Characteristics of study participants
In total, 81 cases with pleural effusion were enrolled in this study. The demographic and clinical characteristics of the study populations are shown in Table 1. Of these, 21 (25.9%) had MPE, and 60 (74.1%) had BPE. With respect to the clinical characteristics, the patients with MPE were older than those with BPE (68.0 versus 58.0 years, P = 0.016). Of the 21 cases with MPE, 19 (90.5%) were lung cancer, and the other two (9.5%) were pleural metastasis of extra-thoracic malignancy. Seven out of 21 cases with MPE (33%) were positive for malignant cells in the cytologic examination of pleural fluid. The other 14 cases were histologically confirmed through biopsies of tissues of primary origin and a clinical course compatible with MPE. Pleural fluid white blood cell counts were lower in the MPE group compared with those of the BPE group (450 versus 1,160 /µl, P = 0.003). In addition, patients with MPE demonstrated significantly higher glucose (114.0 versus 95.5 mg/dL, P = 0.037) and lower adenosine deaminase (17.0 versus 83.0 IU/L, P = 0.001) levels than those with BPE.
Level of pro-cathepsin D and diagnostic accuracy
For all study cases, a significant positive correlation between pleural fluid pro-cathepsin D level and plasma pro-cathepsin D level was shown (Spearman's r =0.870, 95% confidence interval = 0.803 to 0.916, P < 0.0001) (Fig. 1). Though there were no significant differences in plasma pro-cathepsin D between two groups, the level of pleural fluid pro-cathepsin D was significantly higher in the MPE group than the BPE group (0.651 versus 0.590 pg/mL, P = 0.034) (Table 1). There were no differences in pleural fluid pro-cathepsin D level according to causative malignancy of MPE (Fig. 2).
Table 1. Clinical characteristics of two patient groups.
|
Malignant pleural effusion
(n = 21)
|
Benign pleural effusion
(n = 60)
|
P-value
|
Age, years
|
68.0 (59.0–81.0)
|
58.0 (35.5–73.5)
|
0.016
|
Male sex
|
14 (66.7)
|
41 (68.3)
|
0.888
|
Diagnosis of MPE
|
|
|
|
Lung cancer
|
|
|
|
Adenocarcinoma
|
10
|
|
|
Squamous cell carcinoma
|
7
|
|
|
Small cell carcinoma
|
2
|
|
|
Breast cancer
|
1
|
|
|
Cholangiocarcinoma
|
1
|
|
|
Diagnosis of BPE
|
|
|
|
Pleural tuberculosis
|
|
37
|
|
Parapneumonic effusion
|
|
23
|
|
Pleural fluid findings
|
|
|
|
Specific gravity
|
1.020 (1.015–1.020)
|
1.020 (1.015–1.020)
|
1.000
|
pH
|
7.5 (7.5–7.5)
|
7.5 (7.5–7.5)
|
0.870
|
WBC, /µl
|
450.0 (288.0–710.0)
|
1169.0 (397.5–2124.0)
|
0.003
|
Neutrophil, %
|
30.0 (20.0–40.0)
|
30.0 (20.0–54.0)
|
0.521
|
Lymphocyte, %
|
70.0 (60.0–80.0)
|
70.0 (46.0–80.0)
|
0.521
|
Glucose, mg/dL
|
114.0 (106.5–151.0)
|
95.5 (69.3–139.3)
|
0.037
|
Protein, g/dL
|
4.2 (3.7–5.0)
|
4.6 (2.9–5.4)
|
0.845
|
Albumin, g/dL
|
2.3 (2.0–2.9)
|
2.4 (1.6–2.7)
|
0.551
|
LDH, IU/L
|
417.0 (235.5–548.0)
|
447.0 (211.0–881.0)
|
0.552
|
ADA, IU/L
|
17.0 (14.0–24.0)
|
83.0 (17.8–109.2)
|
0.001
|
Pro-cathepsin D
|
|
|
|
Plasma, pg/mL
|
0.469 (0.421–0.554)
|
0.455 (0.405–0.549)
|
0.528
|
Pleural fluid, pg/mL
|
0.651 (0.601–0.716)
|
0.590 (0.511–0.692)
|
0.034
|
Data are presented as the median (interquartile range) or no. (%).
MPE, malignant pleural effusion; BPE, benign pleural effusion; WBC, white blood cell; LDH, lactate dehydrogenase; ADA, adenosine deaminase.
In 21 MPE cases, pleural fluid and plasma pro-cathepsin D levels were also compared between MPE with positive pleural fluid cytology (n = 7) and MPE with negative cytology (n = 14). There was no significant difference in pleural fluid pro-cathepsin D level (median of 0.620 pg/mL and interquartile range [IQR] of 0.547–0.647 pg/mL in positive cytology versus median of 0.684 pg/mL and IQR = 0.615–0.718 pg/mL in negative cytology, P = 0.110). There was also no significant difference in plasma pro-cathepsin D level either (median of 0.438 pg/mL and IQR of 0.390–0.491 pg/mL in positive cytology versus median of 0.478 pg/mL and IQR of 0.423–0.554 pg/mL in negative cytology, P = 0.410).
