid, for rapid diagnosis of spontaneous bacterial peritonitis (SBP) [11,12], meningitis [13,14] and urinary tract infection [15,16] is proposed.

However, the diagnostic function of dipstick in the assessment of pleural fluid has not yet been fully investigated, test strips to determine the amount of protein and leukocyte esterase in pleural fluid, two parameters that are of particular importance in the diagnosis of pleural types, have been less evaluated [6,7].

Considering that the role of urinary dipstick in the diagnosis of pleural effusion has not yet been confirmed, this study was conducted with the aim of investigating the diagnostic value of urine dipstick in quickly differentiating exudative pleural effusion from transudate, as well as infectious exudative pleural effusion from non-infectious.

This prospective study was conducted on patients diagnosed with pleural effusion requiring thoracentesis in the pulmonary department of teaching hospitals under the supervision of Jundishapur University of Ahvaz (including Imam Khomeini, Golestan and Razi hospitals) in 2022. The ethical committee of Ahvaz Jundishapur University of Medical Sciences has approved this study (Ethics number: IR.AJUMS.HGOLESTAN.REC.1401.182). The Declaration of Helsinki outlines basic ethical principles to protect human research subjects.

A. Sample size and sample characteristics

The sample size is based on the same article (17) in which the dipstick sensitivity for detecting infectious pleural effusion was reported to be 90% compared to the gold standard (P=0.9), taking into account the confidence factor of 95% and the accuracy of 0.5 0 was calculated using the following formula equal to 70 individuals.

\[N=\frac{Z^2_{1-\frac{\alpha }{2}}\ P(1-P)}{d^2}\]

At first, the pleural fluid samples of 88 patients with pleural effusion were included in the study by purpose-based sampling method. But based on the inclusion and exclusion criteria, 70 patients were included in the final analysis.

The inclusion criteria were: clinical diagnosis of pleural effusion by a lung specialist and confirmed by imaging findings and requiring thoracentesis, normal coagulation tests (INR less than or equal to 2), absence of thrombocytopenia (platelet level less than 100,000 per microliter) and patient consent to participate in the study. Also, patients with coagulopathy, frequent pleural effusion, antibiotic use in the recent past (one month) and any defects in file information and laboratory results were excluded from the study.

B. Etiological diagnosis of pleural effusion

After obtaining informed consent, history and clinical examination, routine examinations were performed in all patients. Epidemiological and clinical information of the patients, including sex, age, co-morbidities at the time of thoracentesis were recorded. Pleural effusion was diagnosed by a pulmonologist based on the absence of breathing sounds on auscultation, dullness on accuracy and reduction of tactile fremitus and confirmed on the basis of radiological findings.

Based on the type of pleural effusion, the patients were divided into 3 groups: infectious exudative pleural effusion, non-infectious exudative pleural effusion, and transvasive pleural effusion. The cause of pleural effusion was determined based on clinical findings and laboratory results of pleural fluid and serum.

Exudative and transudative effusions were defined based on Light’s criteria. The ratio of pleural fluid protein to serum protein, lactate dehydrogenase ratio less than 0.6 was used to differentiate exudative from transudative [5].

C. Thoracentesis and assessment of pleural fluid by dip stick

All patients underwent ultrasound-guided pleural aspiration (USG). Thoracentesis was performed using an 8 mm needle and a sample of pleural fluid was sent to the laboratory for biochemical evaluations (protein, glucose and lactate dehydrogenase, pH) and microbiological and cytological evaluations. Simultaneously, pleural fluid was evaluated using a leukocyte esterase reagent strip designed for urine testing to measure protein, leukocyte esterase, pH, and glucose.

A drop of non-centrifuged pleural fluid collected in heparinized tubes was placed on the leukocyte label of the strip. After exactly 2 minutes, the color change in the strip was visually read. The results of the reagent strip were read using a calorimeter: on a scale of 6 degrees (grade 0 to +4) for protein evaluation, on a scale of 5 degrees (grade 0 to +3 based on the density of purple color) for leukocyte esterase level, on a scale of 5 degrees (grade 0 to +3) for glucose level, as values for pH (range 8.50-6, with 0.5 intervals) and numerical values for specific gravity (dark green 1.005, light green 1.015, brown 1.02, orange 1.03).

All analysis of the dipstick results was done by a researcher who was unaware of the results of the biochemical and microbiological tests. A dip stick protein grade higher than +3 was considered exudative and a leukocyte grade higher than +2 was considered infectious. Finally, the diagnostic performance of the leukocyte esterase strip test was compared with the standard diagnostic method (laboratory and microbiological findings).

