Pleural diseases are common respiratory conditions that often require hospital admission and have shown an increasing incidence.1 2 In the United States, approximately 1.5 million patients experience pleural effusion annually, with most cases attributed to congestive heart failure, pneumonia, and cancer.3 4 A recent multicentre, cross-sectional study in China estimated the prevalence of pleural effusion at 4684 per 1 million Chinese adults.5 In that study, the most common causes were parapneumonic effusion and empyema (25.1%), malignant neoplasms (23.7%), and tuberculosis (12.3%).5 The median hospitalisation cost was ¥15 534.5 (interquartile range, 9447.2-29 000.0).5 Additionally, an increasing trend in admissions for spontaneous pneumothorax has been observed in England, highlighting the prevalence of the disease and its associated healthcare burden.2

Management of pleural diseases involves various diagnostic and therapeutic procedures that extend beyond the pleural space to include the airway and lung parenchyma. Whether closed or open, these procedures substantially contribute to the overall healthcare burden. However, information about pleural diseases and related respiratory procedures in Hong Kong remains limited, highlighting the need for contemporary, population-based epidemiological data.

The Hospital Authority, which provides healthcare services to over 90% of Hong Kong’s population, maintains extensive healthcare databases. These include the Clinical Management System (CMS) and the Clinical Data Analysis and Reporting System (CDARS), which capture a wide range of longitudinal clinical data. Examples include hospital discharge records, diagnosis and procedure codes for each hospitalisation episode, radiological findings, and laboratory parameters, particularly blood and pleural fluid analyses. This comprehensive dataset provides valuable insights into the burden of pleural diseases and accurately represents the local population.

Before analysing diseases and procedures using administrative data, it is essential to validate the accuracy of diagnosis and procedure codes within the healthcare database. These codes are typically entered by attending physicians, interventionists, or surgeons performing the procedures, which suggests a high degree of reliability. However, no prior local validation study has been conducted. Therefore, we aimed to assess whether diagnosis codes for pleural diseases and procedure codes for relevant respiratory procedures are accurately recorded for each hospitalisation episode within the Hospital Authority systems.

This retrospective, observational validation study of diagnosis and procedure codes utilised data from a territory-wide healthcare database in Hong Kong. Clinical data were obtained from CDARS, provided by the Hospital Authority. Hospitalisation episodes with the targeted diagnosis and procedure codes between 1 January 2013 and 31 December 2022 were retrieved from the system. Each observation represented a hospitalisation episode rather than a unique patient, and no patient recruitment was involved.

Diagnosis and procedure codes were defined using the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM). The basic format of an ICD-9-CM code consists of three to six digits. The Hospital Authority further extends these codes with additional characters after the decimal point to specify particular diagnoses or procedures within an ICD-9-CM code subgroup (‘subcodes’). These subcodes are displayed in CDARS but are not typically accessible to frontline CMS users. All hospitalisation episodes in acute hospitals with a discharge diagnosis code of pneumothorax (codes starting with 512), pleural effusion (codes starting with 012, 197.2, 220.4, 510, or 511), traumatic pneumothorax or haemothorax (trauma-related pleural events, codes starting with 860), or procedure codes for relevant respiratory procedures (codes starting with 33 or 34) were retrieved, regardless of their position in the coding list. Hospitalisation episodes for patients younger than 18 years or from paediatric departments were excluded from subsequent validation analyses. Uninterrupted hospitalisation episodes following the index episodes, including those in acute or convalescent hospitals with the same diagnosis code of interest, were also excluded, as these may represent duplicate entries for the same clinical event. The remaining hospitalisation episodes after exclusions were grouped as the main cohort.