The analysis for assessing diagnostic accuracy of plasma and pleural fluid pro-cathepsin D in discriminating MPE from BPE was done after excluding MPE with positive pleural fluid cytology. Sensitivities, specificities, PPVs, and NPVs of the candidate cut-off values were calculated for BPE versus MPE with negative pleural fluid cytology (Table 2). On ROC curve analysis, the optimal discrimination point between MPE and BPE was defined as a cut-off value of 0.651 pg/mL for pleural fluid pro-cathepsin D (sensitivity, 71.4%; specificity, 73.3%) and 0.469 pg/mL for plasma pro-cathepsin D (sensitivity, 64.3%; specificity, 58.3%). A cut-off value of 0.651 pg/mL for pleural fluid pro-cathepsin D showed a PPV of 38.5% and an NPV of 91.7% (Table 2). The area under the curve (AUC) values for pleural fluid and plasma pro-cathepsin D were 0.698 and 0.577, respectively (Fig. 3). When 100% specificity was achieved, the optimal cut-off value of pro-cathepsin D was 1.087 pg/mL in pleural fluid and 0.736 pg/mL in plasma. At cut-off value of 100% specificity, sensitivity was 0% in both pleural fluid and plasma. All cases with BPE revealed that pleural fluid pro-cathepsin D level was lower than the cut-off value of 1.087 pg/mL (used for rule-in purpose). On the other hand, when 100% sensitivity was achieved, the optimal cut-off value of pro-cathepsin D was 0.375 pg/mL in pleural fluid and 0.311 pg/mL in plasma. At cut-off value of 100% sensitivity, specificity was 0% in both pleural fluid and plasma. All cases with MPE revealed that pleural fluid pro-cathepsin D level was higher than the cut-off value of 0.375 pg/mL (used for rule-out purpose).
Table 2. Diagnostic performance of pleural and plasma pro-cathepsin D in predicting malignant pleural effusion.
Pleural fluid pro-cathepsin D, pg/mL
|
|
Sensitivity %
|
Specificity %
|
PPV %
|
NPV %
|
LR+
|
LR–
|
≥0.605
|
85.7 (56.2–97.5)
|
53.3 (40.1–66.1)
|
30.0 (17.1–46.7)
|
94.1 (78.9–98.9)
|
1.84 (1.30–2.59)
|
0.27 (0.07–0.99)
|
≥0.615
|
78.6 (48.8–94.3)
|
58.3 (44.9–70.1)
|
30.6 (16.9–48.3)
|
92.1 (77.5–97.9)
|
1.89 (1.26– 2.82)
|
0.37 (0.13–1.03)
|
≥0.660
|
64.3 (35.6–86.0)
|
75.0 (61.9–84.9)
|
37.5 (19.6–59.2)
|
90.0 (77.4–96.3)
|
2.57 (1.43–4.62)
|
0.48 (0.23–0.97)
|
Suggested optimal cut-off, pg/mL
|
0.651
|
71.4 (42.0–90.4)
|
73.3 (60.1–83.5)
|
38.5 (20.9–59.3)
|
91.7 (79.1–97.3)
|
2.68 (1.57–4.57)
|
0.39 (0.17–0.90)
|
Plasma pro-cathepsin D, pg/mL
|
|
Sensitivity %
|
Specificity %
|
PPV %
|
NPV %
|
LR+
|
LR–
|
≥0.442
|
71.4 (42.0–90.4)
|
43.3 (30.8–56.7)
|
22.7 (11.9–38.2)
|
86.7 (68.4–95.6)
|
1.26 (0.85–1.88)
|
0.66 (0.28–1.58)
|
≥0.487
|
50.0 (24.0–75.9)
|
65.0 (51.5–76.6)
|
25.0 (11.4–45.2)
|
84.8 (70.5–93.2)
|
1.43 (0.76–2.67)
|
0.76 (0.45–1.32)
|
≥0.517
|
42.9 (18.8–70.4)
|
73.3 (60.1–83.5)
|
27.3 (11.6–50.4)
|
84.6 (71.5–92.7)
|
1.61 (0.77–3.36)
|
0.78 (0.49–1.24)
|
Suggested optimal cut-off, pg/mL
|
0.469
|
64.3 (35.6–86.0)
|
58.3 (44.9–70.7)
|
26.5 (13.5–44.7)
|
87.5 (72.4–95.3)
|
1.54 (0.94–2.52)
|
0.61 (0.30–1.27)
|
Data are presented as percentages (95% confidence interval).
CI, confidence interval; PPV, positive predictive value; NPV, negative predictive value; LR+, positive likelihood ratio; LR−, negative likelihood ratio.