D. Statistical Analysis

Statistical analysis was done by SPSS software version 26 (SPSS Inc, Chicago, IL, USA). Kolmogorov-Smirnov and Shapiro-Wilk tests were used to test for the normality of data. Central and descriptive statistics were reported for quantitative. Analytical analyzes were performed using Kruskal-Wallis non-parametric and chi-square test tests. P value<0.05 was considered statistically significant.

To check the diagnostic performance of urinary dipstick parameters, ROC curve was drawn and areas under the curve (AUC) were determined in 95% confidence interval (CI). To determine the diagnostic performance of the parameters of the reagent strip in the diagnosis of pleural effusions at the optimal cut-off point, sensitivity percentage, specificity, positive predictive value (PPV), negative predictive value (NPV) were reported. ROC analysis was performed using MedCalc software (MedCalc Software Bvba).

In this study, 70 patients diagnosed with pleural effusion requiring thoracentesis, with mean age of 58.64 ± 23.04 years (range 16 to 96 years), including 37 males (52.9%) and 33 females (47.1%) participated (Table 1).

Most of the patients (84.29%) were hospitalized in ICU due to acute respiratory failure. According to laboratory reports, 17 patients (24.29%) had transudative pleural effusion and 53 patients (75.71%) had exudative pleural effusion.

The underlying causes of transudative effusion included kidney failure, pulmonary embolism, heart failure, and liver cirrhosis. Patients with exudative pleural effusion included 30 cases (56.60%) of infectious exudative (pneumonia, tuberculosis, empyema) and 23 cases (43.40%) of non-infectious exudative (including malignancies, lupus erythematosus, kidney disease).

The results of laboratory evaluations showed that the number of leukocytes and protein level of pleural fluid were the highest in patients with pleural effusion and infectious exudate and the lowest in transudative patients (P< 0.0001 and P= 0.001, respectively) (Table 2).

The results of pleural fluid evaluation by dipstick showed that bilirubin (P=0.092) and ketone (P=0.799) tests were not related to the type of pleural effusion (Table 3). Also, the result of nitrite test was negative in all pleural effusion patients (100%). Protein test of reagent strip in all patients with transudative pleural effusion showed +1 and +2 results. Also, in 96.22% of patients with exudative pleural effusion, it was grade +3 or more.

Leukocyte esterase test was negative in all patients with transudative pleural effusion as well as patients with non-infectious exudative effusion. In all patients with infectious exudative pleural effusion, LER grade was +2 and +3. Glucose test strip was +2 and +3 in all patients with transudative effusion. The pH test of the reagent strip showed values greater than or equal to 8 in all patients with transudative pleural effusion and greater than or equal to 7 in patients with non-infectious exudate. In infectious exudative patients, the result of pH test was between 6 and 8.

The results of the analysis of the area under the ROC curve showed that protein tests (AUC: 0.989; P < 0.0001), leukocyte esterase (AUC: 0.783; P < 0.0001), glucose (AUC: 0.858; P < 0.0001) P<0), and the pH of the reagent strip (AUC: 0.906; P<0.0001) were effective in differentiating types of exudative pleural effusions from transudates compared to the standard method (laboratory findings and microbiological culture) (Table 4 and Figure 1). However, pleural fluid specific gravity test was not effective in differentiating types of exudative pleural effusion from transudate (AUC: 0.568; P=0.312).

The results of the analysis of the area under the ROC curve also showed that protein tests (61.5%; AUC: 0.726; P<0.0001), leukocyte esterase (AUC: 1.000; P<0.0001), glucose (AUC: 0.935; P<0.0001) and reagent strip pH (AUC: 0.838; P<0.0001) were effective in differentiating infectious exudative effusion from non-infectious (Table 5 and Figure 2). However, pleural fluid specific gravity test did not have a significant diagnostic function in differentiating infectious exudative from non-infectious (AUC: 0.616; P=0.122).

The results of the present study showed that the leukocyte esterase test was negative in all patients with transudative pleural effusion and non-infectious exudative effusion, and cases with infectious pleural effusion had a leukocyte esterase grade of more than 2.

The analysis of the area under the ROC curve also showed that the leukocyte esterase test was effective in differentiating exudative pleural effusions from transudates (sensitivity and negative nasal value of 100%) as well as differentiating infectious from non-infectious exudatives. These results show that this test can be useful for quick and accurate diagnosis of infection in pleural effusion.