Manual verification of a proportion of the retrieved diagnosis and procedure codes, down to the subcode level, was conducted to ensure data accuracy. The main cohort was first filtered to include only hospitalisation episodes at the authors’ affiliated institution, Prince of Wales Hospital (PWH), forming the PWH cohort. A maximum of 50 hospitalisation episodes for each diagnosis or procedure code were randomly extracted from the PWH cohort to estimate the true positive predictive values (PPVs) within a 13% margin of error at a 95% confidence interval (95% CI). This precision level was chosen pragmatically to balance statistical rigour with the substantial manual effort required for chart review in this validation study. Prince of Wales Hospital is a tertiary care centre with a complex case mix, encompassing a wide range of pleural diseases and advanced respiratory procedures. Within the PWH cohort, the types of pleural disease (pleural effusion, pneumothorax, and trauma-related pleural events) and their underlying aetiologies (eg, non-tuberculous infection, tuberculosis, and malignancy) were determined through retrospective review of clinical notes, discharge summaries, radiological findings, and blood and pleural fluid analysis results using the CMS. Procedure codes were verified by reviewing procedure records within the corresponding hospitalisation episodes. All cases were independently reviewed by two board-certified respiratory physicians. Discrepancies were resolved through joint case review until consensus was reached. Coding accuracy was expressed as PPVs with 95% CIs. The PPV was calculated by dividing the number of true positives (ie, hospitalisation episodes in the PWH cohort where diagnosis and procedure codes were confirmed by manual verification) by the total number of true positives and false positives (ie, episodes where codes were rejected upon manual review). The 95% CI was calculated using the exact binomial method.

We hypothesised that the PPVs for the accuracy of diagnosis and procedure codes would be equal to or greater than 0.700, a commonly used threshold for successful validation.6 7 8 The primary endpoint was the determination of PPVs for the listed diagnosis and procedure codes. All statistical analyses were performed using Python (version 3.12.6).

A total of 26 757 non-traumatic pneumothorax, 218 018 non-traumatic pleural effusion, and 1269 trauma-related pleural events were retrieved from CDARS between 2013 and 2022. Following the exclusion of paediatric patients and uninterrupted hospitalisation episodes, 20 888 non-traumatic pneumothorax, 199 323 non-traumatic pleural effusion, and 1127 trauma-related pleural events remained in the main cohort. Of these, 2451 (11.7%), 24 938 (12.5%), and 251 (22.3%) diagnosis codes for non-traumatic pneumothorax, non-traumatic pleural effusion, and trauma-related pleural events, respectively, were identified from PWH (Fig). Additionally, 185 154 and 106 450 relevant respiratory procedures with ICD-9-CM codes starting with 33 and 34, respectively, were retrieved. After exclusions, 181 770 and 101 336 procedure codes remained, of which 16 078 (8.8%) and 17 299 (17.1%) procedure codes, respectively, were identified from PWH (Fig). Tables 1, 2, and 3 list the diagnosis codes included in the validation analysis for non-traumatic pneumothorax (Table 1), non-traumatic pleural effusion (Table 2) and trauma-related pleural events (Table 3), while Tables 4 and 5 present the procedure codes starting with ‘33’ and ‘34’, respectively; the breakdown of hospitalisation episodes retrieved using these codes, and the numbers remaining after screening, are also shown.

The overall PPVs (95% CIs) for pneumothorax, pleural effusion, trauma-related pleural events, and all diagnosis codes were 0.853 (0.787-0.904), 0.928 (0.903-0.948), 0.957 (0.907-0.981), and 0.919 (0.898-0.936), respectively. The overall PPVs (95% CIs) for procedure codes starting with 33, starting with 34, and for all procedure codes were 0.932 (0.913-0.948), 0.933 (0.916-0.948), and 0.933 (0.920-0.944), respectively.

The PPVs for diagnosis codes related to pneumothorax, pleural effusion, and trauma-related pleural events were all equal to or greater than 0.700, with ranges of 0.700-1.000, 0.833-1.000, and 0.857-1.000, respectively. The lowest PPV (95% CI) was observed for postoperative pneumothorax (procedure code 512.1.2) at 0.700 (0.560-0.812). The highest PPVs were seen for iatrogenic pneumothorax (procedure code 512.1.0) and postoperative haemothorax (procedure code 511.8.7), both at 1.000, with 95% CIs of 0.933-1.000 and 0.762-1.000, respectively. The reasons for false-positive diagnosis codes are summarised in online supplementary Tables 1 to 3, with inappropriate coding of alternative diseases being the most common cause.

