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Clinical Investigations: TECHNIQUES |

Measuring Pleural Fluid pH*: High Correlation of a Handheld Unit to a Traditional Tabletop Blood Gas Analyzer FREE TO VIEW

Gary L. Kohn, MD; William D. Hardie, MD
Author and Funding Information

*From the Division of Pulmonary Medicine, Children’s Hospital Medical Center, Cincinnati, OH.

Correspondence to: William D. Hardie, MD, Children’s Hospital Medical Center, Division of Pulmonary Medicine, OSB-5, 3333 Burnet Ave, Cincinnati, OH 45229; e-mail: bill.hardie@chmcc.org



Chest. 2000;118(6):1626-1629. doi:10.1378/chest.118.6.1626
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Study purposes: To survey hospital laboratories in the United States to determine methods used for measuring pleural fluid pH, and to compare pleural fluid pH values obtained with a traditional tabletop blood gas analyzer (BGA) to those obtained with a handheld analyzer.

Methods: Hospital laboratories nationwide were contacted by telephone to survey the methods used to measure pleural fluid pH. In a second phase, pleural fluid was prospectively collected from 19 pediatric and adult patients with pleural effusions, and pleural fluid pH was measured simultaneously with a traditional tabletop BGA and with a handheld unit.

Results: A total of 220 hospital laboratories were contacted by telephone, and 166 responded (75%). The methods for determining pleural fluid pH for all hospital laboratories were pH meter (35%; n = 59), BGA (32%; n = 53), and litmus paper (31%: n = 51); 2% (n = 3) did not perform the test. University hospitals were more likely to use a BGA, compared to community hospitals (p < 0.014) or children’s hospitals (p < 0.001). In the comparison of pleural fluid measurements, the mean pH for the traditional BGA was 7.358 ± 0.189, and the mean pH for the handheld unit was 7.382 ± 0.203. The absolute difference between the two machines was 0.024 U, and the two methods were correlated (p < 0.01; r = 0.993; degrees of freedom = 36).

Conclusion: Most hospital laboratories in the United States do not measure pleural fluid pH using a traditional BGA and use alternative methods that have previously been shown to be inaccurate. Pleural fluid pH obtained by a handheld unit has a high degree of correlation to that of a traditional tabletop BGA, and it offers a satisfactory alternative for laboratories reluctant to measure pleural fluid pH with a BGA.

Figures in this Article

The pleural fluid pH is an important laboratory test in the diagnosis and management of malignant and parapneumonic effusions. The pleural fluid pH is especially valuable for managing parapneumonic effusions, which have an estimated incidence of 300,000 cases per year in the United States.13 Parapneumonic effusions present with a spectrum of disease severity, ranging from free-flowing, uncomplicated effusions that resolve spontaneously following antibiotic therapy, to complicated effusions that require surgical drainage for resolution. Because the progression from a free-flowing effusion to the formation of intrapleural pus and loculations may occur rapidly, most authors36 recommend prompt pleural drainage when a complicated course seems likely.

Of the common pleural fluid indexes measured to assess parapneumonic effusions, pleural fluid pH has been shown to be the most consistent diagnostic predictor of the need of pleural space drainage, with many authors citing a pleural fluid pH < 7.2 as an indication for chest tube drainage.79 The most accurate method for measuring pleural fluid pH is the blood gas analyzer (BGA).2,1011 Studies by Cheng et al12and Chandler et al13 have demonstrated that other methods of measuring pleural fluid pH, including the pH meter and indicator strip, are far less accurate than the BGA for determining pleural fluid pH. However, many hospital laboratories are reluctant to measure pleural fluid pH with the BGA, probably because of concerns of obstructing the machine with coagulated proteins. A recent analysis of acute-care hospitals in the southeastern United States demonstrated that only 32% of institutions used a BGA for measuring pleural fluid pH.13

The first goal of this study was to survey hospital laboratories nationwide on the methods used to measure pleural fluid pH, and to compare differences between university, community, and pediatric institutions. The second goal was to prospectively compare measurements of pleural fluid pH between the traditional BGA and a new alternative method for analyzing pleural fluid pH, a handheld analyzer (i-STAT Portable Clinical Analyzer; i-STAT Corporation; East Windsor, NJ). This device is used in ICUs for rapid interpretation of blood gas samples. Both machines measure pH with an electrode capable of detecting hydrogen ions by direct potentiometry. In the traditional BGA, this electrode is part of the tabletop unit, whereas in the handheld unit, the electrode is contained in the disposable sample cartridges. The advantage of the handheld device for measuring pleural fluid pH is that the disposable cartridges allow users to avoid potential damage to the base unit by precipitation of pleural proteins. A close correlation in pH measurements between the handheld unit and the tabletop BGA would thus provide an alternative method to measure pleural fluid pH.