The leukocyte esterase dipstick test is designed and considered for the detection of leukocytes in urine, but it has also been used in other biological samples for rapid diagnosis of infection. This test uses the ability of esterase enzyme (which is present in polymorphonuclear leukocytes of pleural fluid) to break down heterocyclic carboxylates and form a pyrrole.

Then pyrrole reacts with diazonium salt and creates purple color in the reagent strip [17]. Since there are many causes of pleural effusion, the importance of ruling out infectious causes is essential for deciding on the use of antibiotics. If the cause of secondary effusion is due to reasons such as heart, liver or kidney failure, the use of diuretics is recommended and the unnecessary use of antibiotics should be avoided. Therefore, this test may be used as a bedside test, until the laboratory results are ready and available, it will be useful in making decisions and choosing the appropriate management strategy (treatment of heart failure or change in antibiotic treatment and pleural drainage).

In past studies, the high diagnostic power of evaluating ascites fluid by the LER reagent strip in diagnosing SBP infection has also been reported, so that false negative results were zero in most studies (specificity 100) [11,18,19].

However, the diagnostic performance of leukocyte esterase dipstick for identifying and differentiating types of pleural effusions has not been fully investigated and there are very few studies in this field; Among other things, in a study, Satheesnath et al. showed that the leukocyte esterase test has a good performance in detecting infectious exudative pleural effusion from non-infectious cases (sensitivity 75%, specificity 100%, positive predictive value 100% and negative predictive value 93.4 %) [20].

In the study by Azoulay et al. on patients with pleural effusion hospitalized in ICU, the evaluation of pleural fluid protein by dipstick had a high accuracy for the diagnosis of exudative pleural effusion from transudate (sensitivity 93.1%, specificity 50%, positive predictive value 3 84.84% and negative predictive value 71.5%). The level of pleural fluid protein evaluated by the reagent strip was in 84.3% of exudative patients grade >3, however, 50% of transudative patients also had protein grade >3 [21].

In the present study, the glucose test strip is useful in differentiating types of exudative pleural effusion from transudate (sensitivity, specificity, positive and negative predictive value, 82.35% and 79.25%, 56.0% and 93.3%, respectively) and It was also quite effective in differentiating infectious from non-infectious exudates (93.33% sensitivity; 93.30% specificity; 93.3% positive predictive value; 91.3% negative predictive value). In other studies, the evaluation of pleural fluid glucose has been confirmed for the diagnosis of parapneumonic effusions and has shown a better diagnostic performance than other pleural fluid biomarkers [22,23].

Assessment of pleural fluid pH and glucose is recommended in common clinical guidelines for the evaluation of pleural effusions [10,24]. Also, these two parameters are closely related, so that the pH of the pleural fluid is determined by the acids produced from the glucose metabolism of the cells in the pleural space, as well as the exit of those acids and the entry of glucose from the pleural membrane. The release of acids in the pleural fluid can be observed in acute inflammatory conditions (such as infection), chronic fibrotic conditions and pleural malignancies [25,26].

In the present study, the pH test of the reagent strip was effective in differentiating types of exudative pleural effusions from transudates and also in differentiating infectious from non-infectious exudates. In other studies, the performance of reagent strips to determine the pH of pleural fluid has been confirmed [27,28].

In Azoulay et al.’s study, pleural fluid pH assessed by urinary dipstick was significantly lower in patients with infectious pleural effusion than non-infectious ones, but it did not have a significant diagnostic function for differentiating types of pleural effusions and classifying exudative effusions as infectious or non-infectious [21]. Therefore, the low pH of the pleural fluid, which is determined through a reagent strip or laboratory tests, can be helpful in the discussion of chest tube drainage.

Another finding of this study was that pleural fluid specific gravity test was not effective in differentiating types of exudative pleural effusion from transudate and also in differentiating infectious exudative from non-infectious exudative. In the study of Abdollahi et al, the evaluation of the specific gravity of pleural fluid by dip stick compared to the standard diagnostic method (Lite criterion) did not have enough sensitivity and specificity to distinguish pleural effusion exudate from transudate [29]. These results show that the use of this method in distinguishing types of pleural effusion is not acceptable and is not recommended.

In total, according to the results of the present study, among the five investigated parameters, the protein strip test had the best diagnostic performance in differentiating exudative pleural effusion from transudate at the ideal cutoff \(\geq\) 2. Also, leukocyte esterase test strip is the most effective and accurate parameter for rapid diagnosis of infection and differentiation of infectious pleural exudation from non-infectious, and leukocyte esterase grade more than 2 indicates infectious pleural effusion.