The PPVs for procedure codes starting with 33 ranged from 0.700 to 1.000. Procedure codes starting with 34 met the PPV benchmark, except for 34.04.3 (indwelling pleural catheterisation) and 34.09.3 (drainage of the pleural cavity, open). The reasons for false-positive procedure codes are listed in online supplementary Tables 4 and 5, with inappropriate coding of alternative but similar procedures being the most common cause. The low PPV for procedure code 34.04.3 (indwelling pleural catheterisation) arose from its misuse to represent non-tunnelled pleural catheter insertion, or to document the presence of an indwelling pleural catheter (IPC) inserted during prior hospitalisations. Procedure code 34.09.3 (drainage of the pleural cavity, open) failed to meet the PPV benchmark because it was misused to represent closed pleural drainage by drain insertion, rather than an open procedure.

This study is the first to validate diagnosis and procedure codes for pleural diseases using a healthcare database in Hong Kong. All diagnosis codes for pleural diseases and the majority of procedure codes for relevant respiratory procedures met the PPV benchmark of 0.700 or higher. Only procedure codes 34.04.3 (indwelling pleural catheterisation) and 34.09.3 (drainage of the pleural cavity, open) failed to meet the validation criteria.

In 2008, the Hong Kong Thoracic Society reported the burden of lung disease in Hong Kong using local data from various governmental sources; however, pleural diseases were not included in the report.9 Over the subsequent decade, the incidence rates of individual pleural diseases were studied in Hong Kong. However, these studies were limited in scope as they focused on single pleural diseases (eg, empyema,10 11 12 malignant mesothelioma,13 and spontaneous pneumothorax14) or were restricted to single-centre settings.10 11

There is a pressing need for contemporary, population-based epidemiological data covering various pleural diseases in Hong Kong. A recent local survey highlighted heterogeneous practices in the management of pleural diseases among medical clinicians and reflected a lack of awareness and dedicated service infrastructure for pleural diseases.15 Given the rapid advancements in diagnostic strategies and therapeutic options for pleural diseases,16 an accurate and up-to-date assessment of their clinical burden is crucial. Such data provide a foundation for guiding future research, benchmarking healthcare standards in Hong Kong against those of other countries, informing the allocation of future healthcare resources for pleural diseases, and estimating the workload of healthcare professionals managing these conditions. All such service developments should be based on an accurate estimation of the current burden and projected future demand. The use of existing healthcare databases offers a practical approach; however, relevant diagnosis and procedure codes must first be validated. A similar research pathway was followed by Arnold et al,17 who validated diagnosis codes prior to assessing the epidemiology of pleural empyema in English hospitals.17 18

Nearly all PPVs of the diagnosis and procedure codes studied exceeded the benchmark of 0.700. Notably, PPVs for procedure codes were generally higher than those for diagnosis codes. This is because diagnosis codes can be carried over from previous hospitalisation episodes, enabling attending physicians to select active or inactive diagnosis codes regardless of their relevance to the current episode. In contrast, procedure codes cannot be carried over and must be entered manually to reflect procedures performed during the corresponding hospitalisation episode. This requirement contributes to the higher accuracy for procedure codes.

The PPV for procedure code 34.04.3 (indwelling pleural catheterisation) was unexpectedly low due to misuse. The absence of a specific diagnosis code indicating the presence of an IPC, combined with the inclusion of the term ‘pleural’ in the code description, contributed to its incorrect use, particularly during searches for non-tunnelled pleural catheter insertion. Updated diagnosis codes to indicate the status ‘presence of IPC’, or a new procedure code for ‘pleural fluid drainage using an existing IPC’, would accurately reflect the clinical scenario. Once available, such codes should be validated before any analyses of IPC use in territory-wide healthcare databases. Alternatively, establishing a clinical registry for IPC use could facilitate more accurate tracking of patients with both malignant and benign causes of pleural effusion.

Some diagnosis codes (eg, hydrothorax related to dialysis [511.8.3] and hydrothorax as complication of peritoneal dialysis [551.8.8]) and procedure codes (eg, video-assisted thoracoscopy for haemostasis [34.09.4] and injection into thoracic cavity [34.92.0]) were used in other hospitals but not at PWH; therefore, they could not be validated in this study. Within the PWH cohort, alternative diagnosis or procedure codes were used and validated. However, the number of hospitalisation episodes associated with these codes was small, and their impact would be minimal in a territory-wide healthcare data analysis where similar codes are grouped together.