Hospital Survey

From August to December 1999, hospital laboratories representing all 48 continental United States were randomly selected to be surveyed by phone. The criteria for selection included listing by the 1999 American Hospital Association Handbook and having at least 10,000 admissions per year. The hospitals were categorized by the American Hospital Association as either community-based, university-based, or children’s hospital, and the criterion for 10,000 admissions was waived for the children’s hospitals. At each institution, the laboratory technician responsible for measuring pleural fluid pH was interviewed about the method used by their laboratory.

Pleural Fluid pH Measurement

After approval from the Institutional Review Board at Children’s Hospital Medical Center, Cincinnati, and The University of Cincinnati Medical Center, samples of pleural fluid were obtained from 19 pediatric and adult patients with pleural effusions requiring thoracentesis or chest tube placement. Samples were collected in blood gas syringes with lithium heparin anticoagulant (SIMS; Keene, NH) and placed on ice. Each sample was measured twice on the BGA and the handheld unit for a total of four pH readings. All measurements were performed using a BGA (CIBA-Corning 278 Blood Gas System[ CC-BGA]; CIBA-Corning; Walpole, MA) and the i-STAT analyzer with a G3+ cartridge at 37°C. Before each measurement, the CC-BGA was calibrated with a standard solutions, and the i-STAT machine was calibrated with its electronic simulating module. Both units measure pH to the third decimal place; the range for pH for the CC-BGA is 6.0 to 8.0, and for the i-STAT analyzer is 6.5 to 8.0.

Statistics

Differences in methods used to measure pleural fluid pH between categories of hospitals were determined with χ2 analysis and z score. The Pearson product moment correlation coefficient and z test were used to analyze the pH data between the traditional BGA unit and the handheld unit. All statistical analyses were performed using Statistical Analysis System Software (SAS Version 6.0; SAS Institute; Cary, NC).

Hospital Survey

Two hundred twenty hospital laboratories were contacted, and 166 laboratories (75%) responded to the telephone survey. Community-based laboratories represented 55% (n = 89), university-based represented 23% (n = 38), and children’s hospitals represented 22% (n = 36) of hospitals surveyed. The methods for determining pleural fluid pH for all hospital laboratories were pH meter (35%; n = 59), BGA (32%; n = 53), and litmus paper (31%; n = 51); 2% (n = 3) did not perform the test. Fifty-eight percent of university hospitals used the BGA for measuring pleural fluid pH, compared to 33% of community hospitals (p < 0.014) and 11% of children’s hospitals (p < 0.001; Table 1 ).

Pleural Fluid pH Measurement

Pleural fluid was obtained from 19 patients (age range, 1 month to 74 years) with pleural effusions of a variety of etiologies, including parapneumonic (n = 10), malignancy (n = 7), and postoperative (n = 2). Each pleural fluid sample was measured twice each for a total of 37 pH readings (one sample was measured only once due to limited amount of pleural fluid). The average (± SD) pH for CC-BGA was 7.358 ± 0.189 (range 6.9 to 7.9), and the average pH for the i-STAT was 7.382 ± 0.203 (range 6.9 to 8.0; Fig 1 ). The absolute difference between the two machines was 0.024 U, which was statistically significant; however, the two machines were highly correlated for pH determination (p < 0.01; r = 0.993; degrees of freedom = 36).

The evaluation of pleural fluid pH is an invaluable test for the clinician and is useful diagnostically and prognostically in the management of patients with parapneumonic and malignant pleural effusions. In parapneumonic effusions, the pleural fluid pH is used to identify patients with a complicated effusion and to predict the need for chest tube drainage.2,11 The original studies that established the clinical guidelines for evaluating and treating parapneumonic and malignant effusions were performed using a BGA to measure pleural fluid pH.89,14 Hence, the BGA is considered the “gold standard” for laboratories measuring and reporting pleural fluid pH values.

Other alternative methods for measuring pleural fluid pH, including the pH meter and pH indicator strips, have been shown to be inaccurate compared with the BGA.1213 The study by Cheng et al12 comparing pleural fluid pH measured simultaneously by a BGA and a pH meter found that the pH meter measured pleural fluid pH 0.16 U higher than the BGA and had a correlation of 0.56. When the BGA was compared with a pH indicator strip, the mean difference was 0.81 U higher than the BGA, and the correlation was 0.32.12 Chandler et al13 had similar results, with a mean difference of 0.3 U between BGA and pH meter for measurement of pleural fluid pH and a difference of 0.16 U between BGA and pH meter. The large differences in pleural fluid pH obtained from the pH meter and indicator strips are clearly enough to affect clinical decision making.1213

Despite the inaccuracies in measuring pleural fluid pH with the pH meter and indicator strip, the majority of all hospital laboratories in our survey did not measure pleural fluid pH with a BGA. While university hospitals were more likely to use the BGA for measuring pleural fluid pH than community or children’s hospitals, we were surprised to find that almost half of all university laboratories used methods other than BGA for measuring and reporting pleural fluid pH. Nationwide, our survey demonstrated that only 32% of surveyed hospitals used the BGA for measuring pleural fluid pH, and this value is identical to that of a recent survey of critical-care laboratories in the southeastern United States.13 In our survey, the most popular method used by laboratories for measuring pH was the pH meter (35%), while in southeastern United States laboratories, the pH indicator paper was most popular (56%). Our study did not assess why hospital laboratories used methods other than the BGA for measuring pleural fluid pH; however, concerns about coagulation of pleural fluid proteins resulting in damage to the analyzer are likely explanations.