Since the value of a diagnostic test lies in its ability to distinguish between disorders that are commonly confused, it seems that the primary use of these reagent strips is the differentiation of infectious (bacterial) effusions.

Therefore, based on these results, the use of urine dipstick can be valuable for quick diagnosis, especially in areas with limited facilities (technical or economic) or there is a time limit to perform standard laboratory analyzes in order to manage and treat patients as best as possible.

Finally, it should be mentioned that the present study was conducted for the first time in examining the diagnostic value of various urinary dipstick parameters in pleural effusion patients and obtained valuable information. But it was also faced with limitations, such as the fact that very few similar studies were found in the literature review, which made a complete and accurate comparison of the results impossible. Other limitations of the study include the small number of samples examined. Therefore, more reliable results can be obtained by conducting more studies with larger sample size.

The results of this study showed that the four parameters of urinary dipstick test including protein, leukocyte esterase, glucose and pH are accurate compared to the standard diagnostic method for differentiating exudative pleural effusion from transudate and also differentiating infectious exudative effusion from non-infectious. Therefore, reagent strips can speed up the diagnosis of the type of pleural effusion at the bed side. As a result, pleural fluid analysis with reagent strips can be used as a quick, easy, practical and inexpensive method to differentiate pleural effusion types, which can be especially helpful in areas with limited resources.

This article is taken from the student’s thesis of the specialized assistant course in internal medicine with the research project number APRD-0103 in the Faculty of Medicine of Ahvaz Jundishapur University of Medical Sciences, and this research was financially supported by the research vice-chancellor of AJUMS.

The authors declare no conflict of interests. All authors read and approved final version of the paper.

All authors contributed equally in this paper.