Duplication of subcodes for similar diagnoses or procedures was also noted. Several diagnoses and procedures were represented by different codes, including:

Hydrothorax related to dialysis (511.8.3) and hydrothorax as complication of peritoneal dialysis (511.8.8);

Fibreoptic bronchoscopy (33.22.0) and bronchoscopy (33.23.0);

Endoscopic ultrasonography of bronchus (33.23.3) and endobronchial ultrasonography (33.23.5);

Closed endoscopic biopsy of bronchus (33.24.0), bronchoscopic biopsy (33.24.1), fibreoptic bronchoscopy with biopsy (33.24.2), and flexible bronchoscopy with biopsy of bronchus (33.24.7);

Lung biopsy via endoscopy (33.27.0), bronchoscopic biopsy under fluoroscopic guidance (33.27.1), and flexible bronchoscopy with biopsy of lung (33.27.2);

Video-assisted thoracoscopy for haemostasis (34.09.4) and video-assisted thoracoscopy, haemostasis (34.21.5); and

Chemical pleurodesis (34.92.1) and pleurodesis, chemical (34.92.2).

Researchers should be reminded to search all relevant diagnosis and procedure codes to minimise the risk of missing data for specific diseases or procedures during code searches. In the long term, reconciling similar codes may help reduce ambiguity and improve data consistency.

This study has several strengths, notably its status as the first validation study conducted using a large healthcare database in Hong Kong. It successfully validated codes for a wide range of pleural diseases and respiratory procedures, thereby laying the foundation for future epidemiological research. However, several limitations should be acknowledged. Not all codes could be adequately validated due to their small case volumes in the PWH cohort. For example, codes for Meigs’ syndrome (220.4), traumatic pneumothorax with open wound into thorax (860.1), and traumatic haemothorax with open wound into thorax (860.3) had small numbers even in the overall cohort, and some codes were duplicated. As such, future research incorporating patient searches based on these diagnosis and procedure codes should take these limitations into account. The single-centre nature of the study represents a further limitation, as disease patterns and coding practices may vary across district general hospitals.

This is the first validation study of diagnosis codes for pleural diseases and procedure codes for relevant respiratory procedures using a territory-wide healthcare database in Hong Kong. All diagnosis codes and the majority of procedure codes demonstrated high PPVs, indicating accurate coding. Given the emergence of new respiratory procedures, diagnosis and procedure codes should be regularly updated. The removal or consolidation of duplicated subcodes within the Hospital Authority system is also necessary to facilitate accurate future research and analysis using clinical codes. Further evaluation and harmonisation of coding practices across different hospitals would be beneficial. These measures will pave the way for future territory-wide studies and enable monitoring of the overall burden of pleural diseases in Hong Kong.

Concept or design: KKP Chan.
Acquisition of data: KKP Chan, TCC Ng, CY Sze, KC Ling.
Analysis or interpretation of data: KKP Chan, TCC Ng, CY Sze, KC Ling.
Drafting of the manuscript: KKP Chan.
Critical revision of the manuscript for important intellectual content: KKP Chan, TCC Ng, C Chan, CHY Lau, SWT Ho, JKC Ng, RLP Lo, WH Yip, JCL Ngai, KW To, FWS Ko, DSC Hui.

All authors had full access to the data, contributed to the study, approved the final version for publication, and take responsibility for its accuracy and integrity.

All authors have disclosed no conflicts of interest.

The authors thank Prof Terry CF Yip from the Department of Medicine and Therapeutics of The Chinese University of Hong Kong for providing statistical support.

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

This research was approved by the Joint Chinese University of Hong Kong–New Territories East Cluster Clinical Research Ethics Committee, Hong Kong (Ref No.: 2022.031). The requirement for patient consent was waived by the Committee due to the retrospective nature of the study.

The supplementary material was provided by the authors and some information may not have been peer reviewed. Accepted supplementary material will be published as submitted by the authors, without any editing or formatting. Any opinions or recommendations discussed are solely those of the author(s) and are not endorsed by the Hong Kong Academy of Medicine and the Hong Kong Medical Association. The Hong Kong Academy of Medicine and the Hong Kong Medical Association disclaim all liability and responsibility arising from any reliance placed on the content.

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