Study subjects
This study was approved by the ethics committee of Occupational Safety and Health Research Institute (OSHRI), Korea Occupational Safety and Health Agency (KOSHA), Incheon, Korea and conducted in accordance with the declaration of Helsinki.
In total, 301 workers who handled indium compounds in the course of their work, volunteered for the study for the prevention of indium lung disease in Korea at 2012 conducted by OSHRI of KOSHA. The serum indium or Krebs von den Lungen-6 (KL-6) concentrations of 78 workers exceeded the Japanese reference values (3 μg/L and 500 U/mL, respectively) [16]. Among them, 34 workers (43.6%) agreed to undergo chest high-resolution computed tomography (HRCT), and the data of these workers were used for our statistical analysis. Health check items included completion of the OSHRI-developed questionnaire with past medical, job, and smoking histories as well as chest symptoms and signs, chest physical examinations, and spirometry as well as serum indium, KL-6, and surfactant protein D (SP-D) levels. SP-D was not used screening health check item to determine for Chest HRCT. Because Usefulness of SP-D in diagnosis of indium induced interstitial lung disease was not as good as KL-6 [6–8], and it had been listed one of secondary health check items for prevention of indium lung disease in Japan.
An occupational physician interviewed each of the workers and confirmed the accuracy of the information on the questionnaires.
Spirometry
Spirometry was performed using PC-10 spirometer (CHEST M.I., Inc., Tokyo, Japan) to get forced vital capacity (FVC) and forced expiratory volume 1 second/forced vital capacity (FEV1/FVC). In the sitting position, the tests were repeated until obtaining appropriate and reproducible results at least three times, and then the best results were selected. All results were confirmed by physician.
Serum KL-6 and SP-D
KL-6 is a mucin-like high molecular glycoprotein, which is strongly expressed on alveolar type II pneumocyte [17]. Serum KL-6 has been known one of sensitive biomarker for interstitial lung diseases such as idiopathic pulmonary fibrosis, radiation pneumonitis, connective tissue disease-associated interstitial lung disease, lung involvement of sarcoidosis, and pulmonary alveolar proteinosis [18, 19].
SP-D is produced and secreted by type II pneumocyte. It is also elevated in patient with inflammatory lung diseases, including idiopathic pulmonary fibrosis and pulmonary alveolar proteinosis [20, 21].
Serum KL-6 levels were measured by enzyme-linked immunosorbent assay (ELISA) kit (EIDIA Co., Ltd., Tokyo, Japan). And the concentration of serum SP-D was measured by human surfactant protein D ELISA kit (BioVendor, Brno, Czech). The methods and steps for analysis of KL-6 and SP-D were followed by guidelines presented by manufacturers. All samples for KL-6 and SP-D were analyzed twice, and mean values were reported and used for statistical analysis.
HRCT scanning and radiologic evaluation
Chest HRCT was performed at the 3 hospitals near the factories when serum indium or KL-6 levels exceeded the Japanese reference values. And then all HRCT films were reviewed by a chest radiology specialist, and the reports were discussed with occupational physician.
All of the subjects underwent volumetric thin section HRCT scan. Patients were scanned caudocranially in one breath hold; 1 mm collimation was used at a table feed of 6 mm/0.75 s scanner rotation (8 mm/s) at 120 kV and 140 mA. Scanning was performed from the lung bases toward the apices. The volumetric axial images with 1 mm thickness and the 1 mm intervals were reconstructed with a high spatial frequency algorithm on supine and prone position. The images were viewed at window levels of −700 HU for analysis of thin section HRCT features. All images were displayed at the lung window setting using a PACS (picture archiving and communication system) workstation (Deja-View, Dongeun Information technology).
The thin section HRCT scans were evaluated for the presence and/or extent of the following features to evaluate indium induced interstitial lung damage:
-
(1)
interlobular or intralobular interstitial thickening; (2) area of ground glass opacity (GGO); (3) area of emphysema. These findings were defined according to the glossary of terms recommended by the Fleischner Society [22].
The lung was divided into six zones (upper, middle, and lower, right and left) by one third and two third of the vertical distance between the lung apices and the domes of the diaphragm. Each of these zones was evaluated and scored separately for the presence and/or extent of the features on the thin section HRCT scan. Interlobular and intralobular interstitial thickening were scored in each of six zones according to the subjective score of the involved area to cross sectional area (0 = no involvement; 1 = minimal involvement; 2 = moderate involvement; 3 = severe involvement) as described in other studies [23, 24]. The scores of the six zones were summated as the total scores for Interlobular and intralobular interstitial thickening from 0 to 18. The remaining features of the thin section HRCT scan (ground glass opacity and emphysema) were evaluated in same scoring system for interlobular or intralobular interstitial thickening. They were expressed as a semi-quantitative scale of grade (score of interlobular and intralobular interstitial thickening + score of ground glass opacity + score of emphysema) as follows: grade 0, <1; grade 1, 1–5; grade 2, 6–10; grade 3, 11–15; grade 4, >16.
Cases of grade 1 or more were regarded positive interstitial changes, grade 0 or other suspected cases were regarded negative.
Statistical analysis
We assessed whether continuous data were normally distributed, and data sets that were not normally distributed were log-transformed before analysis. Mean values of variables were calculated and compared with t-tests or ANOVA, where appropriate. Prevalence data were compared using chi-square test, Fisher’s exact tests, and Mantel-Haenszel trend test.
To compare KL-6 and SP-D levels between groups with low and high serum indium levels, the 34 subjects were divided in 2 groups: subject with serum indium levels of <10 μg/L (S-in <10) and those with ≥10 μg/L (S-in ≥10) were grouped together. Serum indium level of 10 μg/L was simple cut-off value in considering of number of subjects, to divide groups low and high serum indium levels. And To analyze the dose–response relationship between serum indium levels and KL-6 levels, SP-D levels, and HRCT finding, the workers were classified into 6 groups according to their serum indium levels (group 0: less than 1.0, group 1: 1.0–4.9, group 2: 5.0–9.9, group 3: 10.0–29.9, group 4: 30.0–49.9, and group 5: ≥50.0). P values <0.05 were considered statistically significant.