1. Jany, B., & Welte, T. (2019). Pleural effusion in adults—etiology, diagnosis, and treatment. Deutsches Ärzteblatt International, 116(21), 377.
2. Bibby, A. C., Dorn, P., Psallidas, I., Porcel, J. M., Janssen, J., Froudarakis, M., ... & Cardillo, G. (2019). ERS/EACTS statement on the management of malignant pleural effusions. European Journal of Cardio-Thoracic Surgery, 55(1), 116-132.
3. Karki, A., Riley, L., Mehta, H. J., & Ataya, A. (2019). Abdominal etiologies of pleural effusion. Disease-a-Month, 65(4), 95-103.
4. Feller-Kopman, D., & Light, R. (2018). Pleural disease. New England Journal of Medicine, 378(8), 740-751.
5. Light, R. W., Macgregor, M. I., Luchsinger, P. C., & Ball, W. C. Jr. (1972). Pleural effusions: The diagnostic separation of transudates and exudates. Annals of Internal Medicine, 77(4), 507-513.
6. Bedawi, E. O., Hassan, M., & Rahman, N. M. (2018). Recent developments in the management of pleural infection: A comprehensive review. The Clinical Respiratory Journal, 12(8), 2309-2320.
7. Lepus, C., & Vivero, M. (2018). Updates in effusion cytology. Surgical Pathology Clinics, 11(3), 523-544.
8. Mercer, R. M., Corcoran, J. P., Porcel, J. M., Rahman, N. M., & Psallidas, I. (2019). Interpreting pleural fluid results. Clinical Medicine, 19(3), 213-217.
9. Gumus, S., Kaya, H., Ozkisa, T., Turhan, U., Aydogan, M., Deniz, O., & Tozkoparan, E. (2016). Evaluation of pleural fluid total protein, albumin, globulin levels and albumin/globulin ratios in the differential diagnosis of pleural effusions. European Respiratory Society.
10. Davies, H. E., Davies, R. J., & Davies, C. W. (2010). Management of pleural infection in adults: British Thoracic Society pleural disease guideline 2010. Thorax, 65(Suppl 2), ii41-ii53.
11. Gad, A., El-Nemr, N., & Saad, M. (2019). The diagnostic value of leukocyte esterase reagent dip-stick in spontaneous bacterial peritonitis diagnosis in patients with liver cirrhosis. Suez Canal University Medical Journal, 22(1), 56-63.
12. Khairnar, H., Ingle, M., Pandey, V., Kolhe, K., Chauhan, S., Sawant, P., ... & Chaudhary, V. (2020). Accuracy of Leukocyte Esterase Reagent Strip (LERS) test for rapid bedside screening of spontaneous bacterial peritonitis: An observational study. Journal of Family Medicine and Primary Care, 9(11), 5542-5546.
13. Bortcosh, W., Siedner, M., & Carroll, R. W. (2017). Utility of the urine reagent strip leucocyte esterase assay for the diagnosis of meningitis in resource‐limited settings: Meta‐analysis. Tropical Medicine & International Health, 22(9), 1072-1080.
14. Mazumder, S., Ramya, B., & Biligi, D. S. (2018). Utility of urine reagent strips in cerebrospinal fluid analysis: An aid to bedside diagnosis of meningitis. Indian Journal of Pathology and Microbiology, 61(3), 356-359.
15. Laosu-angkoon, S. (2013). The sensitivity and specificity of a urine leukocyte esterase dipstick test for the diagnosis of urinary tract infection in the outpatient clinic of Rajavithi Hospital. Journal of the Medical Association of Thailand, 96(7), 849-853.
16. Najeeb, S., Munir, T., Rehman, S., Hafiz, A., Gilani, M., & Latif, M. (2015). Comparison of urine dipstick test with conventional urine culture in diagnosis of urinary tract infection. Journal of College of Physicians and Surgeons Pakistan, 25(2), 108-110.
17. Kalaisezhian, N., Irsath, S., & Shankar, R. (2018). Validity of leukocyte esterase reagent (LER) strip in diagnosing infectious pleural effusion.
18. Kim, D. Y., Kim, J. H., Chon, C. Y., Han, K. H., Ahn, S. H., Kim, J. K., ... & Moon, Y. M. (2005). Usefulness of urine strip test in the rapid diagnosis of spontaneous bacterial peritonitis. Liver International, 25(6), 1197-1201.
19. Din, Q. U., Rehman, Z. U., Aman, A., Ullah, F., & Rehman, S. U. (2023). Diagnostic accuracy of leucocyte esterase dipstick test for the diagnosis of spontaneous bacterial peritonitis in children with nephrotic syndrome: Diagnostic accuracy of leucocyte esterase dipstick test. Pakistan Journal of Health Sciences, 59-62.
20. Satheeshnath, R., Mehta, A., Haridas, N., George, R., Surendran, D., & Kunoor, A. (2020). A role of dipstick test in rapid diagnosis of differentiating between transudative and exudative effusion compared to standard diagnostic methods.
21. Azoulay, E., Fartoukh, M., Galliot, R., Baud, F., Simonneau, G., Le Gall, J. R., ... & Chevret, S. (2000). Rapid diagnosis of infectious pleural effusions by use of reagent strips. Clinical Infectious Diseases, 31(4), 914-919.
22. Tobin, C. L., Porcel, J. M., Wrightson, J. M., Waterer, G. W., Light, R. W., & Lee, Y. G. (2012). Diagnosis of pleural infection: State-of-the-art. Current Respiratory Care Reports, 1, 101-110.
23. Porcel, J. M. (2013). Pleural fluid biomarkers: Beyond the Light criteria. Clinics in Chest Medicine, 34(1), 27-37.
24. Colice, G. L., Curtis, A., Deslauriers, J., Heffner, J., Light, R., Littenberg, B., ... & Yusen, R. D. (2000) Medical and surgical treatment of parapneumonic effusions: An evidence-based guideline. Chest, 118(4), 1158-1171.
25. Light, R. W., Girard, W. M., Jenkinson, S. G., & George, R. B. (1980). Parapneumonic effusions. The American Journal of Medicine, 69(4), 507-512.
26. Davies, C. W., Kearney, S. E., Gleeson, F. V., & Davies, R. J. (1999). Predictors of outcome and long-term survival in patients with pleural infection. American Journal of Respiratory and Critical Care Medicine, 160(5), 1682-1687.
27. Cheng, D. S., Rodriguez, R. M., Rogers, J., Wagster, M., Starnes, D. L., & Light, R. W. (1998). Comparison of pleural fluid pH values obtained using blood gas machine, pH meter, and pH indicator strip. Chest, 114(5), 1368-1372.
28. Zheng, W. Q., & Hu, Z. D. (2023). Pleural fluid biochemical analysis: The past, present, and future. Clinical Chemistry and Laboratory Medicine (CCLM), 61(5), 921-934.
29. Abdollahi, A., & Nozarian, Z. (2016). Diagnostic value of measurement specific gravity by refractometric and dipstick method in differentiation between transudate and exudate in pleural and peritoneal fluid. Iranian Journal of Pathology, 11(4), 363.