A Simple Method for Differentiating Complicated Parapneumonic Effusion/Empyema from Parapneumonic Effusion Using the Split Pleura Sign and the Amount of Pleural Effusion on Thoracic CT

Abstract

Background: Pleural separation, the "split pleura" sign, has been reported in patients with empyema. However, the diagnostic yield of the split pleura sign for complicated parapneumonic effusion (CPPE)/empyema and its utility for differentiating CPPE/empyema from parapneumonic effusion (PPE) remains unclear. This differentiation is important because CPPE/empyema patients need thoracic drainage. In this regard, the aim of this study was to develop a simple method to distinguish CPPE/empyema from PPE using computed tomography (CT) focusing on the split pleura sign, fluid attenuation values (HU: Hounsfield units), and amount of fluid collection measured on thoracic CT prior to diagnostic thoracentesis.

Methods: A total of 83 consecutive patients who underwent chest CT and were diagnosed with CPPE (n=18)/empyema (n=18) or PPE (n=47) based on the diagnostic thoracentesis were retrospectively analyzed.

Results: On univariate analysis, the split pleura sign (odds ratio (OR), 12.1; p<0.001), total amount of pleural effusion (≥30 mm) (OR, 6.13; p<0.001), HU value≥10 (OR, 5.94; p=0.001), and the presence of septum (OR, 6.43; p=0.018), atelectasis (OR, 6.83; p=0.002), or air (OR, 9.90; p=0.002) in pleural fluid were significantly higher in the CPPE/empyema group than in the PPE group. On multivariate analysis, only the split pleura sign (hazard ratio (HR), 6.70; 95% confidence interval (CI), 1.91-23.5; p=0.003) and total amount of pleural effusion (≥30 mm) on thoracic CT (HR, 7.48; 95%CI, 1.76-31.8; p=0.006) were risk factors for empyema. Sensitivity, specificity, positive predictive value, and negative predictive value of the presence of both split pleura sign and total amount of pleural effusion (≥30 mm) on thoracic CT for CPPE/empyema were 79.4%, 80.9%, 75%, and 84.4%, respectively, with an area under the curve of 0.801 on receiver operating characteristic curve analysis.

Conclusion: This study showed a high diagnostic yield of the split pleura sign and total amount of pleural fluid (≥30 mm) on thoracic CT that is useful and simple for discriminating between CPPE/empyema and PPE prior to diagnostic thoracentesis.

Fig 1. Measurement of total amount of pleural effusion.

The amount of pleural effusion is calculated from the distance between the parietal and visceral pleura layers by drawing a vertical line (Fig 1).

Fig 2. Representative figures for hemi-split pleura sign (A, B) and split pleura sign (C, D).

Thickened visceral or parietal pleura on thoracic CT show the “hemi-split pleura sign” (A, B). When both pleura are thickened and separated by effusion, this is defined as the “split pleura sign” (C, D).

Fig 3. Correlation between serum WBC counts and pleural fluid TCC in the CPPE/empyema and PPE groups.

No apparent associations are seen between serum WBC counts and pleural fluid TCC in the CPPE/empyema group (r = -0.134, p = 0.444) and PPE group (r = -0.172, p = 0.382). CPPE: complicated parapneumonic effusion, PPE: parapneumonic effusion, TCC: total cell count, WBC: white blood cell count.

Fig 4. Correlation between pleural fluid LDH and ADA levels using combined data from the CPPE/empyema and PPE groups.

An intense, strongly positive correlation (r = 0.748, p<0.001) is noted between LDH and ADA levels. ADA: adenosine deaminase, CPPE: complicated parapneumonic effusion, LDH: lactase dehydrogenase, PPE: parapneumonic effusion.

Fig 5. Correlations among radiological parameters in both PPE (A-C) and CPPE/empyema groups (D-F).

A significant correlation between total amount of fluid and thickness of the visceral pleura is seen in both CPPE/empyema (r = 0.394, p = 0.019) and PPE groups (r = 0.318, p = 0.03). Similarly, the thickness of visceral pleura and HU (Hounsfield units) value show moderate positive correlations in both groups (CPPE/empyema group: r = 0.454, p = 0.006; PPE group: r = 0.438, p = 0.002). A significant correlation between total amount of pleural fluid and HU value is seen in the PPE group, but not in the CPPE/empyema group.

Fig 6. Diagnostic accuracy for CPPE/empyema using two parameters.

Receiver-operator characteristic (ROC) curve using the split pleura sign and total amount of pleural effusion (≥30 mm). The split pleura sign (C) shows 80.6% sensitivity, 74.5% specificity, a positive predictive value of 74.5%, and a negative predictive value of 70.7%, with an area under the curve of 0.775. The presence of both split pleura sign and total amount of pleural effusion (≥30 mm) (D) shows 79.4% sensitivity, 80.9% specificity, a positive predictive value of 75%, and a negative predictive value of 84.4%, with an area under the curve of 0.801. A: pleural effusion ≥30 mL; B: pleural effusion ≥30 mL or split pleura sign; C: split pleural sign; D: total amount of pleural effusion ≥30 mL and split pleura sign.