In this study, we compared the portable handheld clinical analyzer to the BGA for determining pleural fluid pH. Both machines use the same technology to measure pH, using an electrode to detect hydrogen ions by direct potentiometry. In the BGA, the electrode is part of the tabletop unit; however, in the handheld unit, the electrode is contained in a disposable cartridge. The benefit of a disposable cartridge is that it limits potential damage from pleural fluid samples and leaves the main unit intact. Our analysis of pleural fluid samples demonstrates that there is a very strong correlation between pleural fluid pH readings obtained with the handheld device compared with the BGA (r = 0.993), with a statistically significant difference between the two units (p < 0.01). This high correlation and statistical significance indicate that the study was well powered, despite the relatively few numbers of pleural fluid samples tested. Although the pleural fluid pH readings were on average 0.02 U higher in the handheld unit compared with the BGA, this bias is unlikely to affect clinical management and is a clear improvement in correlation compared with the alternatives of the pH meter or indicator strip paper.

A general cost comparison between the handheld device and the BGA reveals that the handheld device is affordable for hospital laboratories and is at least comparable in overall cost compared to a BGA. The i-STAT is more expensive to run per sample, with the cost of the G3 i-STAT cartridge at $6, while the CC-BGA costs approximately $0.40 per sample. However, the initial cost of an i-STAT is lower at $5,000 compared to approximately $28,000 for a CC-BGA.

In summary, our study demonstrates that the majority of hospitals throughout the United States continue to use methods other than a BGA for measuring and reporting pleural fluid pH, despite the reported imprecision of these alternative methods. We present data that demonstrate a strong correlation in pleural fluid pH measurements obtained by the handheld device and a tabletop BGA. These data support use of the handheld device for measuring pleural fluid pH as an accurate alternative for laboratories who are reluctant to measure pleural fluid pH with a tabletop BGA.

Abbreviations: BGA = blood gas analyzer; CC-BGA = CIBA-Corning Blood Gas Analyzer

Financial support provided by the Division of Pulmonary Medicine, Children’s Hospital Medical Center, Cincinnati, OH.

Table Graphic Jump Location
Table 1. Comparison of Methods to Types of Hospital Laboratory
* 

p < 0.01 compared to community and children’s hospitals.

Figure Jump LinkFigure 1. Scatter plot with line of regression comparing the pH in pleural fluid samples as measured by the CC-BGA and the i-STAT device. Mean pH for the CC-BGA was 7.358, compared to 7.382 for the i-STAT device. The average difference between the two units is 0.024 (p < 0.01), with a high degree of correlation (r = 0.993; degrees of freedom = 36).Grahic Jump Location

Dr. Paul Succop provided assistance in statistical analysis of our data. Dr. Paul Steele from the Division of Pathology provided laboratory technical support, and Abbott Diagnostics provided the G3+ cartridges and temporary use of the i-STAT unit for the study.

Niederman, MS, Bass, JBJ, Campbell, GD, et al (1993) Guidelines for the initial management of adults with community-acquired pneumonia: diagnosis, assessment of severity, and initial antimicrobial therapy.Am Rev Respir Dis148,1418-1426. [PubMed]
 
Light, RW, Girard, WM, Jenkinson, SG, et al Parapneumonic effusions.Am J Med1980;69,507-512. [CrossRef] [PubMed]
 
Light, RW, MacGregor, MI, Ball, WCJ, et al Diagnostic significance of pleural fluid pH and Pco2.Chest1973;64,591-596. [CrossRef] [PubMed]
 
Light, RW Pleural diseases 3rd ed.1995,129-153 Williams & Wilkins. Baltimore, MD:
 
Steinbrecher, HA, Najmaldin, AS Thoracoscopy for empyema in children.J Pediatr Surg1998;33,708-710. [CrossRef] [PubMed]
 
Hoff, SJ, Neblett, WW, Edwards, KM, et al Parapneumonic empyema in children: decortication hastens recovery in patients with severe pleural infections.Pediatr Infect Dis J1991;10,194-199. [CrossRef] [PubMed]
 
Ewig, JM Pleural effusions: diagnostic considerations. Pediatr Rev. 1995;;16 ,.:79. [CrossRef] [PubMed]
 
Chavalittamrong, B, Angsusingha, K, Tuchinda, M, et al Diagnostic significance of pH, lactic acid dehydrogenase, lactate and glucose in pleural fluid.Respiration1979;38,112-120. [CrossRef] [PubMed]
 
Potts, DE, Levin, DC, Sahn, SA Pleural fluid pH in parapneumonic effusions.Chest1976;70,328-331. [CrossRef] [PubMed]
 
Heffner, JE, Brown, LK, Barbieri, C, et al Pleural fluid chemical analysis in parapneumonic effusions: a meta-analysis.Am J Respir Crit Care Med1995;151,1700-1708. [PubMed]
 
Light, RW Management of parapneumonic effusions.Arch Intern Med1981;141,1339-1341. [CrossRef] [PubMed]
 
Dong-Sheng Cheng, M, Rodriguez, RM, Rogers, JR, et al Comparison of pleural fluid pH values obtained using blood gas machine, pH meter, and pH indicator strip.Chest1998;114,1368-1372. [CrossRef] [PubMed]
 
Chandler TM, McCoskey ED, Byrd RB Jr, et al. Comparison of the use and accuracy of methods for determining pleural fluid pH. South Med J 199; 92:214–217.
 
Potts, DE, Taryle, DA, Sahn, SA The glucose-pH relationship in parapneumonic effusions.Arch Intern Med1978;138,1378-1380. [CrossRef] [PubMed]
 

Figures

Figure Jump LinkFigure 1. Scatter plot with line of regression comparing the pH in pleural fluid samples as measured by the CC-BGA and the i-STAT device. Mean pH for the CC-BGA was 7.358, compared to 7.382 for the i-STAT device. The average difference between the two units is 0.024 (p < 0.01), with a high degree of correlation (r = 0.993; degrees of freedom = 36).Grahic Jump Location

Tables

Table Graphic Jump Location
Table 1. Comparison of Methods to Types of Hospital Laboratory
* 

p < 0.01 compared to community and children’s hospitals.

References

Niederman, MS, Bass, JBJ, Campbell, GD, et al (1993) Guidelines for the initial management of adults with community-acquired pneumonia: diagnosis, assessment of severity, and initial antimicrobial therapy.Am Rev Respir Dis148,1418-1426. [PubMed]
 
Light, RW, Girard, WM, Jenkinson, SG, et al Parapneumonic effusions.Am J Med1980;69,507-512. [CrossRef] [PubMed]
 
Light, RW, MacGregor, MI, Ball, WCJ, et al Diagnostic significance of pleural fluid pH and Pco2.Chest1973;64,591-596. [CrossRef] [PubMed]
 
Light, RW Pleural diseases 3rd ed.1995,129-153 Williams & Wilkins. Baltimore, MD:
 
Steinbrecher, HA, Najmaldin, AS Thoracoscopy for empyema in children.J Pediatr Surg1998;33,708-710. [CrossRef] [PubMed]
 
Hoff, SJ, Neblett, WW, Edwards, KM, et al Parapneumonic empyema in children: decortication hastens recovery in patients with severe pleural infections.Pediatr Infect Dis J1991;10,194-199. [CrossRef] [PubMed]
 
Ewig, JM Pleural effusions: diagnostic considerations. Pediatr Rev. 1995;;16 ,.:79. [CrossRef] [PubMed]
 
Chavalittamrong, B, Angsusingha, K, Tuchinda, M, et al Diagnostic significance of pH, lactic acid dehydrogenase, lactate and glucose in pleural fluid.Respiration1979;38,112-120. [CrossRef] [PubMed]
 
Potts, DE, Levin, DC, Sahn, SA Pleural fluid pH in parapneumonic effusions.Chest1976;70,328-331. [CrossRef] [PubMed]
 
Heffner, JE, Brown, LK, Barbieri, C, et al Pleural fluid chemical analysis in parapneumonic effusions: a meta-analysis.Am J Respir Crit Care Med1995;151,1700-1708. [PubMed]
 
Light, RW Management of parapneumonic effusions.Arch Intern Med1981;141,1339-1341. [CrossRef] [PubMed]
 
Dong-Sheng Cheng, M, Rodriguez, RM, Rogers, JR, et al Comparison of pleural fluid pH values obtained using blood gas machine, pH meter, and pH indicator strip.Chest1998;114,1368-1372. [CrossRef] [PubMed]
 
Chandler TM, McCoskey ED, Byrd RB Jr, et al. Comparison of the use and accuracy of methods for determining pleural fluid pH. South Med J 199; 92:214–217.
 
Potts, DE, Taryle, DA, Sahn, SA The glucose-pH relationship in parapneumonic effusions.Arch Intern Med1978;138,1378-1380. [CrossRef] [PubMed]
 
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